The influence of autoionizing Rydberg states on the H2+ X 2Σg+ v+ = 0, 1, 2 state rotationally resolved photoelectron angular distributions and branching ratios

Abstract

The effect of vibrational autoionization on the H2+ X 2Σg+ v+=0, 1, 2, N+ state rotationally resolved photoelectron angular distributions and branching ratios has been investigated with a velocity map imaging spectrometer and synchrotron radiation. Measurements have been made in the energy regions close to the v+=0, 1 or 2 ionization thresholds where the photoabsorption spectrum is dominated by structure due to autoionizing Rydberg states. The photoelectron anisotropy parameter associated with the X 1Σg+ v″=0, N″=1→X 2Σg+ v+=0, 1 or 2, N+=1 transition has a high value, characteristic of emission predominantly along the polarization axis of the incident radiation, when ionization occurs directly but in the vicinity of an autoionizing Rydberg state a more isotropic angular distribution is observed. For the v+=1 level, the present experimental data are compared with existing theoretical predictions in the energy range encompassing the R(1) 8pσ v′=2 and the Q(1) 8pπ v′=2 states. Qualitative agreement has been obtained between the measured and the predicted rotationally resolved photoelectron anisotropy parameters. The experimental values of the rotationally unresolved S-branch photoelectron anisotropy parameter are found to lie considerably higher than that (0.2, independent of excitation energy) predicted under the assumption of p-wave emission, and, moreover, exhibit deviations which appear to correlate with autoionizing Rydberg states. These observations suggest that a proper description of the photoionization dynamics requires the inclusion of partial waves higher than l=1. In the neighbourhood of an autoionizing resonance, the variations occurring in the rotationally resolved branching ratios depend upon the rotational level of the Rydberg state.