HIGH-INTENSITY MULTIPHOTON IONIZATION OF H-2

A tunable, high-intensity picosecond-dye-laser system has been employed with electron energy analysis to investigate the dynamics of (3 + 1) resonance-enhanced multiphoton ionization via various vibrational levels of the B 1-SIGMA-u+ and C 1-PI-u electronic states in H-2. At the intensities studied [(0.2-6) x 10(13) W/cm2], we find evidence for production of molecular ions in various vibrational levels; at the lower intensities the population distribution of final vibrational states varies with wavelength in a manner consistent with resonant enhancement at the three-photon level, followed by ionization into a vibrational level of H-2+ roughly predictable by a Franck-Condon analysis of ionization out of the C state. At higher intensities, there is a shift to increased population of lower vibrational states of H-2+, consistent with an ac Stark shift of the correspondingly lower vibrational levels of the C state into resonance with the three-photon energy of the laser. Clear evidence of direct dissociation of H-2 followed by single-photon ionization of the excited H atom is observed as well. Above-threshold ionization of these two processes occurs readily. We also find that dissociative ionization is an increasingly important ionization pathway as the wavelength is increased. Finally, we see evidence of a fourth ionization pathway, which we tentatively assign to photoionization into a transient bound state created by the avoided crossing of the first repulsive electronic state of H-2+, \2p-sigma-u, n >, with the single-photon-dressed ground state of H-2+, \1s-sigma-g, n + 1 >.