page_title: Xenon monochloride
section_title: The Xe2Cl molecule
hierarchical_section_title: Xenon monochloride / The Xe2Cl exciplex molecule / The Xe2Cl molecule
caption_attribution_description: Français&#160;: xe2cl
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: Initial studies on the Xe
  ₂Cl molecule, found:
  Its most stable configuration in the excited state has a triangular geometry C₂ᵥ.
  The Xe
  ₂Cl
  * 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. 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. 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 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. 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 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.
  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.* At the ground state, 1²Γ forms a Van der Walls 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°.* The second excited state 2²Γ is also a Van der Walls 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.
  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. 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 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.
caption: Figure 15. Potential curves of Xe
  2Cl according to Huestis et al..[190]
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