Abstract

A tunable, high-intensity picosecond dye laser system has been employed with electron energy analysis to investigate the dynamics of multiphoton ionization, with vibrational levels of the B${}^1{\displaystyle {\sum }_{\text{u}}^{+}}$ and C ${}^1{\displaystyle \prod {}_{\text{u}}}$ electronic states in H₂ as resonant intermediate states. At the intensities studied (0.2 - 6 x 10¹³ 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 ${\text{H}}_2^{+}$ 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 ${\text{H}}_2^{+}$, consistent with an a. c. 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 the H₂ followed by single photon ionization of the excited H atom is observed as well. Above threshold ionization (ATI) 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 new 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 ${\text{H}}_2^{+}|2{\sigma }_u,n〉$, with the single-photon-dressed ground state of ${\text{H}}_2^{+}|1s{\sigma }_g,n+1〉$.

© 1991 Optical Society of America