Pulsed-Field-Ionization Zero-Kinetic-Energy (PFI-ZEKE) Photoelectron Spectroscopic Study of the Renner−Teller Effect in the Ã+ 2Π State of OCS+

The pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of OCS in a cold supersonic expansion has been measured in the region of the X̃ 1 Σ + → à + 2 Π and X̃ 1 Σ + → B̃ + 2 Σ + bands between 122 500 and 129 500 cm -1 . The spectrum of the X̃ → A + transition consists of extensive and strongly perturbed vibrational progressions in the CO- and CS-stretching modes ν 3 and ν 1, respectively. The perturbations, which arise from Fermi interactions between CS-stretching and bending levels, are revealed by (i) the observation of several members of the bending progression, (ii) the very irregular spacings between the two spin−orbit components of the stretching levels, and (iii) numerous anomalies in the vibrational intensity distribution. A complete analysis of the spectrum is presented that takes the spin−orbit interaction, the Renner−Teller effect, and Fermi interactions in the à + ionic state into account. This analysis provides a fully satisfactory explanation for the strongly perturbed positions of the vibrational levels of the à + state and for the anomalous intensity distribution of the photoelectron spectrum. The analysis also enabled the derivation of reliable information on the bending levels and of a set of spectroscopic parameters describing the Renner−Teller effect in the à + state of OCS + . Two closely spaced strong lines have been observed near the origin of the X̃ → B̃ + band. The upper one was assigned to the origin of the X̃ → B̃ + transition by extrapolation of the autoionizing Rydberg series.