page_title: Xenon monochloride
section_title: Emission spectra
hierarchical_section_title: Xenon monochloride / XeCl exciplex / Pathways of decomposition / Radiation / Emission spectra
caption_attribution_description: Français&#160;: structure vibrationnelle xecl
context_page_description: Xenon monochloride 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 Xe
  ₂Cl, 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.
context_section_description: The corresponding spectra demonstrated in Figure 11 was observed by virtually all authors who studied mixtures that were based on xenon and a chlorine donor.
  Two theoretical studies have enabled identification of the emission spectra. Five transitions have heightened intensities that correspond to ΔΩ = 0 i.e., a parallel polarization to the internuclear axis. The starting states are always ionic and the product states are covalent. The characteristics of these emissions are as shown in Table 17.
  The most probable UV transitions are the B→X and D→X. They have the Σ→Σ type. The other transitions, B→A, C→A and D→A, have the nature Π→Π and are much less probable.
  Other theoretically weaker transitions have not yet resulted in an observation with the exception of Hay and Dunning, who made provisions for four transitions that are perpendicularly polarized at the internuclear axis; in other words, with ΔΩ = ±1. Only Ewing and Brau noted an emission centered at 425 nm attributed to a ²Σ→²Π transition. Finally, Krauss suggested the possibility of an emission of the D→B type whose transition period is itself very weak. Table 6 places this at 931 nm.
  The principal emissions were observed and reported as in Table 17.
  The B→X line is observed at 308 nm (Figure 11) while the theoretical prediction of its existence was clearly weak. This is the narrowest emission and the final state shows a somewhat shallow potential well. Just like the rare gas halides, this emission has the strongest transition period. That is why it is the preferred emission in XeCl lasers.
  Experimentally, the (C→A) and (B→A) lines overlap, producing a continuum centered at 345 nm, often of low amplitude as can be observed in Figure 11. The width of the emission depends on the transition tending to a strongly repulsive state. Koltz et al. placed this continuum at between 312 and 460 nm. The weak observed intensities are attributed to the weakness of the probabilities of the transition of the two emissions opposite that of the B→X and by the small amounts of states C formed with respect to states B as was previously seen. Other authors have called attention to the absorption phenomena of molecule Xe
  ₂Cl at this wavelength. According to Kannari et al., reaction (3) is the principal pathway for synthesis of states B and C. Tsuji et al. estimated the proportions of states B and C formed: 38% for state C and 62% state B. The value of the transition probabilities (theoretical value of IB→A/IB→X = 0.07; experimental value of 0.05), so the contribution of (B→A) emission is about 10%. Several authors claimed that a laser based on the 345 nm emission could be developed, especially at pressures of about 10 atmospheres when states B and C are thermalized. Meanwhile, no concrete result had been discovered as of 2014.
  The (D→X) transition centered at 235.5 nm has not been systematically observed. The corresponding line appears weak as in the case in Figure 12. Its optical width is similar to that of (B→X) emission because it leads to the same weakly bound state of X. In contrast, the relative intensity of the (B→X) and (D→X) emissions considerably vary from one author to the other: ID→X/IB→X = 1/3 by Shuker, 1/25 to 1/50 by Sur et al. and 0.14 by Taylor et al.. The latter authors noted that the relation is pressure-independent. It remains unlikely that a laser could be developed using this transition as Shuker had predicted.
  The spectra did not show any D→A emission. Nevertheless, Hassal et Ballik saw a line at 246 nm with very weak intensity (figure 12) without attributing it to the transition under consideration.
  State D emissions are negligible for XeCl spectroscopy. Attributing the absence of D→A as for D→B to the weakly associated transition probability, the same cannot be said for D→X. From Table 17, the D→X emission should be of lesser intensity than B→X. In this case, the possible explanation could be due to the weak production of state D, either by the ionic pathway or by the harpoon reaction using states Xe(³P). The principal path of XeCl
  * synthesis is reaction (3) and
caption: Figure 13. XeCl(B→X) emission spectrum.[94]
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