Hispanopedia WikiXenon monochloride (XeCl) is an exciplex which is used in excimer lasers and excimer lamps emitting near ultraviolet light at 308 nm. It is most commonly used in medicine. Xenon monochloride was first synthesized in the 1960s. Its kinetic scheme is very complex and its state changes occur on a nanosecond timescale. In the gaseous state, at least two kinds of xenon monochloride are known: XeCl and Xe2Cl, whereas complex aggregates form in the solid state in noble gas matrices. The excited state of xenon resembles halogens and it reacts with them to form excited molecular compounds.
Introduction
Molecules that are only stable in electronically excited states are called excimer molecules, but may be called exciplex molecules if they are heteronuclear. The exciplex halides constitute an important class of rare gas halides with formula RgX. Rg is the noble gas, and X is the halogen. These molecules are de-excited by emitting a photon whose energy is some Electronvolts. Therefore, the wavelength of the light produced is in the visible or ultraviolet spectra. Gas or gaseous mixtures that may lead to the formation of these molecules is a quasi-ideal laser medium since the population inversion is directly obtained when the excimer is formed. The other consequence of the unstable ground state is that the excimer or exciplex species must be generated by an external excitation (either through a discharge, an electron beam, microwave, or radiation). At least two gases must be used to generate exciplexes: a halogen donor and a rare gas.[1] However, as shown in Table 1, not all rare gas halide molecules lead to the development of lasers; some may not even exist. Multiple molecules and applications have been developed.[2][3][4][5][6][7][8][9][10]
Se han publicado varios artículos de revisión relacionados con la tecnología láser de cloruro de xenón y sus aplicaciones. [ 11 ] [ 12 ] [ 13 ] [ 14 ]
Algunos autores [ 11 ] [ 14 ] destacan la importancia de determinar con precisión la cinética del medio láser cuando intervienen haluros de gases nobles. Resultados recientes han aportado información sobre la fisicoquímica del medio láser. [ 15 ] [ 16 ] [ 17 ] Las investigaciones espectroscópicas se limitan a la región visible-ultravioleta cercana donde operan los láseres de exciplex. Solo se considerarán mezclas binarias de gases de xenón y un donador de cloro, o mezclas ternarias que también incluyan un gas amortiguador (un gas noble indicado por Rg). Los donantes de cloro más interesantes son CCl4 yHCldebido a su uso en tecnología láser, yCl2 (véase la figura 1).
XeCl y XeLos 2 Clson los más importantes en aplicaciones láser entre los cloruros de xenón. Aunque las lámparas de descarga basadas en mezclas de baja presión de xenón y un donante de cloro emiten luz incoherente, son fiables y fáciles de operar. [ 18 ]
Historia
La idea de que los gases nobles pueden formar haluros surgió a principios de la década de 1920: [ 19 ] A. von Antropoff [ 20 ] y Oddo [ 21 ] sugirieron que el kriptón y el xenón pueden formar bromuros y cloruros . En 1933, Yost y Kaye [ 22 ] intentaron sin éxito sintetizar cloruro de xenón iluminando una mezcla de xenón (70 torr de presión) y cloro (225 torr) con una lámpara de vapor de mercurio .
Los monocloruros de xenón se sintetizaron por primera vez en 1965. [ 23 ] Posteriormente, se sintetizó XeCl sólido .2 yXeClSe sintetizaron 4 compuestos a bajas temperaturas. En 1991, Prosperio et al. [ 24 ] demostraron la existencia deXeCl2 en estado gaseoso, lo cual es importante para la cinética láser, aunque emite unaluz infrarroja. [ 25 ]
En 1973 Riveros et al. [ 26 ] sintetizaron XeCl − ions in the gaseous phase at a pressure of 10−4torr. This ionic molecule attracted little interest.
Systematic studies of XeCl were initiated in 1975 by Velazco and Setser,[27] who demonstrated 304 nm emission from XeCl*. This emission was obtained by mixing xenon atoms (Xe3P2) with chlorine gas Cl2 or other chlorinated compounds (NOCl and SOCl2). The excitation was provided by a cold cathode discharge; the total pressure was a few torr. Months later, Ewing and Brau[28] reported lasing from a XeCl film 2Σ1/2+ → 2Σ1/2+ at 308 nm, which was most promising for industrial applications. The preferred chlorine donor for XeCl laser is HCl. The reasons given are:
- Low absorption cross section at 308 nm of the order of 10−19 cm2.[29] The HCl concentration does not substantially affect the laser's output energy. This is not the case for Cl2 which has a very strong absorption at about 300 nm.[28][30]
- Less toxic than chlorine.
- Generates a post-dissociation excimer laser, which is much better than other chlorine donors. 16,000 consecutive excimer lasers pulses were obtained without affecting energy output.[31]
- The constant rate of vibrational excitation and dissociative electron attachment are more favorable for HCl than for other chlorine donors.[32] These processes help the formation of XeCl*.
Three years later Lorentz et al.[33] performed experiments at high pressures (a few atmospheres) in a mixture containing (Ar/XeCl2) and found an emission centered at 450 nm which was attributed to XeCl2.
The first XeClEl láser 2 se desarrolló en 1980. [ 34 ] [ 35 ] Es probable que este tipo de láser sea sintonizable en un amplio rango de longitudes de onda (30 nm) en elespectro visible. Esto es cierto incluso si los fenómenos de absorción ocurren en el lado de las longitudes de onda más cortas y, por lo tanto, limitan la acción del láser en la región roja delespectro electromagnéticoa partir dela emisión de luz. Experimentos de estado sólido conXe2 Cl* [ 36 ] sugieren que el estado gaseoso es más adecuado para el desarrollo de este tipo de láser. La amplificación medida fue correcta en estado sólido. [ 37 ] El estado líquido [ 38 ] parece unláser de colorante, aunque su implementación parece compleja y costosa. Actualmente, elXeEl láser de 2Cl no se ha desarrollado industrialmente. A diferencia del XeCl, el mejor donador de cloro esel CCl4.4 [ 39 ] mientras que no se produce ninguna acción láser al usar HCl. [ 34 ]
En las mezclas se sintetizan a priori cuatro moléculas . Cabe destacar la posibilidad de sintetizarlas bajo las condiciones experimentales de los láseres y sus funciones.
Se observó XeHCl en un medio gaseoso. Sin embargo, esta molécula solo se ha detectado mediante espectros de emisión en las regiones de microondas, radio e infrarrojo lejano, [ 40 ] pero con emisión predicha por dos estudios teóricos a 232 nm [ 41 ] y 129 nm. [ 42 ] Cabe señalar, sin embargo, que cuando está casi agregada, es más probable que sea estable en estado sólido . Lo mismo ocurre con Xe3 Clque teóricamente puede emitir a 500 nm, [ 43 ] mientras que esta actividad nunca se ha observado en estado gaseoso.
El XeH tiene tres líneas de emisión conocidas. Se observaron a 190 nm, [ 44 ] 250 nm [ 45 ] y 660 nm. [ 46 ] Sin embargo, nunca se han manifestado en los espectros láser, lo que lleva a suponer que el XeH no se forma bajo las condiciones experimentales. Por el contrario, el XeH +El ion se forma en las mezclas utilizadas en los láseres. Juega un papel importante en la cinética de la síntesis de XeCl.*,[47] through a reaction that competes with the creation of Xe+ ions (shown below):
HCl+ + Xe → Xe+ + HCl (80 ± 10%)
HCl+ + Xe → XeH+ + HCl (20 ± 10%)
The rate constant of the entire process is 6.4×10−10 cm3s−1 (± 20%).
Xe+ ion is a pivotal precursor in the formation of the exciplex molecule.
XeCl exciplex
The structure of the XeCl molecule
The potential curves presented in Figure 2 are the results of theoretical[48][49][50] and experimental[51] works.
Common characteristics for all halide states of the noble gases includes a group of related excited states B, C and D and a lower group of dissociatively or weakly bound states A and X. States B, D and X have Σ symmetry (Λ = 1/2) while the C state has π symmetry (Λ = 3/2). The state A is itself split into two sub-states, a symmetry Σ, A1/2 and the other symmetry π, A3/2.
The ionization potential of noble gases in their lowest excited state is close to the electron affinity of halogen atoms. Thus, the rare gas halide molecules are formed by an ionic bond since the excited electron of the noble gas is partly transferred to the halogen atom. The molecule so formed is therefore stable as is the case of the states B, C and D.
This electron transfer does not occur with ground state atoms. As the rare gas atoms are not reactive. This is the case for states A and X.
The B, C and D states
These states are correlated with ground state Xe+ ions and Cl−. The spin-orbital splitting of the Xe+ ion into two states (2P3/2 and 2P1/2) is important; also the states B and D to which they are correlated are significantly far away. For the minimum potential curves corresponding to almost the same value of the internuclear distance (re#0.3 nm), the energy difference measured experimentally is about 9940 cm−1.[52][53][54] This is in agreement with the energy of separation of Xe+ (2P3/2) and Xe+ (2P1/2) states valued at 10574 cm−1.
Potential curves of the states B and C intersect adiabatically with a potential curve correlated to Xe* + Cl at large internuclear distances: 7.1 nm experimentally[55] and 7.19 nm[56] and 6.3 nm[10] theoretically. A more recent theoretical investigation specifies these intersection phenomena.[57] States B and C merging at long-distance, intersect two successive potential curves correlated to Xe* + Cl. The lowest correlated to Xe (3P2) + Cl (2P3/2) is 7.25 nm and after that, the next correlated to Xe (3P1) + Cl (2P3/2) is intercepted at 18.68 nm. As this intersection occurs at a great distance, the ionic character of the binding of these states near the equilibrium internuclear distance re is virtually unaffected.
This situation is slightly different for state D that crosses these two potential curves at a much shorter distance.[57] Indeed, state D intersects Xe (3P2) + Cl (2P3/2) only at 0.89 nm and Xe (3P1) + Cl (2P3/2) at 1.02 nm.
The distinction between states B and C is that they are correlated with Xe+ ions whose semi-occupied orbital p is in a plane parallel to the internuclear axis for the state B and perpendicular to this axis for the state C.[58]
On an examination of the energy position of the potential curve of states B and C, their proximity results in some difficulty. the values of the energy gap (EB – EC) between the two states is enumerated in Table 2. The data is highly dispersed; computed values, in particular, are far removed from all experimental values. These were determined mostly from the intensity ratios of the two emissions XeCl* centered at 308 nm and 345 nm, either with or without corrections by the participation of transition (B → A).[59] The most direct measure is given by Jouvet et al.[60] Excitation spectra of XeCl* directly provide the energy difference between the vibrational levels v′=0 and v″=0 which correspond respectively to the B and C states. This value of 90 cm−1 is close to other measurements from studies in kinetics.[16][61][62]
I: measurement derived from the value of the intensity ratios of the XeCl emissions centered at 308 and 345 nm (see § 3-1-1)
C: measurement derived from a kinetic study providing the coupling constants between these two states.
*: Emission at 345 nm is not corrected for the contribution XeCl (B → A)
**: XeCl is in the solid state.
Positioning of state B in relation to state C is theoretically justified by considering the configuration interaction between the ionic and covalent character states of similar symmetry.[65][73][74] In a state 2Σ (as states B and X), a simply occupied orbital is located closer to an orbital of another atom such that the interaction or exchange of charges between two atoms are larger and easier than in a state 2π (like states C and A3/2), where a simply occupied orbital is perpendicular to the molecular axis and far away from another atom. The correction introduced by this phenomenon in terms of energy values is much more important for Σ states than for the π states.[73] This interaction greatly increases the energy of state B relative to that of state C. Hence, the positioning on the observed potential curves from Figure 2.
The X and A states
The lowest states are correlated with ground state xenon and chlorine atoms.
Due to spin-orbital splitting of the chlorine atom level at 881 cm−1[75] into two states, (2P3/2) and (2P1/2), state A is divided into two sub-states. However, the effect of the spin-orbital coupling here is significantly weaker than in the case of Xe+ ion. At large internuclear distances, an energy gap of 882 cm−1 between A1/2 and A3/2 was experimentally measured in the solid state in a neon matrix.[76] Thus, this value is very close to the energy separation of states Cl (2P3/2) and Cl (2P1/2). This confirms the theoretical assumptions of state correlations between XeCl state A and Cl. At large distances state A3/2 is similar to state X. Becker et al.,[77] who laid out the interaction potentials of 35Cl (2P3/2 and 2P1/2) and Xe (1S0) from the analysis of quasi–elastic scattering in collisions produced from crossed beams, has experimentally confirmed this result. Unlike some other noble gas halides, XeCl has a non-dissociative ground state. This bonding character was demonstrated experimentally well before theoretical studies of XeCl in solid state argon matrices at 20K[54] and later in the gaseous state.[53][55]
The Van der Waals force between atoms[78] is not strong enough in state X to explain the presence of a potential well that when low (the depth is in the order of kilotorr) can contain between 12 and 20 vibrational levels (see Table 3). The relative increase in the binding energy of state X compared to state A can also be explained by taking into account the configuration interaction.[79] State A is also very lightly bound with binding energy half that of state X.
Spectroscopic constants
The energy Ev'j'M of a known state M with a vibrational level v' with the rotational quantum number j is:
Ev'j'M = Te(M) + EVib(M) + ERot(M) where Te(M), EVib(M) and ERot(M) respectively denote the molecule's vibrational and rotational electronic energies.
Electronic structure
The main features of the electronic states of a known state M are usually the dissociation energy De, the inter-atomic distance re and energy of the bottom of the potential well EM. For XeCl, different reported values of these quantities are summarized in Tables 4, 5 and 6. They were determined theoretically or experimentally for isotope 35Cl in the solid or gaseous state.
Dissociation energies
Dissociation energies have been calculated or measured for different states of the excimer. Some states have more measurements than others. States A, C and D have too few measurements for a statistical analysis. For state B, the four values are not consistent with each other
For state X, there are six values, two of which are outliers. That of Flannery[10] is an old, imprecise theoretical estimate. That of Tellinghuisen et al.[55] is the first experimental determination made in 1976. Seven years later[83] the same team corrected this value and closed the gap on the most recent estimates. The remaining four values seem to be the only reliable ones. De is (with a probability of 95%) between 278.3 cm−1 and 285.3 cm−1. The interval corresponds to a 1.3% fluctuation around 281.5 cm -1. Indeed, among the selected determinations are two measures with high uncertainty,[77][81] and a third which the author does not indicate.[79] The value of De of state X, depends on the number of vibrational levels contained in the well and sets the number of transitions bound → bound that can be achieved. This result is fundamental to a better understanding of XeCl laser spectroscopy.
Equilibrium atomic distances
The interatomic distance for states A, C and D has few measurements, but they are close. On average, state A is 0.408 nm, state D, 0.307 nm and state C, 0.311 nm.
For the state X, the theoretical determination of Adrian and Jette[86] is statistically far from the others. Omitting it, using confidence level of 95% state X re, will be in the range: 0.318 nm < r e < 0.326 nm.
The value of Tellinghuisen et al.[55] is at the limit of the interval. If ignored, the other three authors announce an identical value of 0.323 nm.
Tellinghuisen's value for state B is far from others for re. It is the same for Ewing et Brau,[56] the oldest study of the noble gas halides which is based on the analogy of the excited rare gas with alkali metals. These are only estimates. These two values will be discarded to give a confidence interval at 95% for the interatomic distance of state B: 0.2993 nm < re < 0.3319 nm.
Bottom of the potential well energies
Table 6 shows that there is very little information for states X, A and D. For state X, Sur et al.[81] arbitrarily took bottom of the well X as the origin of their energy scale. It is therefore not a direct measurement. Therefore, the state X as well as state A have been the subject of only one study; that of Aquilanti et al..[79] For state D, two quite different determinations exist.
As was seen in an earlier section, the location of states B and C is problematic.
State B has the most attention from researchers. Two measures are statistically far from the others. Besides the previously mentioned study by Ewing and Brau,[56] the old theoretical work of Hay and Dunning are among the doubtful determinations[49] which will be broached soon. Without considering these values, the experimental work provides a confidence interval at a very narrow 95% threshold: from 32380.1 cm−1 to 32415.3 cm−1.
In contrast, no conclusion can be drawn statistically given the small number of measurements of state C. However, further analysis will illuminate despite the non-matching character values in Table 6. Indeed, the positioning of the C states with respect to state B has resulted in many publications.
Un análisis estadístico de los valores de la Tabla 2 proporciona un enfoque paso a paso al intervalo de confianza al 95% que es el siguiente: 76,8 cm −1 < (E B - E C ) < 100,2 cm −1 . Solo cuatro medidas pertenecen a este intervalo. Esta es la determinación directa de Jouvet et al. [ 60 ] y tres valores deducidos de estudios cinéticos . [ 50 ] [ 61 ] [ 62 ] Por otro lado, una estimación puntual da 88,5 cm −1 y la única medida que es consistente con ella (dado el error absoluto indicado) es de Jouvet et al. . [ 60 ] en (90 ± 2 cm −1 ). El estudio estadístico confirma entonces las conclusiones alcanzadas en el párrafo 1.1.
Los intervalos de confianza enumerados anteriormente para el estado B y la diferencia de energía (E B – E C ) producen un intervalo para E C : 32279,9 cm −1 < E C < 32338,4 cm −1 .
En estas condiciones, solo el valor de Jouvet et al. [ 60 ] en la Tabla 6 es consistente con este rango. Las tres determinaciones dudosas incluyen la de Hay y Dunning [ 49 ] con un valor defectuoso para E B. Otro estudio teórico temprano de Clugston y Gordon [ 87 ] también arrojó este intervalo. Lo mismo ocurre con el trabajo experimental de estado sólido realizado por Fajardo y Apkarian. [ 72 ]
Calculating the mean of the two values in Table 6 yields 43838.45 cm -1. The energy gap of state B is then of the order of 11400 cm−1. Shostak and Strong[52] experimentally determined the energy difference between states A and B. They found 9900 cm−1. The difference between these values (EB – ED) is very sharp. Considering only work by Sur et al.,[81] the energy difference between states B and D becomes of the order of 9950 cm−1 which is close to that of Shostak and Strong.[52] This observation casts fresh doubts on the theoretical work of Hay and Dunning[49] for which (EB – ED) is 10888 cm−1.
With respect to the electronic structure, it appears that older studies pose a problem regarding some of their results.[10][49][55][56][87] On the other hand, work carried out by Fajardo and Apkarian[72] is not always consistent with observations of the gaseous state. Moreover, recent theoretical studies do not eliminate significant differences with experimental results.[42][43]
The removal of the values of Hay and Dunning,[49] reduces to a determination the values of De for states C and D, and makes the three other values relating to state B homogenous. Among these Tellinghuisen et al.[55] poses a problem for other values. The energy De for state B then has an average value of 36184 cm−1.
Vibrational Structure
The vibrational energy of level v’ of any state M can be calculated as:
EVib(M) = ωe (v’+1/2) – ωexe (v’+1/2)2
where ωe and (ωexe) indicates respectively, the basic vibrational frequency and the anharmonicity constant. Their respective determinations are collected in Tables 7 and table 8.
The basic vibrational frequencies
The values of ωe are grouped together in Table 7.
Los estados X, C y D solo tienen cuatro determinaciones. Ninguna medida puede considerarse estadísticamente distante de las demás, a pesar de las disparidades.
El estado B ofrece nueve determinaciones. Un análisis estadístico conduce a un intervalo de confianza del 95%: 194,7 cm −1 < ω e < 195,4 cm −1 .
Seis valores de la Tabla 7 resultan extraños. Tres de ellos, notablemente. Se trata de publicaciones antiguas, dos de las cuales (Hay y Dunning [ 49 ] y Brau y Ewing [ 89 ] ) fueron fundamentales para la sección anterior. Los resultados de Golde [ 92 ] se basaron en el mismo método que el utilizado por Brau y Ewing [ 89 ] .
Las otras tres medidas que están fuera del rango son más recientes. Kvaran et al. [ 90 ] investigaron el estado sólido. Al igual que Fajardo y Apkarian, [ 72 ] observaron diferencias significativas en el estado gaseoso. En contraste, lo más sorprendente son las discrepancias entre Jouvet et al. [ 60 ] y Tamagake et al. [ 74 ] , que fueron estudios con buenos resultados. Finalmente, entre los valores que coincidieron con estos rangos hay muchos estudios que fueron más teóricos [ 42 ] [ 88 ] que experimentales. [ 55 ] [ 81 ]
En conclusión, Tellinghuisen et al. [ 55 ] ofrece muy buenos resultados tanto en el estado B como en el estado X.
Los resultados reportados sobre el estado C son bastante cuestionables. [ 49 ] [ 72 ] [ 87 ] El trabajo de Jouvet et al. [ 60 ] es extremo comparado con otras medidas del estado B.
As for state D, excluding results by Hay and Dunning[49] makes it more cohesive than the other three values.
Finally it is necessary to specify the values of ωe for states X, C and D. The main interest of this clarification would be a better resolution of the vibrational structure of the transition used in the laser, which requires a better knowledge of state X. On the other hand, the structure of state C is important because it plays a fundamental role in laser kinetics .
Anharmonicity constants
Table 8 shows anharmonicity constant measurements for the various states. The measurements for constants of anharmonicity for states X, C and D are very inconsistent.
Six measurements for state B produce the confidence interval at 95%:
0.532 cm−1 < ωexe < 0.669 cm−1.
The work by Jouvet et al.[60] is statistically far from the others and the authors cannot explain this difference. Hay and Dunning[49] give correct forecasts, as does the vibrational structure study by Tellinghuisen et al..[55]
Rotational structure
The following expression denotes rotational energy: Erot(M) = B’.K’ef – D’.(K’ef)2, where K’ef = j’(j’+1) ± (1/2).δ(j’+1/2) ;
B’ and D’ respectively are the rotational constant and the first centrifugal distortion constant. Their values are indicated in table 9 and table 10. δ is a parameter that is equal to 2.0 for state B[62] and 0.4 for state X.[93]
Therefore, the rotational structures is very poorly known. Despite that, one should notice the consistency of some measurements made on B’.
Synthetic pathways
Cuando se encuentran en una configuración perteneciente a estados metaestables np 5 (n+1)s 1 , (n = 5 para el xenón), los gases nobles poseen propiedades de polarizabilidad y dispersión elástica similares a las de los metales alcalinos . [ 95 ] El electrón de valencia , s, del gas noble excitado tiene una energía de enlace cercana a la del metal alcalino que le sigue en la tabla periódica . En publicaciones anteriores, [ 56 ] [ 92 ] [ 96 ] [ 97 ] esta analogía que es aplicable solo para los gases nobles más pesados, se utiliza para estudiar el comportamiento de estos gases con donadores de halógeno. Los metales alcalinos tienen buena afinidad química por los halógenos y deberían tener afinidad por los gases nobles excitados. Experimentalmente, la sección transversal de colisión de los estados metaestables de los gases nobles con los halógenos es similar a la de los metales alcalinos con los halógenos. [ 96 ] [ 97 ] [ 98 ] Por lo tanto, un xenón excitado tiene una estructura electrónica cercana a la del cesio , de modo que puede reaccionar con un donador de cloro para formar XeCl* .
Existen diferencias significativas entre los metales alcalinos y los gases nobles excitados en cuanto a su simetría molecular . El número de estados de los haluros de gases nobles es mayor que el de las sales de metales alcalinos. Esto se debe al desdoblamiento espín-órbita de los átomos e iones de los gases nobles.
La primera condición para producir XeCl es hacer que el xenón sea reactivo. Para ello, debe ser excitado, ionizado o ambas cosas. Se han utilizado varios métodos de excitación externa. Los más comunes son las descargas eléctricas, [ 27 ] los haces de electrones, [ 39 ] la excitación láser, [ 99 ] las microondas [ 100 ] y las partículas α. [ 15 ]
La excitación no es selectiva y se produce la formación de XeCl.* pueden seguir muchas rutas. Su importancia relativa varía con las condiciones, especialmente con la presión, el modo de excitación y el donador de halógeno. Cuando intervienen mezclas ternarias, el proceso de creación de XeCl es más complicado. Sin embargo, la adición de ungas amortiguadorofrece muchas ventajas. Otros gases nobles son más baratos que el xenón, pero (junto con sus especies excitadas y sus iones) absorben menos que el xenón a 308 nm. Por lo tanto, el gas amortiguador puede utilizarse en proporciones muy altas sin alterar mucho la potencia de salida del láser. En estas condiciones, las proporciones de xenón y HCl deben coincidir con las necesarias para producir la cantidad deseada de molécula de exciplex. La función esencial del gas amortiguador es transferir a los átomos de xenón la energía de excitación necesaria. Esta transferencia puede considerarse instantánea. Puede dar lugar a la excitación o ionización del xenón o a la formación de un ion RgXe+. [ 4 ] Cada una de estas especies puede reaccionar con el donador de cloro para formarXeCl* . Por otro lado, la formación de especies neutras de RgXe no parece ser importante. [ 5 ]
Las dos principales vías de síntesis de exciplexos son la colisión (entre moléculas de cloro y xenón, donde al menos una especie se excita) y la recombinación iónica. El gas portador interviene a veces en la primera y casi siempre en la segunda.
La formación de XeCl* es extremadamente eficaz ya que Konovalovet al. [ 101 ] observaron emisión de XeCl enkriptónmientras que el xenón estaba presente solo en cantidades traza (0,2%).
Vía fotoasociativa
XeCl* La síntesis ocurre cuando una mezcla que contiene xenón y cloro (Cl2 ) se excita mediante un láser que emite entre 304 nm y 312 nm. [ 99 ] Luego se inducen dos reacciones: [ 102 ]
- excitación de un átomo o molécula de xenón electrónicamente aislado seguida de colisiones reactivas
- La interacción simultánea de un par en colisión y uno o dos fotones introducidos por láser genera un estado intermedio que luego da como resultado el producto deseado sin una colisión intermedia.
En este último caso, se forma un complejo transitorio [ 103 ] (Xe- Cl2 )*en el estado (1Πu). [ 104 ] Por lo tanto, son posibles dos vías de disociación desde el momento en que un fotón es absorbido por el par Cl-Cl o el par Xe-Cl de (Xe-Cl2 )*en el estado (1Πu). [ 104 ] [ 105 ]
Xe-Cl2 (1Πu) + hν →Xe-Cl2 (1Πg) →Xe +Cl2 −→ XeCl(B,C) + Cl
Xe- Cl2 (1Πu) + hν → Xe-Cl(X)-Cl + hν → Xe-Cl(B)-Cl → XeCl(B) + Cl
La constante de velocidad de la reacción se midió considerando el fotón como un tercer participante. Es 6 × 10−29 cm 6 s −1 . [ 106 ]
Se obtuvieron resultados similares con otros donantes de cloro, incluidos HCl y CCl4.4 .
En todos los casos, las moléculas de XeCl(B, C) se producen siempre en estados con una fuerte excitación vibracional.
Trayectoria de colisión
La importancia de numerosos procesos radica en el tipo y la excitación de las especies que colisionan. El principal residuo en todos los casos son las emisiones que resultan de las colisiones binarias.
colisiones de arpones
Estas reacciones involucran al donador de cloro en el estado fundamental y un átomo excitado de xenón, tanto en el primer 6s, Xe * como en niveles superiores Xe ** como el nivel 6p.
Mecanismo
Generalmente, estas reacciones pueden describir el resultado de colisiones de átomos de gases nobles (Rg) y donadores de halógenos (RX), donde X es un átomo de halógeno y R una molécula radical. [ 107 ] Los productos de las reacciones dependen en gran medida del tipo de gas noble y donador de halógenos. En nuestro caso, donde Rg = Xe y X = Cl, la naturaleza de los productos sigue esta regla. [ 50 ] [ 108 ] En algunos casos, esta colisión puede no producir ningún gas noble haluro. [ 50 ]
El átomo Rg y la molécula RX se siguen cuando se aproximan al potencial adiabático más bajo y la reacción procede mediante el mecanismo orbital controlado en el cruce iónico-covalente. Los reactivos (Rg y RX) se aproximan en una superficie diabática covalente. Luego forman un complejo Rg* ...RX a una distancia internuclear bastante grande. Su potencial es V(Rg, RX). Cuando la distancia se vuelve suficientemente pequeña, puede ser que V(Rg, RX) interseque una superficie de potencial iónico (Rg +...RX − ). El cruce puede ocurrir a través de la transferencia de un electrón de Rg a RX. Esto se conoce como mecanismo de arpón. En este caso, los átomos continúan en la nueva superficie. Esto conduce a una reacción de difusión y a la formación de RgX * .
Figure 3 shows the process of creating XeCl* which involves Rg=Xe and X=Cl. After its transfer, the electron occupies an antibonding orbital of RCl. In the presence of Xe+, RCl− splits into R and Cl−. Xe* ions and Cl− then recombine to form XeCl in states B, C and D because there is no new force between Cl− and R. The vibrational excitation of XeCl* is always important. In total, everything takes place according to the reaction equation:
Xe* + RCl → XeCl*(B,C,D) + R with rate constant of kMX
However, the competitive formation of XeCl* reactions occur before or after the intersection. They correspond to the interactions of the V potential (Rg*, RX*) and V (Rg + RX *).
In general, this situation occurs when the ionic surface is intersected by covalent surfaces where RX is in its lowest excited state. The distribution of output depends on the number and nature of output channels that are possible following collisions.[107][109] The most common occurs at the intersection of the potential surfaces by a transfer of electronic energy that can cause a dissociation of the excited acceptor:
Rg* + RX → (Rg+...RX−) → Rg(B,C,D) + RX*with rate constant kET
Rg* + RX → (Rg+...RX−) → Rg + R + X with rate constant kD
This pathway tends to become less important as the complexity of RX increases
It is also possible that the transfer took place in a state that is not correlated to the RX* ion but at very high Rydberg states in the neutral molecule and lying just below the limits of ionization. Critical factors regulating the branching ratios are the potential energies interrelated with the molecular ion (VI), the Rydberg group close to the ionization (VII) or an initial excited atom (VIII). The importance of these pathways increases with the depth of hole V (Rg*, RX*).
When the highly separated asymptotic energy levels are in the order VI > VII > VIII and the potential energies (VII) are attractive, the first failed intersection is encountered when the approach of reacting atoms favors output of (VII) rather than the anionic (VI). Since (VII) has a cationic center that remains strongly linked, it preferentially leads to a transfer of excitation. This is the dissociative excitation reaction:
Rg* + RX → Rg + R* + X or Rg + R + X*with rate constant kDE
If VIII > VII at long distance, the Penning ionization pathway or associative ionization are possible:[107]
Penning ionization : Rg* + RX → Rg + RX+ + e−with rate constant kPI
Associative ionization: Rg* + RX → (RgRX)+ + e−with rate constant kAI
In (VI) bonding with an halogen atom is in principle, weak and atomic transfer is enhanced between Rg and R. This potential thus leads to the formation of the exciplex.
There are therefore a priori five competitive ways of synthesizing RGX. For XeCl* an excited xenon atom collides with a chlorine donor. These five reactions were all observed for various chlorine donors.[109] To quantify the proportion of produced exciplex, it is customary to define the branching ratio. It shows the rate of formation of XeCl, as denoted by Γ XeCl:
ΓXeCl = kMX / (kMX + kAI + kPI + kET + kDE + kD)
ΓXeCl measurements were effectuated for several chlorine donors and principally for the 6s and 6p states of xenon.
Xe(6s or 6p) + RCl → products with rate constant kQ
kQ is the total rate constant and is calculated as: kQ = kMX + kAI + kPI + kET + kDE + kD
The results for Cl2, CCl4 and HCl (v = 0) are summarized in Tables 11–13. ΓXeCl is set equal to 1 by Setser Ku[102] where the chlorine donor is Cl2. This decision is justified by the fact that for Xe* + Cl2 we have VII > VI > VIII, which according to Simons[107] fixes an unlikely channel for the excitation transfer.
Un primer análisis de las Tablas 11-13 muestra que los resultados concuerdan bien cuando se realizaron varias mediciones para la misma reacción. Observamos que la mayoría de las colisiones solo tuvieron sus constantes de velocidad medidas una vez. Además, con raras excepciones, estas determinaciones para K Q y Γ XeCl se limitan a los estados excitados más bajos del xenón atómico. Esto evidencia la necesidad de nuevas mediciones para confirmar los resultados experimentales disponibles y estimar el papel de otros estados que se forman si se utilizan, como en el caso de los láseres, modos de excitación no selectivos.
Un resultado importante para los láseres de XeCl se evidencia en un análisis inicial. Xe(6s) + HCl (v = 0) no produce XeCl. Sin embargo, según las estimaciones de Kannari et al. [ 118 ], el 5% de la síntesis de exciplex se produce a través de la reacción de arpón. Además, los estados Xe(6p) producen el 2,5% de esta cantidad.
Estados iniciales: Xe(6s)
El cloro molecular reacciona eficientemente con estos estados de xenón. Dado que Cl2 se forma en mezclas gaseosas (Figura 1), esta reacción es importante en lacinéticade los láseres de XeCl.
Reacción con CCl4 es más rápido queCl2 por un orden de magnitud, pero sigue siendo eficaz. Esta reacción es importante en lacinéticadelXe2 láseres.
Si el donante de cloro es HCl, la situación es más compleja. Se aprecian dos situaciones:
- HCl en el estado fundamental con nivel vibracional v=0. Los valores de K D son muy similares independientemente del estado inicial del xenón; la relación de ramificación para los estados 6s es muy baja. La contribución de estos estados de xenón a la formación de XeCl* es insignificante. Además, se producen reacciones competitivas antes de la intersección de las curvas de potencial V(Xe* + HCl) y V(Xe++ HCl −). [ 115 ] El extintor Xe (6s) HCl es importante en la cinética láser . Destruye los estados de xenón capaces de formar un exciplex.
- HCl en el estado fundamental con nivel vibracional v=1. Para el Xe( 3PAG2 ) estado, Chang [ 68 ] identificó un marcado aumento en la tasa de producción de XeCl. La constante de velocidad para la síntesis de XeCl se midió con un valor mínimo de 2×10−10 cm 3 s −1 y Γ XeCl = 35%. La primera estimación realizada por Levin et al. [ 116 ] y basada en correspondencia se publicó en 6 × 10−11 cm 3 s −1 y Γ XeCl = 11%, pero esta reacción quedó obsoleta con las mediciones directas de Chang.A medida que aumenta la excitación vibracional del HCl, aumenta la velocidad de formación de XeCl. No se dispone de una medición directa, pero existen estimaciones analógicas. Para v=2, los valores de las constantes de velocidad de síntesis incluyen: 5,6 × 10−10 cm 3 s −1 [ 119 ] y 2.0 × 10−10 cm 3 s −1 . [ 120 ]
Según otros autores, se tiene en cuenta el conjunto de niveles vibracionales. Y para V ≥ 1, Kannari et al. [ 121 ] propusieron una constante de velocidad de síntesis de 5,6 × 10−10 cm 3 s −1 y Γ XeCl = 26%. Son necesarios experimentos para aclarar este aspecto de la cinética láser . [ 118 ]
Estados iniciales: Xe(6p)
Las reacciones sintéticas del XeCl son generalmente más efectivas que en el estado 6s. Esto se aplica a los tres donadores de cloro indicados gráficamente en las tablas 11, 12 y 13.
Las constantes de velocidad son dos veces más rápidas para el cloro que para el HCl y el CCl4.4 .
Para el HCl, la situación es diferente al caso anterior. Si las constantes de velocidad totales son del mismo orden de magnitud que las de los estados 6s, las relaciones de ramificación Γ XeCl son altas. El resultado explica la predicción de Kannari et al. [ 118 ] con respecto a la efectividad de la velocidad de síntesis de XeCl* de Xe(6p).
Con referencia a las curvas de potencial de la Figura 3, las curvas de potencial de V( Xe** + RX) y V( Xe + + RX − ) se intersecan a una mayor distancia internuclear que los estados 6s en una región de interacciones fuertes. [ 115 ] Esto explica por qué la producción de XeCl es más efectiva después de la intersección que en los estados 6s [ 102 ] [ 115 ] independientemente del donador de cloro, como se observa para Cl2 , HCl,CCl4 , y también para los clorofluorocarbonos [ 122 ] en los estados 6p[1/2]0y 6p[3/2]2.
Se producen reacciones competitivas. Una de ellas ha sido observada y cuantificada experimentalmente: la relajación colisional inducida por HCl: [ 123 ]
Xe(6p[3/2] 2 ) + HCl → Xe(6s[5/2] 2 0 ) + HCl con constante de velocidad k a o k a = 4,3 × 10−11 cm 3 s −1 .
Esto representa solo el 6% del valor de k Q de la tabla 12 para el estado (6p[3/2] 2 ). Dado que la proporción de síntesis de exciplex se sitúa en el 60%, cabe concluir que existen otros procesos competitivos importantes en juego.
Los resultados resumidos en la Tabla 12 se refieren al HCl (v=0). Para los estados 6p, el papel de la excitación vibracional del HCl en la cinética de la formación de XeCl no se comprende bien. Algunos autores argumentan que las constantes de velocidad son cercanas al estado v=0 si el HCl está vibracionalmente excitado, pero estos resultados se basan en analogías. Por lo tanto, se necesita una aclaración experimental. La constante de velocidad para v=1 se sitúa en 5,6 × 10−10 cm 3 s −1 . [ 116 ] Se utiliza el mismo valor para v=2. [ 120 ] Kannari et al. [ 121 ] todavía no es probable que reduzca los diferentes niveles vibracionales del HCl y para v≥1, 8.2 × 10Se propone −10 cm 3 s −1 .
Estados fuertemente excitados del xenón
Experimentos realizados con Cl2 muestran que la efectividad de la formación de XeCl aumenta con la energía de excitación del átomo de xenón; la constante de velocidad de síntesis se multiplica por tres cuando se pasa de los estados 6s a los estados 7p (tabla 11).
La tasa de XeCl* synthesis increases by an order of magnitude when one goes beyond the 6s states to the 6p states when CCl4 (table 13) is utilized.
HCL is ambiguous. An examination of Table 12 shows that the increase in kQ does not appear to increase significantly with the xenon excitation. So far, no measurements go beyond the 5d[3/2] state that is roughly of the same energy as the 6p state. The rate of synthesis also seems very effective from the 7s[3/2] states[70] without there being any known numerical value. The available information does not support assuming a more efficient rate of synthesis of the exciplex as the excitation of xenon gradually increases. Indeed, for the state 5d[5/2]30, there is only an excitation with a reaction rate constant of 3.2×10−12 cm3s−1:[123]
Xe(5d[5/2]20) + HCl → Xe(6p[3/2]2) + HCl
Also, the Rydberg states do not appear to have produced XeCl. The observed reactions for Xe(31f)[124] are the following:
Xe(31f) + HCl(J) → Xe(31l) + HCl(J) (α)
Xe(31f) + HCl(J) → Xe(nl) + HCl(J-1) if J≤5 (β)
Xe(31f) + HCl(J) → Xe+ + e− + HCl(J-1) if J>5 (γ)
The total rate constant is kT = (11.3 ± 3.0)×10–7 cm3s−1, divided into the following:
kα = (5.5 ± 2.5)×10–7 cm3s−1 (l-changing)
kβ = (4.8 ± 2.4)×10–7 cm3s−1 (n-changing)
kγ = (0.9 ± 0.4)×10–7 cm3s−1 (ionisation)
Note that the reaction (γ) produces an important XeCl precursor, namely Xe+.
Conclusions
Harpoon reactions play an important role in laser kinetics.
For Xe2Cl lasers, the situation is simple when reacted with CCl4. For the XeCl laser, the harpooning kinetics is more complex. Despite its weak proportion in a gaseous mixture, Cl2 is produced much effectively from the exciplex through harpooning. The 6s states do not come into play in the production of XeCl* to the extent that they give rise to collisions with molecules of vibrationally excited HCl.
La cinética de la excitación vibracional del HCl es, por lo tanto, fundamental. Al menos los primeros seis niveles de vibración deben tenerse en cuenta para construir un modelo satisfactorio. [ 125 ] [ 126 ] [ 127 ] [ 128 ] Esta excitación vibracional es producida por los siguientes electrones:
HCl(v) + e − → HCl(v') + e − (EV) con constante de velocidad K .
Se midieron las constantes de velocidad de (EV) para las siguientes transiciones: v=0→v'=1, v=0→v'=2, v=1→ v'=2 y v=2→v'=3. Entonces se puede proponer una ley empírica: [ 127 ]
K v→v+1 = v K 0→1
K v→v+2 = v K 0→2
Los valores de K dependen de la distribución de energía de los electrones, como se muestra en la Figura 4.
En las reacciones de arpón, la velocidad de síntesis del estado B con respecto a la del estado C se encuentra entre 1 y 2, independientemente de la naturaleza del haluro de gas noble. [ 58 ] Sin embargo, se observa un claro aumento en la proporción del estado B con respecto al estado C cuando aumenta la presión. [ 96 ] Esta relación también está fuertemente influenciada por la naturaleza del donador de cloro. Es 1,2 para CCl4 [ 96 ] y 1,3 paraCl2. [ 58 ] El estado deexcitación del xenón es importante. Para el caso delCl2 , se observó [ 112 ] que la tasa de síntesis del estado B podría ser cinco veces mayor que la del estado C si Xe(6p[1/2]0) participa en la reacción que si se encuentran en estados fuertemente excitados.
En las colisiones reactivas entre especies neutras intervienen otras reacciones, pero su papel es insignificante.
Reacciones que involucran especies moleculares excitadas
El papel de las moléculas de xenón
Resulta difícil encontrar reacciones que involucren las moléculas de xenón y HCl en la literatura publicada.
Lorents [ 70 ] solo midió la constante de velocidad de descomposición de Xe 2 * por HCl como (8,2 ± 0,8) × 10 –10 cm 3 s −1 sin indicar los productos resultantes.
Por el contrario, Bibinov y Vinogradov [ 108 ] observaron la siguiente reacción con Cl2 :
Xe 2 * + Cl2 →XeCl* + Cl + Xe
Exciplex synthesis was by harpooning. The rate constant was estimated at 7.1×10−10 cm3s−1.[121]
The role of excited HCl
Castillejo et al.[129] observed an HCl emission between 200 and 240 nm due to the B transition B(1Σ+) → X (1Σ+) (see figure 5). This emission disappears with increase in the pressure of xenon and XeCl(B) appears. In other words, XeCl(B) could be synthesized by the reaction:
HCl (B 1Σ+) + Xe (1SO) → XeCl(B) + H
The rate constant is estimated at 5×10−10 cm3s−1.[130]
Another output pathway seems competitive to exciplex synthesis within the same collision which product should be:
Xe+ + H + Cl + e− and the associated rate constant associated is 1×10−10 cm3s−1.[121]
The role of excited Cl2
Cl2 is synthesized in the laser through the following reaction:
Cl* + HCl → Cl2* + Cl
The rate constant is 1×10−10 cm3s−1.[121] Exciplex synthesis occurs through the following reaction:
Xe + Cl2*(1Σu+) → XeCl*+ Cl with rate constant ku
The values of ku are given in table 14. The results from Zuev et al.[131] is statistically distant from the others although recent. Ignoring it, the average value should be ku = 2.6×10−10 cm3s−1.
A corresponding reaction could be found for the Cl2* (D’ 3π2g)[108] state.
Termolecular reactions
They are essentially produced in ternary mixtures and are of the type:
Xe** + Cl2 + M → XeCl* + Cl + M with rate constant kc
The rate constant kc is given in table 15. Notice only the processes where M=Ar are negligible.
As for helium, there are two reactions:
Xe* + Cl + He → XeCl* + He
Xe** + Cl + He → XeCl* + He
The rate constants are respectively, 10−27 cm6s−1 and 3×10−27 cm6s−1.[134]
There also exist data where the xenon atoms are at the ground state:
Xe + Cl + M → XeCl (X) + M where M = Ne or Xe
In both cases, the rate constant is: 1.2×10−33 cm6s−1.[135]
Other reactions
Chlorine, Cl2, synthesized in a gaseous mixture could induce the following reactions:
Xe + Cl2 → XeCl2
Xe* + Cl2 + Xe → Xe+ + Cl2− + Xe → (XeCl2)* + Xe[136]
As the sublimation temperature of XrCl2 is ts= 80 °C, this molecule is synthesized at room temperature, in the solid state within the gaseous mixture. This causes a parasitic lasing phenomenon called "laser snow".[137]
Some authors have proposed increasing the temperature to make XeCl2 sublime. It then becomes reactive and actively participates in the synthesis of XeCl* :
XeCl2* → XeCl* + Cl
Xe* + XeCl2 → 2 XeCl*
The temperature increase procures two advantages: to eliminate the parasitic laser phenomenon and increase XrCl production. However, the increase should not be of much importance so that XeCl2 does not dissociate which would destroy the preceding reaction.
In ternary mixtures, RgCl exciplexes could be synthesized, possibly leading to the formation of XeCl* through so-called displacement reactions. They have been observed when the Rg is Ar or Kr:[135][138]
RgCl* + Xe → XeCl* + Rg with rate constant kd or kd=1.5×10−10 cm3s−1 for Rg = Ar
Inversely, RgCl synthesis consumes the available chlorine reducing the rate of XeCl production. The laser quality may be negatively affected as was the case with krypton.[139]
This review will be limited to synthetic reactions of XeCl*, excluding ionic recombination. A second pathway exists and will be considered.
Ion recombination
According to several authors[115][140][141]bimolecular reactions (Xe+ + Cl−, Xe2+ + Cl− and RgXe+ + Cl−) are not involved.
Ternary reactions are typically:
Xe+ + Cl− + Rg → XeCl* + Rg (3)
Xe+2 + Cl− + Rg → XeCl* + Rg + Xe (4)
RgXe+ + Cl− + Rg → XeCl* + 2 Rg (5)
Xenon ions are synthesized directly in the discharge or through successive reactions that involve Rg+, Rg2+ as well as other ionic or excited species. Figure 1 gives an example where Rg=Ne and figure 6 where Rg=He.[116][119][142][130][143][144]
The Cl− ions are basically formed by dissociative attachment from an HCl electron:[32]
HCl(v) + e− → H + Cl− (AD)
En ese mismo caso, la constante de velocidad (AD) depende de la distribución de energía de los electrones como se ilustra en la Figura 4.
El tercer elemento Rg es químicamente pasivo. Su única función es estabilizar la reacción. [ 145 ] Por lo tanto, los autores solo consideraron las tasas de recombinación de los iones positivos y negativos. Estas varían significativamente con la presión total de la mezcla gaseosa, el gas portador y la temperatura.
Las reacciones (3) y (4) se demostraron experimentalmente para todos los gases nobles. Las figuras 7 y 8 muestran la influencia del gas amortiguador y la presión en la velocidad de recombinación de estas reacciones cuando se utilizan helio y neón como gases amortiguadores. Esta velocidad de recombinación es del mismo orden de magnitud en ambos casos, de aproximadamente 10⁻⁶ cm³ s⁻¹ . Aparentemente, la influencia de la temperatura solo se ha estudiado para el neón. (Véase la figura 9). La velocidad de recombinación α₃ en la reacción (3) es máxima a 180 K para una presión absoluta de 294,2 kPa. [ 146 ] Por lo tanto , α₃ es 4,2 × 10⁻⁶ .−6 cm 3 s −1 .
El análisis más refinado de la reacción (4) fue llevado a cabo por Bates et Morgan. [ 147 ] quienes encontraron que el método de Monte-Carlo , la ecuación de Flannery y la teoría de Langevin pueden dar buenos resultados solo cuando la presión es superior a 1 atm . Esta es la norma para los láseres. La teoría "mareal" propuesta concuerda con las mediciones experimentales de Mezyk et al. [ 140 ] lo cual es evidente en la Figura 10. La tasa de recombinación α 4 para la reacción (4) es del mismo orden de magnitud que α 3 .
La reacción (5) solo se observa cuando Rg es neón o argón. Para esta reacción, la evolución de la tasa de recombinación α 5 en presencia de neón presurizado se muestra en la figura 6. Imada et al. [ 148 ] estudiaron la influencia de la temperatura para una presión total fija de 294 kPa. El valor máximo de α 5 se obtiene a 120 K y α 5 = 7,5 × 10−6 cm 3 s −1 .
Para el argón, solo se dispone de dos estimaciones a temperatura ambiente. A una presión de 2 atm, α 5 = 2,10 −6 cm 3 s −1 [ 149 ] y a una presión de 1 atm, α 5 es 1 × 10−6 cm 3 s −1 . [ 65 ]
Reaction (5) does not favor a transitory complex RgXeCl* as an intermediate stage.[57] The following reaction, therefore, plays a minor role:
RgXe+ + Cl− + Rg → RgXeCl* + Rg → XeCl* + 2 Rg
On the contrary, the principal synthetic pathway is given by:
RgXe+ + Cl− + Rg → 2 Rg + Xe+ + Cl− → XeCl* + 2Rg
Kannari et al..[142] estimated the contribution of each of the three recombination and harpooning reactions for three types of mixtures. The results are shown in Table 16. Reaction (3) provides the bulk of the exciplex molecules and generally the harpooning reactions play a secondary role. When helium is used, in contrast, the harpooning reactions contributes about 10–15% of XeCl* synthesis.[144][150] Other authors only estimate this contribution at 1% when the ionic pathway is involved.[125] These theoretical conclusions are confirmed by experimental methods for the generality of the buffer gases and for other chlorine donors.[144][151] The "harpoon" reactions, notwithstanding, are important despite their low contributions. These harpoon reactions are the reactions which are set in motion after the first excitation. Ionic recombinations, which then provide the bulk of the exciplex molecules, kick off 20 ns later.[144]
In table 16, the column named "others" shows 5.8% for neon, meaning that other recombination pathways are possible.
Xe3+ ions are synthesized in the gaseous mixtures used in lasers. These ions react with Cl-10− in order to produce XeCl. Nevertheless, this reaction is only a little contribution to the kinetics of the laser.[152]
Xe+* ions react with Cl− in order to produce XeCl*.[15][153] Alekhin et al.[153] have also synthesized XeCl* using NaCl vapors. XeCl* is the product of the lowest vibrational states (v≤20) using highly excited Xe* ions in a bimolecular reaction. The rate of synthesis is estimated to be between 2×10−10 and 1×10−9 cm3s−1. A corresponding reaction is proposed using HCl.[15] This conclusion is based on the presence of the states which are responsible for the third continuum of xenon – only Xe2+ ions, since XeCl* is not produced.[146][148] On the contrary, Xe* ion participation in the reaction is compatible with the observations of other authors. Several authors[144][150][154] have confirmed the presence of Xe* ions (6s 4P3/2) in the laser mixtures. Their concentration is a thousand times greater than that of Xe* ions in the harpoon reaction.[125] On the other hand, the concentration of these ions and that of XeCl* and Cl− as a factor of time is not incompatible with the synthesis of exciplex molecules using Xe+. The beginning of the decline in Xe+* and Cl− is related to an increasing acceleration of the rate of synthesis of XeCl*. The distribution during harpoon reactions between states B and C occurs in random proportions in experimental conditions.
The first estimate of the ionic pathways was made by Tysone and Hoffman[155] who suggested 76% for states B and 24% for states C. Successively, the buffer gases are neon, argon and krypton. Ohwa and Kushner[156] published similar values: 77% for states B and 23% for states C. They used a quaternary mixture containing a buffer gas (using neon) from hydrogen, H2.
A recent and more detailed study was conducted by Tsuji et al.[141] in a mixture of helium as buffer gas. They found that:
– Los estados D se forman especialmente a partir del ion Xe + , ( 2 P 1/2 ) ;
Los estados B y C se producen exclusivamente a partir del ion Xe + ( 2P3 / 2 ) en las siguientes proporciones: Estados B: 62,6% y Estados C: 38,4%. La tasa de producción de XeCl* es 98%. [ 157 ] Entonces hay pocas reacciones competitivas.
En experimentos de laboratorio, el número de estados Xe + ( 2 P 1/2 ) y Xe + ( 2 P 3/2 ) es el mismo. Además, las constantes de velocidad de la reacción (3) relativas a estos dos estados de xenón son similares. Sin embargo, bajo estas condiciones, el número de estados D formados es muy bajo con respecto al número de estados B y C. La velocidad de formación de XeCl(D) con respecto a XeCl(B, C) se estima en 0,033±0,006. La disociación más rápida de [ Xe + ( 2 P 1/2 ) Cl −] * con respecto al de [ Xe + ( 2 P 3/2 ) Cl −] * es responsable de esta situación.
Vías de descomposición
Radiación
Espectros de emisión
Los espectros correspondientes que se muestran en la Figura 11 fueron observados por prácticamente todos los autores que estudiaron mezclas basadas en xenón y un donante de cloro.
Dos estudios teóricos han permitido identificar los espectros de emisión. [ 42 ] [ 49 ] Cinco transiciones presentan intensidades elevadas que corresponden a ΔΩ = 0, es decir, una polarización paralela al eje internuclear. Los estados iniciales son siempre iónicos y los estados producto son covalentes. Las características de estas emisiones se muestran en la Tabla 17.
Las transiciones UV más probables son B→X y D→X. Son del tipo Σ→Σ. Las otras transiciones, B→A, C→A y D→A, son del tipo Π→Π y son mucho menos probables. [ 73 ]
Otras transiciones teóricamente más débiles aún no han dado lugar a una observación, con la excepción de Hay y Dunning, [ 49 ] quienes propusieron cuatro transiciones que están polarizadas perpendicularmente en el eje internuclear; es decir, con ΔΩ = ±1. Solo Ewing y Brau [ 89 ] observaron una emisión centrada en 425 nm atribuida a una transición 2 Σ→ 2 Π. Finalmente, Krauss [ 73 ] sugirió la posibilidad de una emisión del tipo D→B cuyo período de transición es en sí mismo muy débil. La Tabla 6 la sitúa en 931 nm.
Las principales emisiones se observaron y se registraron como se indica en la Tabla 17.
La línea B→X se observa a 308 nm (Figura 11), mientras que la predicción teórica de su existencia era claramente débil. Esta es la emisión más estrecha y el estado final muestra un pozo de potencial relativamente poco profundo. Al igual que los haluros de gases nobles, esta emisión tiene el período de transición más fuerte. Por eso es la emisión preferida en los láseres de XeCl. [ 4 ]
Experimentalmente, las líneas (C→A) y (B→A) se superponen, [ 59 ] produciendo un continuo centrado en 345 nm, a menudo de baja amplitud, como se puede observar en la Figura 11. El ancho de la emisión depende de la transición que tiende a un estado fuertemente repulsivo. Koltz et al. ubicaron este continuo entre 312 y 460 nm. [ 50 ] Las débiles intensidades observadas se atribuyen a la debilidad de las probabilidades de la transición de las dos emisiones opuestas a la de B→X y a las pequeñas cantidades de estados C formados con respecto a los estados B, como se vio anteriormente. Otros autores han llamado la atención sobre los fenómenos de absorción de la molécula Xe2 Cla esta longitud de onda. [ 158 ] Según Kannariet al., la reacción (3) es la vía principal para la síntesis de los estados B y C. [ 142 ] Tsujiet al.estimaron las proporciones de los estados B y C formados: 38% para el estado C y 62% para el estado B. [ 141 ] El valor de las probabilidades de transición (valor teórico de IB→A/IB→X= 0,07; valor experimental de 0,05), [ 50 ] por lo que la contribución de la emisión (B→A) es de aproximadamente el 10%. Varios autores [ 6 ] [ 59 ] [ 159 ] afirmaron que se podría desarrollar un láser basado en la emisión de 345 nm, especialmente a presiones de aproximadamente 10atmósferascuando los estados B y C se termalizan. Mientras tanto, no se había descubierto ningún resultado concreto hasta 2014.
La transición (D→X) centrada en 235,5 nm no se ha observado sistemáticamente. La línea correspondiente aparece débil, como en el caso de la Figura 12. Su anchura óptica es similar a la de la emisión (B→X) porque conduce al mismo estado débilmente ligado de X. [ 53 ] En contraste, la intensidad relativa de las emisiones (B→X) y (D→X) varía considerablemente de un autor a otro: I D→X /I B→X = 1/3 según Shuker, [ 53 ] de 1/25 a 1/50 según Sur et al. [ 81 ] y 0,14 según Taylor et al. . [ 160 ] Estos últimos autores señalaron que la relación es independiente de la presión. Sigue siendo improbable que se pueda desarrollar un láser utilizando esta transición, como predijo Shuker. [ 53 ]
Los espectros no mostraron ninguna emisión D→A. Sin embargo, Hassal et Ballik [ 100 ] observaron una línea a 246 nm con una intensidad muy débil (figura 12) sin atribuirla a la transición en consideración.
Las emisiones del estado D son insignificantes para la espectroscopia de XeCl. Atribuyendo la ausencia de D→A como para D→B a la probabilidad de transición débilmente asociada, [ 42 ] [ 49 ] [ 73 ] no se puede decir lo mismo para D→X. Según la Tabla 17, la emisión D→X debería tener menor intensidad que B→X. En este caso, la posible explicación podría deberse a la débil producción del estado D, ya sea por la vía iónica [ 141 ] o por la reacción de arpón utilizando estados Xe( 3 P). [ 97 ] La vía principal de XeCl* La síntesis es la reacción (3) y la relación del número de estados B con respecto al estado D es 0,053. Según la Tabla 17, es probable que el estado D se desexcite exclusivamente hacia el estado X. Las probabilidades de transición de la Tabla 17 muestran ID→X/IB→X≈6,2%, con resultados del orden de magnitud de Suret al. [ 81 ] y no muy lejos de los de Tayloret al.[ 160 ] .
Estas emisiones se degradan en mayor o menor medida para longitudes de onda cortas, como muestra el espectro de emisión de la línea (B→X) en la figura 13. En los espectros de absorción se observó un fenómeno de oscilación correspondiente con la misma longitud de onda. [ 52 ] Además, la emisión (D→X) tiene la misma estructura de línea que (B→X). [ 81 ]
El ancho y la naturaleza oscilatoria de estas líneas están ligados a la existencia de transiciones que surgen de altos niveles vibracionales de estados radiativos excitados. [ 50 ] [ 74 ] [ 92 ] La excitación vibracional es resultado de la energía que queda después de la formación de la molécula de exciplex. Esta energía depende tanto del estado del átomo/ión de xenón involucrado en la reacción como del donador de halógeno. [ 58 ] [ 74 ] [ 115 ] Para la emisión de 345 nm, las transiciones en un alto nivel vibracional están más dispersas hacia la región roja para C→A 3/2 que para B→A 1/2 porque la barrera repulsiva de A 3/2 es más pronunciada y está más cerca del estado superior de la emisión que A 1/2 . [ 74 ]
The oscillatory nature of these spectra tends to disappear with an increase of pressure, showing only the peaks arising from the v≤2 level when the pressure is above 1 atm. This shows that the vibrational relaxation effectively depopulates the highest vibrational levels.[10][92] On the other hand, the disappearance of the elevated levels is faster for state B than for state C because state C has a much longer lifetime.[74] The vibrational relaxation of states B and C play an important role in the chemical kinetics of XeCl lasers.
Beyond 5 atm, these lines increase in width, possibly due to collisional enlargement induced by rays or due to the entire rotational structure.[161]
The isotopic effects are negligible for xenon but marked for chlorine. The vibrational lines associated with the heaviest isotope 37Cl are lightly displaced towards the greatest wavelengths. For example, the gap reads 1.51Å for the 4-0 line of B→X.[55]
Radiative lifetimes of excited species
Values for states B, C and D are shown in Table 18 for the vibrational level v=0. These are states B and C which have resulted in more determinations.
In state B, two values are statistically distant from the others.[67][162] They correspond to the oldest measurements. Without taking them into account, the confidence interval obtained in ns is: 8<τB<12.3.
For state C, the dispersion is more important. Grieneisen et al.'s determination[67] is still statistically distant from the others as well as the two theoretical values[42][49] along with a measurement obtained at the solid state.[76] When the above is disregarded, the confidence interval, in ns, then becomes: 129.1<τC<135.9.
Using average values, the relation τB/τC is 0.0764. It is adequately comparable with a direct measure which is 0.087 ± 0.009.[64] This relation is important because it plays an important role in the vibrational relaxation of states B and C.
A systematic study of the lifetimes of several vibrational levels (v≤136) of states B and C was conducted as reported in Table 19.[163]
Lifetimes increase by a factor of 4 when v goes from 0 to 100. A graphical extrapolation of the data relative to state B is shown in Figure 14.
For state D, only three determinations are relatively close to one another. At the gaseous state, Shuker[53] noted that D→X emission has a time-based dependence similar to B→X emission, which is in line with the previous magnitudes as the lifetime of the B state is of the order of 10 ns. However, other measures are necessary to precisely value τD.
The collisional pathway
The influences of xenon and HCl will be discussed first, followed by the role of the diverse buffer gases and of the chlorine donors.
Destruction of the XeCl* molecule
In Xe/HCl mixtures
The only process of destruction of states B and C of XeCl, other than the radiative process, which has been proved is:
XeCl* + HCl → Other products and not XeCl (6) with rate constant of kH
XeCl* + Xe → Other products and not XeCl (7) with rate constant of kX
XeCl* + 2 Xe → Other products and not XeCl and Xe2Cl or → Xe2Cl* + Xe (8) with rate constant of kDX
XeCl* + Xe + HCl → Other products and not XeCl (9) with rate constant of kM
XeCl* + e− → Xe + Cl + e− (10) with rate constant of ke
As of 2014 no result had been found for state D.
The values obtained for states B and C are collected in Table 20. The authors assume that the reaction rates are identical for the two states.
Reaction (9) has been observed only once, recently.[16] Comparison data are therefore not available. In contrast, the other reactions have been repeatedly observed and quantified.
For kH, three measures are statistically distant from the others.[16][155][164] The last (older) two are superior to the others. The first, a recent measure, is the only experiment which proved process (9) which had been neglected. Measurements made by Rives et al.,[16] kH must be multiplied by 2 which puts them at the same level as the other values. Taking reaction (9) into account, the set of values of kH must be revised downward except for Rives et al..[16] A confidence interval is difficult to obtain in these conditions.
For kX, a statistical analysis is very difficult because of the high dispersion of significant absolute values of doubled uncertainties. Lorents[70] provided only an upper limit. Rives et al.[16] results leave open to question whether this process is computable, considering its weak rate constant. Statistically, kX, should not surpass 6.12×10−12 cm3s−1.[61] One other (old) measure,[164] had already provided an erroneous value for kH. Another measure[61] was strongly revised downwards six years later.[62]
Reaction (8) which does not lead to the production of Xe2 Cl*es de importancia insignificante. [ 62 ] [ 111 ] Las mediciones dadas para kDXestán bien dispersas y el intervalo de confianza contiene solo tres valores. [ 16 ] [ 162 ] [ 165 ] Dos de las mediciones excluidas son estimaciones dudosas, [ 135 ] [ 168 ] mientras que las otras son medidas directas correspondientes [ 62 ] [ 70 ] [ 71 ] [ 131 ] [ 155 ] que proporcionaron buenos resultados. Sobre kDXuna gran incertidumbre, pero el valor promedio es representativo de los resultados generales, es decir, 9.1×10−31 cm 6 s −1 .
Los valores medidos de k e muestran una fuerte dispersión. Solo cuatro valores son estadísticamente cercanos [ 119 ] [ 130 ] [ 155 ] [ 166 ] El valor promedio de 9,6 × 10−8 cm 3 s −1 está relativamente cerca de la única medida directa. [ 166 ]
Lou [ 171 ] también sugirió otros productos para la reacción (10):
XeCl* + e−→Xe++ Cl−(ke1= 1,8×10−7 cm 3 s −1 ) o → Xe* + Cl + e − (k e2 = 1.2 × 10−7 cm 3 s −1 )
Se observaron algunas diferencias para las reacciones de tipo (6) teniendo en cuenta los niveles vibracionales de los socios de colisión:
XeCl* (v=0) + HCl(v=1) → Xe + HCl + Cl + Cl (6a)con constante de velocidad de kHa
XeCl* (v=0) + HCl(v=2) → Xe + HCl + Cl + Cl (6b)con constante de velocidad kHb
XeCl(B,C;v≠0) + HCl(v=0) → Otros productos y no XeCl (6c) con constante de velocidad k Hc
Los valores de las constantes de velocidad se resumen en la Tabla 21. Presentan una buena dispersión y no corresponden a ninguna medición directa. Estos valores se basan específicamente en estimaciones análogas.
Reactions that correspond to reactions (6) and (7) are evident when XeCl is in the ground state of X(v=0). These phenomena affect laser performance and are therefore important. The rate constants are assembled in Table 22. These rates do not vary with the vibrational level of the colliding molecules. Only one direct measurement exists;[30] the others are estimates.
Role of the buffer gas
The addition of a third gas in significant quantities also affects the kinetics of disappearance of XeCl(B,C). It induces reactions which are similar to those produced by xenon:
Double collision (11) : XeCl(B,C) + Rg → Xe + Cl + Rg rate constant of k11
Triple collision (12) : XeCl(B,C) + 2 Rg → Xe + Cl + 2 Rg rate constant of k12
Mixed triple collision (13) : XeCl(B,C) + Xe + Rg → 2 Xe + Cl + Rg rate constant of k13
The rate constants of the three processes are grouped in tables 23–25.
Las reacciones (11) y (13) son siempre importantes, mientras que la reacción (12) tiene una contribución insignificante. Los resultados presentan una gran dispersión. Las diferencias pueden alcanzar órdenes de magnitud. Cuatro referencias [ 62 ] [ 70 ] [ 164 ] [ 175 ] han proporcionado mediciones directas de las velocidades de reacción. Las demás son estimaciones. Estas se basan en correspondencias y son meramente indicativas. No se dispone de información para el criptón.
En el conjunto de estas reacciones se observan reacciones competitivas.
Las reacciones de (11) son competitivas para las reacciones de desplazamiento. En este caso, los productos son RgCl(B). Solo se han observado en el caso donde Rg = Kr: [ 138 ]
XeCl* + Kr → KrCl + Xe
La constante de velocidad es 0,7 × 10−9 cm 3 s −1 . [ 139 ] Por lo tanto, esta reacción es más efectiva que el enfriamiento. Juega un papel importante en la cinética láser. También es tan rápida como el proceso de creación de XeCl* por reacción de arpón. La tabla 20 trata sobre una de las principales vías de destrucción de la molécula de exciplex.
Para Brashears et al. , [ 177 ] es posible obtener el complejo triatómico, Rg XeCl*, as product. This is a competitive reaction when collisions that produce dissociated atoms occur. Emissions of KrXeCl at 370 nm have been observed,[177] along with ArXeCl at 326 nm[178] and NeXeCl at 434 nm.[91] The rate constants have not been measured, except for Rg=Kr, which is 9×10−33 cm6s−1.[62]
However, the creation of ArXeCl seems to be preferential by a competitive reaction (13):
Xe* + Ar + Xe → ArXeCl*
The rate constant is 4×10−30 cm6s−1.[9] It is then of the same order of magnitude as (13).
However, the synthesis of the Xe2Cl* trimer is the most frequent competitive reaction of (13).
For helium, Baginskii et al.[174] provided a solution using Xe*2 + Cl + He of which the rate constant is 1.5×10−31 cm6s−1.
A corresponding reaction for (11) was demonstrated for XeCl at the ground state. The rate constants are summarized in Table 26. The measurements are greatly dispersed (only one is direct) and data on krypton are absent.[30] The others are based, more or less, on estimates. Amongst these, one[179] is statistically distant from the others. On using neon, the rate constant for XeCl(X, v=1) has been estimated as 1×10−11 cm3s−1.[156]
Other chlorine donors and other reactions
The main reactions are those corresponding to reaction (6):
XeCl* + RCl → Other products and not XeCl (14) rate constant of kR
The values of the rate constants through RCl = Cl2 or CCl4 are summarized in table 27. The three chlorine donors studied (HCl, Cl2 and CCl4) have rates of quenching of the same order of magnitude.
All the measurements in Table 27 were experimental. For chlorine, only one (recent) value is statistically distant from the others.[61] The absolute difference is not very great versus the other determinations. An average value for kR for chlorine is 5×10−10 cm3s−1, which is very close to a measure relative to CCl4.
For chlorine, Grieneisen et al.[67] pointed to two different values for the rate constant for states B and C. They were respectively estimated as (8.8 ± 1.5)×10−10 cm3s−1 and (3.3 ± 0.3)×10−10 cm3s−1. This is a direct measure of the process of destruction through binary collision with Cl2 that includes all the phenomena and not just quenching. As states B and C are energetically close, collisional coupling is acting on the two states. A similar result for xenon seems to reinforce this hypothesis.
Some atoms of free chlorine exist in the conditions which matter for lasers. The following quenching reactions is provided for:
XeCl* + Cl → Xe + 2Cl
Two authors have estimated the rate constant: 1.4×10−9 cm3s−1[119] and 8×10−10 cm3s−1.[135]
La presencia de impurezas, I m , como los clorocarbonos (consecuencia de la corrosión [ 181 ] ), NO, CO2 ,O2 , CO,N2 O,H2 O podría tener un efecto en la cinética química de desaparición delXeCl* dado que las colisiones binarias son Im–XeCl* poseen constantes de velocidad del orden de 3×10−10 cm 3 s −1 , [ 167 ] lo que los hace comparables al XeCl* + reacción RCl. Sin embargo, dados los niveles habituales de impurezas, las frecuencias de reacción son insignificantes. Se ha propuesto una solución práctica para eliminarlas que implica la introducción de 1 torr deH2. [ 181 ]
Proceso de acoplamiento colisional entre los estados B y C.
En mezclas binarias de Xe/HCl
La escasa diferencia energética (aproximadamente 100 cm⁻¹ ) entre estos dos estados (Tabla 2) sugiere que se produjo un acoplamiento. Sin embargo, este resultado no se cuantificó con precisión ni se confirmó posteriormente. Recientemente no se ha detectado ningún fenómeno de acoplamiento por colisión inducido por el cloro.
El papel de los electrones tampoco se conoce bien en el proceso de acoplamiento. Según Finn et al. , [ 164 ] su papel es insignificante, aunque Johnson et al. [ 135 ] dieron una constante de velocidad elevada. Esta velocidad es la misma, según ellos, para las transferencias de B a C y de C a B. La diferencia de energía entre B y C no es cero (véase la Tabla 2). La velocidad de reacción se estimó en 2 × 10−8 cm 3 s −1 .
Estos acoplamientos se demuestran mediante colisiones binarias utilizando un átomo de xenón:
XeCl(B ; v' = 0) + Xe → XeCl(C ; v' = 0,1) + Xe (15) constante de velocidad de k BC
XeCl(C ; v' = 0, 1) + Xe → XeCl(B ; v' = 0) + Xe (16) constante de velocidad de k CB
Las mediciones de las constantes de velocidad no son muy consistentes, como se puede observar en la Tabla 28.
En los experimentos de Inoue et al. , [ 61 ] los niveles vibracionales v'=0,1 se excitaron directamente. Este no es el caso en otros experimentos. [ 16 ] [ 71 ] El último valor [ 121 ] es solo una estimación teórica basada en similitudes con otras reacciones. La brecha energética ΔE = E B – E C deducida de k CB y k BC , sugiere que podría haber más información. Suponiendo que los estados E B y E C se termalizaron:
k BC /k CB = exp(ΔE/kT) ya que los pesos estadísticos de los dos estados son los mismos. [ 49 ]
ΔE también fue inferido por Inoue et al. [ 61 ] como 85 cm −1 , y como 119 cm −1 por Rives et al. , [ 16 ] mientras que 22 cm −1 fue la medición dada por Le Calvé et al. [ 71 ] (ver Tabla 2). Solo los dos primeros valores son valores de ΔE que son compatibles con 100 cm −1 , el orden de magnitud aceptado. Existe una clara diferencia entre estos dos; un orden de magnitud separa los valores de k BC y k CB en los dos experimentos. [ 16 ] [ 61 ] Grieneisen et al. [ 67 ] proporcionaron solo la tasa global de destrucción de los estados B y C, en otras palabras, extinción y acoplamiento. Para la destrucción del estado C, encontraron (15,5 ± 0,9) × 10−12 cm 3 s −1 y para el estado B (10,3 ± 0,9) × 10−12 cm3s−1, which are intermediate values between those of Inoue et al.[61] and Rives et al..[16] Recall that quenching by xenon only has a weak influence (Table 20). Inoue et al.[61] notably did not take account of reaction (9). If the same approach is taken for the results by Rives et al.,[16] the values of kBC and kCB are close to those of Inoue et al..[61] As was explained for kx and kH, taking account of the process (9) modifies the values of the reaction rate. On this point, Rives et al.[16] is more precise than Inoue et al..[61]
The advantage of Inoue et al.'s[61] result was in vibrational resolution, as kBC and kCB vary with the vibrational level v. For level v=70 to 130, rate constants between 15 and 20×10−11 cm3s−1 were observed.[163] kBC and kCB seems to then grow with v.
Since most of the time XeCl(B, C) is formed with a strong vibrational excitation, knowledge of the exact estimate of the variation of kBC and kCB with v; and the chemical kinetics of the vibrational relaxation and its importance relative vis-à-vis to the coupling process are important.
The role of the buffer gas
Collisional coupling is produced by binary collisions with an atom of a rare gas, Rg:
XeCl(B) + Rg → XeCl(C) + Rg (17) rate constant of kBCRg
XeCl(C) + Rg → XeCl(B) + Rg (18) rate constant of kCBRg
Dreiling and Setser[163] provide order of magnitude values for kBCRg and kCBRg for a given vibrational level. The results are shown in Table 29. This shows that the rate constants increase regularly when the vibrational level, v, of XeCl* is higher and the rare gas, Rg, is heavier.
Using helium, experiments have been made at low and high pressures.[66] At high pressures, the transfer constants are of the order of (1.5 ± 0.7)×10−12 cm3s−1 and at low pressures 3.0×10−11 cm3s−1. A strong pressure induces a vibrational relaxation such that the values of v involved in the transfer are weak and vice versa for weak pressures. The only available direct determination for kBCHe gives a value less than 3×10−13 cm3s−1.[69]
For neon, the values of the rate of transfer at low and high pressure are respectively, 3.0×10−11 cm3s−1 and (0.8 ± 0.4)×10−12 cm3s−1.[66] They are inferior to those of Table 29. The direct measurement of the rate constant kBCNe gives a value less than 3.10−13 cm3s−1.[69] Finally, according to Ohwa,[156] the order of magnitude of the two rate of coupling constants would be 4.8×10−12 cm3s−1 for v=4.
For argon, the results increase. At low pressures, the order of magnitude would only be 6.0×10−11 cm3s−1.[66] Other authors[65] published rates of transfer of 1.2 ± 0.4×10−4 cm3s−1 for a pressure interval starting from 10 to 1000 torr. Direct measurements of kBCAr and kCBAr are available without specifying the vibrational levels involved:[50]
kBCAr = 36×10−4 cm3s−1 and kCBAr = 21×10−11 cm3s−1
Meanwhile, Yu et al.[69] noted a variation with temperature of kBCAr:
kBCAr = (4 ± 2)×10−12 cm3s−1 at 300K and kBCAr = (2 ± 1)×10−12 cm3s−1 at 230K.
For krypton, we can only make an estimation:
kBCKr = (4)×10−12 cm3s−1.[69]
It is clear that the collisional coupling process induced by the rare gases are not well established. Different authors give different order of magnitudes. The uncertainty on the rate constants is therefore as important as for that of xenon. The vibrational excitation seems to play a role that is still not well defined. Direct measurements for kBCRg and kCBRg are not available. From the first estimations, the phenomena seem important in the kinetics of gaseous mixtures.
Vibrational relaxation
XeCl* is more often synthesized with strong vibrational excitation and can reach vibration quantum numbers as high as v=100.[182] This induces some vibrational relaxation that is formed by binary collision with an atom of a rare gas.[183]
Only a single measurement for xenon and level v=2 has been published.
XeCl(B; v = 2) + Xe → XeCl(B; v’ = 0.1) + Xe rate constant of kv
where kv = (2 ± 1)×10−10 cm3s−1.[61]
Most of the known results are related to buffer gases. Yet, only Dreiling and Sester[163] completed measurements. The vibrational relaxation can be written as:
XeCl*(v) + Rg → XeCl*(v’) + Rg (19)
The orders of magnitude of kvRg are summarized in Table 30. kvRg increases with the vibrational level of XeCl* and heavier rare gases, Rg. Values of kvRg are assumed to be the same for states B and C.
For helium and krypton, no comparison is available.
For neon, only the reaction with first two vibrational levels of B have been documented:
XeCl(B; v = 1) + Ne → XeCl(B ; v = 0) + Ne with rate constant of kvNe=(0.3 to 0.5)×10−11 cm3s−1.[184]
For argon, the values of kvAr has been determined for v=33, 60 and 75.[90] Their values, respectively, are (17 ± 5)×10−11; (31 ± 9)×10−11 and (43 ± 10)×10−11 cm−11. Other authors placed the figure for kvAr between (10 and 15)×10−11[155] agreeing on the order of magnitude.
Disappearance pathways of the exciplex molecule
The chemical kinetics due to collisional coupling of states B and C and vibrational relaxation are not well known. The few available results often disagree, although a general idea of the situation is possible. For high vibrational levels, coupling overrides the vibrational relaxation while the contrary is true for the lowest levels,[58] even if a rare gas is involved.
The various destructive processes of XeCl(B), differ in importance. A mixture optimized for lasers is used. Neon is favored over argon because the latter strongly absorbs via the Ar+2 ion at 308 nm.[135] Therefore, a ternary mixture (Ne/Xe/HCl) is used. The total pressure is fixed at 3 atm, the respective partial pressures is 2268.6 torr, 10 torr and 1.4 torr. The rate constants are the average values of the most reliable estimates.
The results are summarized in Table 31. For reaction (19), only the lowest vibrational levels are accounted. The lower frequency of disappearance limit is 0.40 ns−1. This process induces the highest destruction, indicating that XeCl(B) synthesized with high vibrational excitation is quickly relaxed by binary collision with neon and (probably) also by xenon. This suggests that other processes are really noticeable only after XeCl(B) is on the v=0 level, which is why reaction (17) uses the value of k BC Do relative to a low v. Once the relaxation is complete other processes take over. Depopulation by spontaneous emission is very important as well as reactions (11) and (17). These two processes lack refined measurements and determinations overall. The role of the xenon coupling is not better known but has less influence than the destruction by binary collision with HCl. Other better known processes are negligible. In particular all termolecular reactions are negligible.
The Xe2Cl exciplex molecule
Generally, Rg2X molecules are less stable than RgX.[7]Xe2Cl is of double interest. It can cause perturbations in laser XeCl performance because it absorbs well at 308 nm and enables the development of another type of laser based on an Xe2Cl emission.
The Xe2Cl molecule
Initial studies on the Xe2Cl molecule[33][185] found:
- Its most stable configuration in the excited state has a triangular geometry C2v.[186]
- The Xe2Cl* excited states are complexes formed from the association of a molecular ion of Xe+2 and an atomic ion of Cl−.
- The observed emission of the molecule is broad; the corresponding transitions result in a very repulsive ground state.
The potential curves calculated by Huestis et al.[187] from the DIM (Diatomics In Molecules) method are presented in Figure 15.
The three lowest states are covalent and repulsive. They are correlated to XeCl(X or A) and to an atom of xenon at the ground state. The experimental value of the energy at state 12Γ is 0.273 eV.[33] It is compatible with these potential curves. The following three states are ionic. The bound state 42Γ is correlated to XeCl(B) + Xe; the following, 52Γ, a repulsive state, is correlated to XeCl(C) + Xe.
Last and George[43] made a determination of the potential curves using another method, the DIIS (Diatomics In Ionic Systems) method without considering spin-orbital coupling. They found, like Huestis et al.[187] that the 42Γ state is the lowest ionic state. At the bottom of the well, this state has the configuration of an isosceles triangle, such that the distance between the equilibrium positions of Xe and Cl is 3.23 Å. According to Adams and Chabalowski[42] the Xe–Cl distance is 3.39 Å.
Initially, the potential curves of the different states were plotted by maintaining a constant and equal Xe-Xe distance at 3.25 Å (figure 16). Last and George discovered nine states (three covalent and six ionic). The potential curves of the antisymmetric states 42Γπ and 62Γπ are almost coincident with the potential curves of the symmetric states 52Γ and 62Γ. The 32Γ and 72Γ states highlighted by Huestin et al. are absent since the spin-orbital coupling were not taken into account. Inversely, three states, (22Γπ, 42Γπ and 62Γπ) with the π symmetry, were not included in their diagrams.[187]
A second study kept the separation of Xe-Cl at 3.23 Å (figure 17).
* In 42Γπ state, the molecule with isosceles triangle configuration such as the Xe-Cl and Xe-Xe distances are respectively 3.13 and 4.23 Å. The state is 0.8 eV above the 42Γ state.[43]* At the ground state, 12Γ forms a Van der Waals complex. It has a bond-dissociation energy of 0.075eV and a dissymmetric triangular configuration. The Xe–Cl distances are 3.23 and 4.06 Å and the Xe–Cl–Xe angle is 74.4°.[43]* The second excited state 22Γ is also a Van der Waals complex. It has a symmetrical geometry and an Xe–Cl distance of 3.99 Å with an Xe–Cl–Xe angle of 68.4°. Its dissociation energy is 0.055 eV.[43]
Another way of describing Xe–Cl–Xe finds the stable state to be linear and symmetric. At the ground state, the Xe-Cl distance should be 3.24 Å and the dissociation energy 0.076 eV. An excited state could exist with a geometric distance of Xe-Cl of 3.06 Å.[43] This state, which is not shown in Figures 16 and 17, would possess an energy higher than 0.72 eV to that of the 42Γ state. The bonding would be ionic.
Only an experiment conducted at the solid state[72] can be compared to these theoretical results. The special state studied was the 42Γ state. The isosceles triangle structure of this state was confirmed. Three quantities can be compared with theoretical predictions. The Xe-Xe distance is measured at 3.17 Å and that of Xe-Cl at 3 Å. The agreement in values is best for the energy at the bottom of the well that was evaluated at 3.15 eV. The fundamental vibrational frequencies for Xe–Xe, is ωx = 123 cm−1 and for Xe–Cl, ωc = 180 cm−1.
Synthetic pathways
Three principal pathways of Xe2Cl* synthesis are energetically possible through collisions and two others through photodissociation:
Xe*2(A1Σ) + Cl2 → Xe2Cl* + Cl (20)
Xe* + Xe + Rg → Xe2Cl* + Rg (21)
Xe2+ + Cl− + Rg → Xe2Cl* + Rg (22)
XeCl*(X) + Xe + hν → Xe2Cl* (23)
Xe + Cl + Xe + hν → Xe2Cl* (24)
where Rg is a rare gas, probably xenon or a buffer gas.
The authors disagree on the relative importance of these synthetic processes. The processes depend on experimental conditions.
Through harpoon reactions
Reaction (20) is a very energetic harpoon reaction. It involves Xe*2 excited state. According to Bruce et al.,[112] this is the dominant synthetic pathway. Other authors though do not share this view since they believe that this reaction is weak,[187] or indeed negligible.[188] Its rate constant has not yet been measured.
The photoassociative pathway
Reactions (23) and (24) were only recently discovered.[106]
The ionic pathway
According to a theoretical computation,[147] the rate of recombination α’ of the Xe+2 and Cl− ions when Rg = Xe (reaction (22)) was, at the first instance, estimated as 1×10–7 cm3s−1. The same authors later revised this value downward as: α’ = 5×10–8 cm3s−1.[189] This result was confirmed experimentally.[187][190] According to computations, this reaction could become important at high pressures at which Xe2Cl* becomes the principal reaction product, to the detriment of XeCl* (reaction (4)).
The ternary reactions
The synthesis of Xe2Cl* is principally through pathway (21). According to a recent study,[62] the reaction can be interpreted as the result of two successive reactions, the second reaction corresponding to a vibrational relaxation through collision using Rg:
XeCl(B,C) + Xe ↔ Xe2Cl*(v)
Xe2Cl*(v) + Rg → Xe2Cl* + Rg
The starting vibrational levels of Xe2Cl*(v) are above the limit of dissociation of the state in XeCl* + Xe.
In contrast, Yu et al.[69] believe that the formation of Xe2Cl* is through a triatomic complex, RgXeCl*, mainly :
XeCl* + Rg → RgXeCl* where Rg≠Xe
RgXeCl* + Xe → Xe2Cl* Rg
These reactions have been observed in only argon and krypton.
The second reaction is one of displacement. Another reaction is competitive to it when xenon is replaced by krypton. This quenching process should have a rate constant higher than 1×10−13 cm3s−1.[69][177]
The lifetime of the RgXeCl* complex is not well known. It is estimated at 200 ns for KrXeCl[69][177] and 40 ns for NeXeCl.[91] This interval in time is sufficient for the second collision to have a chance of being produced.
The rate constants have been measured as summarized in table 32. If Rg≠Xe, only two direct measurements have been carried out.[39][62] The last[191] is only an evaluation.
As for xenon, notice that the totality of the kDX constants of table 20 could be taken as those of the fifth column of table 32 since kDX could be merged with reaction (21).[62]
Paths of disappearance
The radiative pathway
Emission spectra
Theoretical studies[158][185] show that the allowed transitions are (figure 15) :
42Γ → 12Γ (A)
42Γ → 22Γ (B)
42Γ → 32Γ (C)
The starting states are always the same and the corresponding wavelengths, λTh, are indicated in Table 33. They can be compared to experimental values, λObs.
Experimentally, Fajardo and Apkarian[72] observed two transitions (A) and (B) in the spectral domain, even while there was a significant wavelength shift. In most cases, a very large continuum (approximately 80 nm) was observed covering the three emissions. The maximum positioning oscillated between 450 and 500 nm. An example of this sort of spectrum is given in Figure 11. On computation, the limits of short wavelength emissions were evaluated at 443 nm.[101]
According to Last and George,[43] the Xe–Cl–Xe linear molecule ought to have produced an emission approaching the ground state at 321 nm and the transition moment should be elevated to 3.9 D. As of 2014, however, no experiment confirms this prediction.
At the solid state, the Xe2Cl* emission shifts towards the red range and is centered around 570 nm.[192][193] A corresponding result is observed in the liquid state.[194] This phenomenon should be owed to a distortion of the potential curves arising from molecular interactions which are closest to themselves than at the gaseous state. A theoretical study[195] attributes this to the polarization of the xenon matrix by Xe2+Cl− and by Van der Waals forces.
Emission of Xe2Cl* trimer is only observed at high pressures of the rare gas (xenon or buffer gas) and fluorescence increases with the pressure of xenon.[33] These results follow because the synthetic pathway of Xe2Cl* is similar to that of reaction (21). Considering the values of the rate constant of reactions of type (21), the reaction frequency does not deviate in a significant way even when the rare gas pressure is close to 200 torr. Reaction (22) only takes place under pressure of several atmospheres.[189]
Lifetime of Xe2Cl (42Γ)
The only state where Xe2Cl is the original parent of a luminous emission is 42Γ). Several determinations of its lifetime obtained at the gaseous state are summarized in Table 34. The results vary and the uncertainties involved are important. The confidence interval obtained within a threshold of 5% lies between 240 and 253 ns. Of these, four values are not included.[62][80][167][190] Given the strong absolute uncertainty, another measure[111] has a common interval within the confidence interval.
Measurements realized at the solid state provide values that are yet more dispersed such as is shown in Table 35.
The collision pathway
The role of chlorine donors (RCl)
Beyond the radiative disexcitation, the Xe2Cl (42Γ) state is destroyed by a double collision with RCl. In practical terms, every author agrees that double collision is the dominant destruction pathway of Xe2Cl when collision is involved, whatever the chlorine donor. Therefore, Xe2Cl* emissions are only observed at weak concentrations of RCl.[15][112][167] The values of the rate constants for reactions (24) are given in Table 36.
Xe2Cl* + RCl → Other products except Xe2Cl (24)
There are only two determinations for CCl4 and these are coincident. For HCl, two values are statistically distant from others.[152][198] Giving an explanation for this distance remains difficult. The confidence interval at a threshold of 5% is from 4 to 7×10−10 cm3s−1.
In the case of chlorine, Cl2, only one half of measurements are statistically close.[80][111][165][167] Even so, this closeness is difficult to explain. Its confidence interval at the threshold of 5% varies from 3.7 to 4.5×10−10 cm3s−1. The three chlorine donors appear to have a corresponding influence on the collisional destruction of Xe2Cl*.
To estimate the rate constant of the reaction:
Xe2Cl* + Cl → 2 Xe + 2 Cl
The value is 1×10−9 cm3s−1.[200]
The role of rare gases
These are uniquely binary reactions:
Xe2Cl* + Rg → Other products except Xe2Cl (25)
The disappearance of Xe2Cl* by collision on a xenon atom was observed by Grieneisen et al.,[67] the reaction constant was estimated at 6×10−15 cm3s−1. However, this reaction has not been demonstrated by other authors.[39][70][165][197][199] The upper bound of the rate constant of reaction (25) is 1×10−17 cm3s−1,[197] although other authors placed this limit at 4 to 7×10−14 cm3s−1[165][199] or 5×10−13 cm3s−1.[39] The value used by Kannari et al.,[121] 8×10−12 cm3s−1, has no basis.
For ternary mixtures, the role of the buffer gas is not well known.
For argon, (3 ± 1)×10−14 cm3s−1[39] and (1.5 ± 0.4)×10−14 cm3s−1 are available.[196]
For helium, 5×10−13 cm3s−1[152] and 3×10−14 cm3s−1 are available.[119]
The role of electrons and impurities
The rate of reactions of Xe2Cl* + e− → 2 Xe + Cl + e− (26) does not have consistent estimates. They are summarized in Table 37.
The impurities have a lesser influence in the chemical decay of Xe2Cl than XeCl*.[167] The bimolecular rate constants of disappearance of Im–Xe2Cl* are an order of magnitude lower than the relative rate constants for binary collisions ImXeCl*. Yet, for CO2 and nitric oxide, NO, the rate constants are of the same order of magnitude, about some 10−10 cm3s−1. Impurity levels, most often low, may influence the measurements. The reaction frequencies are negligible.
See also
References
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{{cite journal}}: Missing or empty|title=(help) - 1234P.S. Julienne & M. Krauss (1979). "Role of the III(1/2) -II(1/2) transition in rare-gas–halide kinetics". Applied Physics Letters. 35 (1): 55–57. Bibcode:1979ApPhL..35...55J. doi:10.1063/1.90929.
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{{cite journal}}: Missing or empty|title=(help) - 12345M.F. Golde (1975). "Interpretation of the oscillatory spectra of the inert-gas halides". J. Mol. Spectrosc. 58 (2): 261–273. Bibcode:1975JMoSp..58..261G. doi:10.1016/0022-2852(75)90112-5.
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{{cite journal}}: Missing or empty|title=(help) - 1234567D.W. Setser & J. Ku (1985). F. Lahmani (ed.). Photophysics and Photochemistry above 6 eV. Amsterdam: Elsevier. pp. 621–637. ISBN 978-0-444-41699-5.
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{{cite journal}}: Missing or empty|title=(help) - 1234J.E. Velazco; J. H. Kolts & D. W. Setser (1976). "Quenching rate constants for metastable argon, krypton, and xenon atoms by fluorine containing molecules and branching ratios for XeF* and KrF* formation". J. Chem. Phys. 65 (9): 3468. Bibcode:1976JChPh..65.3468V. doi:10.1063/1.433573.
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- 12T. Letardi; H. Fang & S. Fu (1992). "Theoretical modeling of an X-ray preionized self-sustained XeCl laser". IEEE J. Quantum Electron. QE-28 (7): 1647–1652. Bibcode:1992IJQE...28.1647L. doi:10.1109/3.142551.
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{{cite journal}}: CS1 maint: multiple names: authors list (link) - ↑T. Hammer & W. Bötticher (1989). "Spectroscopic investigation of the ionization kinetics in XeCl laser discharges by Xe* density measurements". Appl. Phys. B. 48 (1): 73–84. Bibcode:1989ApPhB..48...73H. doi:10.1007/BF00694421. S2CID 120499689.
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- 12G. Imada; K. Masugata; K. Yatsui & W. Masuda (1993). "Numerical analysis on temperature dependence of XeCl-lasing characteristics". Appl. Phys. Lett. 63 (10): 1313. Bibcode:1993ApPhL..63.1313I. doi:10.1063/1.109715.
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{{cite journal}}: Missing or empty|title=(help) - 1234Q. Lou (1988). "The effect of specific input energy on the performance of an X-ray preionised XeCl discharge laser". Opt. Commun. 65 (1): 26–32. Bibcode:1988OptCo..65...26L. doi:10.1016/0030-4018(88)90435-X.
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- 1234H.C. Brashears, Jr.; D.W. Setser & Y.C. Yu (1981). "Emission spectra of KrXeCl*, KrXeBr*, KrXeI*, ArKrF*, and ArKrCl*". J. Chem. Phys. 74 (1): 10. Bibcode:1981JChPh..74...10B. doi:10.1063/1.440863.
- ↑B. Forestier; B. Fontaine & P. Gross (1980). "Supersonic Flow Low Temperature Electronic Transition Excimer Lasers". Journal de Physique Colloques. 41, C9-455: C9-455–C9-462. doi:10.1051/jphyscol:1980962. S2CID 98293236.
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- ↑Y.A. Kudryavtsev & N.P. Kuz’mina (1977). "Excimer ultraviolet gas-discharge XeF, XeCl, and KrF lasers". Sov. J. Quantum Electron. 7: 131–133. doi:10.1070/QE1977v007n01ABEH008848.
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- ↑F. Okada & V.A. Apkarian (1991). "Electronic relaxation of Xe2Cl in gaseous and supercritical fluid xenon". The Journal of Chemical Physics. 94: 133. doi:10.1063/1.460387.
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- ↑G. Marowsky; F.K. Tittel; W.L. Wilson, Jr. & R. Sauerbrey (1983). "Experimental study of chlorine donors for the triatomic exciplex Xe2Cl". AIP Conf. Proc. 100: 334. doi:10.1063/1.34066.
- 12W.L. Morgan & D.R. Bates (1992). "Tidal termolecular ionic recombination". J. Phys. B. 25 (24): 5421–5430. Bibcode:1992JPhB...25.5421M. doi:10.1088/0953-4075/25/24/020. S2CID 250873929.
- 123A.W. Mc Cown; M.N. Ediger; S.M. Stazak & J.G. Eden (1983). "Photodissociation of Xe+2 and Kr+2 in the ultraviolet: Application to Xe2Cl formation kinetics". AIP Conf. Proc. 100: 222. doi:10.1063/1.34054.
- 12M. Ohwa & M. Obara (1986). "Theoretical analysis of efficiency scaling laws for a self-sustained discharge pumped XeCl laser". J. Appl. Phys. 59 (1): 32. Bibcode:1986JAP....59...32O. doi:10.1063/1.336835.
- 12M.E. Fajardo & V.A. Apkarian (1988). "Charge transfer photodynamics in halogen doped xenon matrices. II. Photoinduced harpooning and the delocalized charge transfer states of solid xenon halides (F, Cl, Br, I)". J. Chem. Phys. 89 (7): 4102. Bibcode:1988JChPh..89.4102F. doi:10.1063/1.454846.
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- 123Zuev, V. S.; Kanaev, A. V.; Mikheev, L. D. (1987). "Determination of the absolute quantum efficiency of the luminescence of Xe2Cl* in Cl2–Xe mixtures". Soviet Journal of Quantum Electronics. 17 (7): 884–885. doi:10.1070/QE1987v017n07ABEH009473.
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- Xenon compounds
- Chlorides
- Nonmetal halides
- Diatomic molecules