Absorption of electronically excited Xe₂Cl in the ultraviolet

The absorption cross section for the lowest-lying, bound excited state (4 ²Γ) of Xe₂Cl has been measured at several wavelengths in the ultraviolet (UV). Temporally isolating the excited Xe ₂Cl species in Xe/Cl₂ gas mixtures has been accomplished by producing XeCl molecules in the B state by photoassociation [Xe+Cl+ℏω→XeCl(B)] at λ=308 nm. Collisional mixing of the lowest lying ion pair states of the excimer subsequently forms the XeCl species in its C state. Following the formation of Xe₂Cl* from XeCl(B,C) by a three-body collision, the long radiative lifetime of the trimer (>200 ns) relative to those for the XeCl B and C levels is exploited to ensure that the triatomic molecule is the predominant species when the experiments are performed. A second UV laser pulse depletes the Xe ₂Cl(4 ²Γ) population by photoexcitation [as evidenced by the sudden suppression of the 4 ²Γ→1 ²Γ blue-green (λ∼485 nm) fluorescence] and the absorption cross section is determined from the dependence of the degree of fluorescence suppression on the laser intensity. Contrary to the known Xe ₂⁺ 1(1/2)_u→2(1/2)_g UV absorption profile, Xe₂Cl(4 ²Γ) exhibits significant absorption at wavelengths as low as 193 nm, whereas the dimer ion absorption at 248 nm is only a fraction of its peak (λ∼340 nm) value. Photoionization of excited Xe₂Cl at wavelengths below ∼275 nm may be responsible for the cross sections measured for photon energies as large as 6.4 eV.