Xenon 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 , 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. 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.
{| class="wikitable" style="text-align:center" |+ Table 1. Properties of rare-gas halides. D – molecule is dissociative and does not exist. F – observed fluorescence. L – molecule with laser effect. ! Halogen/Rare gas !! Helium !! Neon !! Argon !! Krypton !! Xenon |- | Fluorine || D || F & D || L || L || L |- | Chlorine || D || D || F & D || L || L |- | Bromine || D || D || D || F & D || L |- | Iodine || D || D || D || D || F & D
Several review articles related to xenon chloride laser technology and its applications have been published.
Some authors stress the importance of accurately determining the kinetics of the laser medium when rare-gas halides are involved. Recent results have provided insight into the physical chemistry of the laser medium. Spectroscopic investigations are limited to the visible-near ultraviolet region where exciplex lasers operate. Only binary gas mixtures of xenon and a chlorine donor, or ternary mixtures that also include a buffer gas (a rare gas indicated by Rg) will be considered. The most interesting chlorine donors are and HCl because of their use in laser technology, and (see Figure 1).
XeCl and are most important in laser applications amongst the xenon chlorides. Although discharge lamps based on low-pressure mixtures of xenon and a chlorine donor emit incoherent light, they are reliable and easy to operate.
History
The idea that the noble gases can form halides arose in the early 1920s: A. von Antropoff and Oddo suggested that krypton and xenon may form bromides and chlorides. In 1933, Yost and Kaye unsuccessfully tried to synthesize xenon chloride by illuminating a mixture of xenon (70 torr of pressure) and chlorine (225 torr) with a mercury-vapor lamp.
Xenon monochlorides were first synthesized in 1965. Later, solid and compounds were synthesized at low temperatures. In 1991, Prosperio et al. demonstrated the existence of in the gaseous state, which is important for lasing kinetics, although it emits an uninteresting infrared light.
In 1973 Riveros et al. synthesized ions in the gaseous phase at a pressure of 10−4 torr. This ionic molecule attracted little interest.
Systematic studies of XeCl were initiated in 1975 by Velazco and Setser, who demonstrated 304 nm emission from . This emission was obtained by mixing xenon atoms () with chlorine gas or other chlorinated compounds (NOCl and ). The excitation was provided by a cold cathode discharge; the total pressure was a few torr. Months later, Ewing and Brau 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. The HCl concentration does not substantially affect the laser's output energy. This is not the case for which has a very strong absorption at about 300 nm. 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. The constant rate of vibrational excitation and dissociative electron attachment are more favorable for HCl than for other chlorine donors. These processes help the formation of .
Three years later Lorentz et al. performed experiments at high pressures (a few atmospheres) in a mixture containing (Ar/) and found an emission centered at 450 nm which was attributed to .
The first laser was developed in 1980. This type of laser is likely to be tunable over a wide range of wavelengths (30 nm) in the visible spectrum. This is true even if the absorption phenomena occur on the side of shorter wavelengths and therefore limits the laser action at the red region of the electromagnetic spectrum from light emission. Solid state experiments with suggest that the gaseous state is more suitable for the development of this type of laser. The measured amplification was correct in the solid state.Vartkess A. Apkarian, Mario E. Fajardo, N. Schwentner, Lawrence Wiedeman , Priority date 18 March 1987 The liquid state seems like an ideal dye laser although implementation seems complex and costly. Presently, the Cl laser has not been industrially developed. Unlike XeCl, the best chlorine donor is while no laser action occurs when using HCl.
Four molecules are a priori synthesized in the mixtures. Note particularly the possibility of synthesizing these under the experimental conditions of lasers and their roles.
XeHCl was observed in a gaseous medium. However, this molecule has only been detected via emission spectra in the microwave, radio and far infrared regions, but with emission predicted by two theoretical studies at 232 nm and 129 nm. Note, however, that when nearly in aggregate, it is more likely to be stable at the solid state. It is the same for which can theoretically emit at 500 nm, while this activity has never been observed in the gaseous state.
XeH has three known emission lines. They were observed at 190 nm, 250 nm and 660 nm. However, they have never manifested in the laser spectra, which leads to the assumption that XeH is not formed under the experimental conditions. Conversely, the ion is formed in the mixtures used in lasers. It plays a significant role in kinetics in the synthesis of , through a reaction that competes with the creation of ions (shown below):
+ Xe → + HCl (80 ± 10%)
+ Xe → + HCl (20 ± 10%)
The rate constant of the entire process is 6.4 cm3s−1 (± 20%).
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 and experimental 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 ions and . The spin-orbital splitting of the ion into two states ( and ) 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. This is in agreement with the energy of separation of (2P3/2) and (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 and 7.19 nm and 6.3 nm theoretically. A more recent theoretical investigation specifies these intersection phenomena. States B and C merging at long-distance, intersect two successive potential curves correlated to Xe* + Cl. The lowest correlated to Xe () + Cl () is 7.25 nm and after that, the next correlated to Xe () + Cl () 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. Indeed, state D intersects Xe () + Cl () only at 0.89 nm and Xe () + Cl (2P3/2) at 1.02 nm.
The distinction between states B and C is that they are correlated with 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.
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 centered at 308 nm and 345 nm, either with or without corrections by the participation of transition (B → A). The most direct measure is given by Jouvet et al. Excitation spectra of 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.
{| class="wikitable" style="text-align:center" |+ Table 2 Energy gap (EB – EC) between the states B and C of XeCl. ! EB – EC (cm−1) ! Process ! Year ! Reference |- | −1489 | C | 1977
| −560 | C | 1978
| 7 | I | 1979
| 81 | I & C | 1979
| 128 ± 35 | I | 1980
| −5.4 ± 25 | I | 1980
| 200 | I | 1980
| 230 | I | 1980
| 180 | C | 1981
| 289 | I* | 1982
| 220 ± 40 | I | 1983
| 85 | C | 1984
| 0 | C | 1984
| −22 | C | 1985
| > 50 | I** | 1986
| 230 ± 40 | I | 1987
| 90 ± 2 | Absorption | 1989
| 98 +30−40 | I & C | 1990
| 118 ± 40 | I | 1992
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.