Photoionization of rare-gas-dimer excited states: Experimental measurements of absolute cross sections for Xe2*(0u+,1g,2g) in the ultraviolet (248351 nm)

Photoionization of excited states of the xenon dimer (Xe2) 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 Xe2+ 1(12)u ions, collisional and radiative relaxation of the Xe 6p and 6s 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 N2) 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(12)0 and 6s(32)1 levels confirm that a molecule is being photoionized. The molecular states involved are 0u+,1g, and 2g which correlate with the 6s(32)1 excited level and a S01 (ground-state) atom. The optical transitions associated with each laser wavelength studied appear to be Xe2*0u+Xe2+1(32)g (=248.4 nm); 1g (or 2g)1(32)u (=277.0 nm); and 1g (or 2g)1(12)u for =307.9,337.1, and 351.1 nm. The discrepancy between the measured cross sections and the values calculated previously (Lorents, Eckstrom, and Huestis, 1973) from a quantum-defect approach is roughly a factor of 2 at 308 nm but increases rapidly at shorter wavelengths. Also, in contrast to theoretical predictions for the 6s atomic Xe excited states in which the photoionization cross section falls monotonically with photon energy, the measured peaks at 7×10-18 cm2 for =308 nm (=4 eV). The profile of the photoionization cross-section spectrum that is reported here for Xe2* is, however, remarkably similar to that calculated by Rescigno et al. [J. Chem. Phys. 68, 5283 (1978)] for the 1 excited state of the Ar2 excimer (Ar2*).