Vibrational autoionization in H2: Vibrational branching ratios and photoelectron angular distributions near the v+=3 threshold

Vibrational branching ratios and photoelectron angular distributions are reported for photoionization of normal (the equilibrium ortho/para mixture) hydrogen in the region just above the v+=3 ionization limit. This region contains a number of vibrationally autoionizing Rydberg states, and the goal of the work was to observe experimentally the characteristic behavior associated with the decay of these states that was predicted by Raoult and Jungen [J. Chem. Phys. 74, 3388 (1981)] on the basis of an MQDT (multichannel quantum defect theory) calculation on para-H2 (J″=0). We have indeed observed that vibrational autoionization strongly favors the ionization channel representing the minimum change in the vibrational state of the ion core, as predicted. We also observed a sharp reduction in the photoelectron asymmetry of the v+=3 (and, to a lesser extent, the v+=2) ionization channel for resonant photoionization. Hence, qualitative agreement with theory is observed; however, a quantitative comparison now requires an extended calculation that includes the additional rotational levels that were populated under the present experimental conditions.