The effect of autoionization on the N2+ X 2Σg+ state vibrationally resolved photoelectron anisotropy parameters and branching ratios

Vibrationally resolved photoelectron anisotropy parameters and branching ratios for the N2+ X 2Σg+ state have been measured between the ionization threshold and ∼16.7 eV to study the influence of autoionization on the photoionization dynamics. In this energy range the ion yield curve exhibits extensive structure due to autoionizing Rydberg or valence states. Some of these Rydberg states belong to series converging onto the A 2Πu limit but, close to the ionization threshold, states belonging to series converging onto vibrationally excited levels of the X 2Σg+ state are also observed. At photon energies free from the influence of neutral excited states, the measured vibrational branching ratios correlate with the Franck–Condon factors for direct ionization. However, strong deviations are observed when the excitation energy corresponds with a transition into an autoionizing state. An attempt to interpret these variations using Franck–Condon factors connecting the neutral excited state and the final ionic state proved unsuccessful. This failure suggests that the intermediate states might not be pure Rydberg states. Electronic and, close to threshold, vibrational autoionization affects the vibrationally resolved photoelectron anisotropy parameters, leading to a significant reduction in the fairly high β-values determined for non-resonant ionization. Photoelectron images of the N2+ X 2Σg+ state recorded close to the v+ = 1 or the v+ = 2 ionization thresholds suggest that electronic autoionization results in rotational branch populations that differ from those due to direct, non-resonant, photoionization. (paper)