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

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.

8 pages, 5 figures, 1 table