Superexcited State Dynamics of OCS: An Experimental Identification of Three Competing Decay Channels among Autoionization, Internal Conversion, and Neutral Predissociation.

The 7sσ and 6pσ superexcited Rydberg states of OCS belonging to series converging onto the B̃2Σ+ ionic limit have been successfully prepared by three-photon UV excitation, and their ensuing competing relaxation processes have been probed by a time-delayed IR ionization pulse. The time profiles of S+ ions, which encode their fragmentation mechanism, are only observable at high pump intensities, thus providing unique experimental identification of the neutral predissociation channel producing S* atoms. Benefiting from this feature and by comparison with the time behavior of OCS+ ions, three competing relaxation channels are identified: autoionization associated with both X̃2Π and Ã2Π ionic states; internal conversion to isoenergetic RA states, the deactivation of which manifests as a picosecond decay in the time profile of OCS+ ions; picosecond neutral predissociation appearing as a nondecaying plateau in the time profiles of S+ ions.