Spectroscopic investigation of excimer molecules by photoabsorption and photoassociation methods. I. XeCl

The excitation spectrum of the XeCl (B, $v'=0-21$) molecule was determined in a wide range of wave-lengths ($315-275$ nm). The molecules were formed as a result of flash photolysis by excimer laser radiation of gaseous Xe–Cl$_2$ mixtures of natural isotopic composition. They were then pumped by light from a frequency-tunable dye laser. A comparison of the intensities of free-bound and bound-bound transitions was used to estimate the rate constant of photoassociation, regarded as recombination of Xe and Cl atoms in which a photon acts as the third body: the rate was found to be $k_a^\lambda\simeq6\times10^{-29}$ cm$^6$ s$^{-1}$ at $\lambda\simeq310.5$ nm. A comparative analysis of the dependence of the intensity of the luminescence emitted by XeCl (C–A) and Xe$^2$CI$^*$, pumped in the region of the long-wavelength absorption edge of Xe–Cl$^2$–CI mixtures ($\lambda\ge 309$ nm), on the pressure of xenon made it possible to identify a continuum-type absorption band with a minimum at 310.5 nm. This band was attributed to the photoassociation process XeCl(X)$+$Xe$+h\nu\to$ Xe$^2$Cl$^*$ or Xe$ +$ Cl $+$ Xe $+h\nu\to$ Xe$^2$Cl$^*$.