Mode-specific photoionization dynamics of a simple asymmetric target: OCS

Vibrationally resolved photoelectron spectra of OCS+(C (2)Sigma(+)) are used to probe coupling between photoelectron motion and molecular vibration for a simple asymmetric system. Spectra are reported over the photon energy range of 21 <= h nu <= 55 eV. Vibrational branching ratios for all of the normal modes are determined and the results exhibit mode-specific deviations from Franck-Condon behavior. Schwinger variational calculations indicate the presence of four shape resonances, two k sigma resonances and two k pi resonances. All of the resonances play a role in the observed vibrationally resolved behavior. Two results are striking; first, the resonances are more sensitive to the C-O stretch than to the C-S stretch, particularly for photon energies above 30 eV. This relative insensitivity of the resonance to geometry changes involving a third-row element is similar to other systems studied. Second, theoretical results lead to the counterintuitive conclusion that bending the molecule suppresses the high energy resonance, even though there is an enhancement in the vibrational branching ratio curve for the single quantum bending excitation. The agreement between the theoretical and experimental branching ratio curves is good. Finally, the results unambiguously demonstrate that the forbidden bending excitation is caused by photoelectron-mediated vibronic coupling, i.e., the variation in the electronic transition matrix element with geometry, rather than the traditional explanation of interchannel vibronic coupling with intensity borrowing between ionic states.