page_title: Xenon monochloride
section_title: Ion recombination
hierarchical_section_title: Xenon monochloride / XeCl exciplex / Synthetic pathways / Ion recombination
caption_attribution_description: Français&#160;: hexecl
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: According to several authors 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.
  The Cl⁻
   ions are basically formed by dissociative attachment from an HCl electron:
  HCl(v) + e⁻ → H + Cl⁻
   (AD)
  In that same case, the rate constant (AD) depends on the energy distribution of the electrons as illustrated in Figure 4.
  The third element Rg is passive chemically. It only serve to stabilize the reaction. Therefore, the authors only took the recombination rates of the positive and negative ions into consideration. These vary in a significant way with the total pressure of the gaseous mixture, the buffer gas and temperature.
  Reactions (3) and (4) were experimentally demonstrated for all the rare gases. Figure 7 and Figure 8 show the influence of the buffer gas and pressure on the rate of recombination of these reactions when helium and then neon are utilized as buffer gases. This rate of recombination is of the same order of magnitude in both cases, of about some 10⁻⁶ cm³s⁻¹. Apparently the influence of temperature has only been studied for neon. (See Figure 9.) The rate of recombination α₃ in reaction (3) is at maximum at 180K for an absolute pressure of 294.2 kPa. α₃ is therefore 4.2×10⁻⁶ cm³s⁻¹.
  The more refined analysis of reaction (4) was carried out by Bates et Morgan. who found that the Monte-Carlo method, Flannery's equation and Langevin's theory can give good results only when the pressure is above 1 atm. This is the norm for lasers. The proposed "tidal" theory agrees with the experimental measurements of Mezyk et al. which is evident in Figure 10. The rate of recombination α₄ for reaction (4) is of the same order of magnitude as α₃.
  Reaction (5) is only observed when Rg is neon or argon. For this reaction, the evolution of the rate of recombination α₅ in the presence of pressurized neon is shown in figure 6. Imada et al. studied the influence of temperature for a fixed total pressure of 294 kPa. The maximum value of α₅ is obtained at 120K and α₅ = 7.5×10⁻⁶ cm³s⁻¹.
  For argon only two estimations are available at room temperature. At a pressure of 2 atm, α₅ = 2.10⁻⁶ cm³s⁻¹ and at a pressure of 1 atm, α₅ is 1×10⁻⁶ cm³s⁻¹.
  Reaction (5) does not favor a transitory complex RgXeCl
  * as an intermediate stage. 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.. 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. Other authors only estimate this contribution at 1% when the ionic pathway is involved. These theoretical conclusions are confirmed by experimental methods for the generality of the buffer gases and for other chlorine donors. 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.
  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.
  Xe+* ions r
caption: Figure 7. Ion-ion recombination termolecular reaction[147]  constants for XeCl lasers using helium as buffer gas, and calculated according to Flannery's equation. N is the density of the buffer gas and NL is the Loshmidt constant.[148]
exif: {"Image Orientation": "Horizontal (normal)", "Image XResolution": "96", "Image YResolution": "96", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop", "Image DateTime": "2012:02:02 23:15:01", "Image ExifOffset": "150", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "346", "EXIF ExifImageLength": "440"}