Photoionization of rare-gas-dimer excited states: Experimental measurements of absolute cross sections for ${\mathrm{Xe}}_2^{*}(0_u^{+},1_g,2_g)$ in the ultraviolet ($248\le \lambda \le 351$ nm)

Photoionization of excited states of the xenon dimer (${\mathrm{Xe}}_2$) has been observed and absolute ionization cross sections have been measured for several laser wavelengths between 248 and 351 nm. Production of the excimers is accomplished by two-photon ionization of ground-state Xe atoms at 193 nm, followed by formation and the subsequent dissociative recombination of $\mathrm{Xe}_2^{}{}_{}{}^{+}$ $1{\left(\frac{1}{2}\right)}_u$ ions, collisional and radiative relaxation of the Xe $6p$ and $6s^{\prime }$ manifolds, and formation of low-lying excimer states by three-body collisions. Absolute photoionization cross sections are subsequently determined at 351.1, 337.1, 307.9, 277.0, and 248.4 nm by combining a second rare-gas—halide excimer (or ${\mathrm{N}}_2$) laser pulse with a microwave-absorption technique to monitor the absolute photoelectron density (in real time) as a function of the intensity of the second laser pulse. The use of microwave absorption allows for the detection of photoelectrons in the presence of a high-pressure background gas. Laser-induced fluorescence and spontaneous emission studies of the temporal behavior of the populations of all of the Xe $6p$ states as well as the $6s^{\prime }{\left(\frac{1}{2}\right)}_0$ and $6s{\left(\frac{3}{2}\right)}_1$ levels confirm that a molecule is being photoionized. The molecular states involved are $0_u^{+},1_g$, and $2_g$ which correlate with the $6s{\left(\frac{3}{2}\right)}_1$ excited level and a ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ (ground-state) atom. The optical transitions associated with each laser wavelength $\lambda$ studied appear to be ${\mathrm{Xe}}_2^{*}{0}_u^{+}\to {\mathrm{Xe}}_2^{+} 1{\left(\frac{3}{2}\right)}_g (\lambda =248.4\mathrm{} \mathrm{nm})$; $1_g (or 2_g)\to 1{\left(\frac{3}{2}\right)}_u (\lambda =277.0\mathrm{} \mathrm{nm})$; and $1_g (or 2_g)\to 1{\left(\frac{1}{2}\right)}_u$ for $\lambda =307.9,337.1, \mathrm{and} 351.1$ nm. The discrepancy...