High-intensity multiphoton ionization ofH2

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 ${\mathit{B}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ and ${\mathit{C}}^1$ ${\mathrm{\Pi }}_{\mathit{u}}$ electronic states in ${\mathrm{H}}_2$. At the intensities studied [(0.2–6)×${10}^{13}$ W/${\mathrm{cm}}^2$], 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 ${\mathrm{H}}_2^{+}$ 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 ${\mathrm{H}}_2^{+}$, 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 ${\mathrm{H}}_2$ 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 ${\mathrm{H}}_2^{+}$, ‖2p${\mathrm{\sigma }}_{\mathit{u}}$,n〉, with the single-photon-dressed ground state of ${\mathrm{H}}_2^{+}$, ‖1s${\mathrm{\sigma }}_{\mathit{g}}$,n+1〉.