Multiplet-specific shape resonance and autoionization effects in (2+1) resonance enhanced multiphoton ionization of O_2 via the d ^1Π_g state

Abstract

In this paper we discuss the single‐photon ionization dynamics of the d  ^1Π_g Rydberg state of O_2. Comparison is made with vibrationally resolved measurements of photoelectron spectra which employ (2+1) resonance enhanced multiphoton ionization (REMPI) through the d  ^1Π_g state. A σ_u shape resonance near the ionization threshold leads to non‐Franck–Condon vibrational branching ratios and a substantial dependence of photoelectron angular distributions on the vibrational state of the X  ^2Π_g ion. Significant differences exist between our one‐electron predictions and experiment. These are mainly attributed to electronic autoionization of repulsive ^1∑^−_u , ^1∑^+_u , and ^1Δ_u states associated with the 1π^3_u 1π^3_g configuration. A proposed singlet "K" ^1Π_u Rydberg state converging to the A  ^2Π_u ion probably also contributes to autoionization in the d  ^1Π_g state spectrum. We also show that autoionizing H and J  ^3Π_u Rydberg states of O_2 converging to the a  ^4Π_u and A  ^2Π_u ionic thresholds, respectively, may play a previously unsuspected role in the C  ^3Π_g state one‐color REMPI spectra. We discuss multiplet‐specific (spin‐dependent) effects via comparison of these results with recent experimental and theoretical studies of O_2 C  ^3Π_g photoionization.