Distinguishing resonance symmetries with energy-resolved photoion angular distributions from ion-pair formation in O$_2$ following two-photon absorption of a 9.3 eV femtosecond pulse

TL;DR

This study combines experimental and theoretical approaches to analyze how ion-pair formation in O$_2$ following two-photon excitation reveals resonance symmetries through energy-resolved photoion angular distributions.

Contribution

It provides new insights into the resonance symmetries involved in ion-pair formation in O$_2$ by analyzing energy-dependent angular distributions of photoions.

Findings

Identification of $^3\,\Sigma^-_g$ and $^3\Pi_g$ symmetry states involved.

Observation of KER-dependent angular distribution variations.

Evidence for two nearly degenerate continuum resonances with different symmetries.

Abstract

We present a combined experimental and theoretical study on the photodissociation dynamics of ion-pair formation in O$_2$ following resonant two-photon absorption of a 9.3 eV femtosecond pulse, where the resulting O$^+$ ions are detected using 3-D momentum imaging. Ion-pair formation states of $^3\Sigma^-_g$ and $^3\Pi_g$ symmetry are accessed through predissociation of optically dark continuum Rydberg states converging to the B $^2\Sigma^-_g$ ionic state, which are resonantly populated via a mixture of both parallel-parallel and parallel-perpendicular two-photon transitions. This mixture is evident in the angular distribution of the dissociation relative to the light polarization, and varies with the kinetic energy release (KER) of the fragmenting ion-pair. The KER-dependent photoion angular distribution reveals the underlying two-photon absorption dynamics involved in the ion-pair production mechanism and indicates the existence of two nearly degenerate continuum resonances possessing different symmetries, which can both decay by coupling to ion-pair states of the same total symmetry through internal conversion.