Pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopic study of the Renner-Teller effect in the (A)over-tilde+ 2II 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 (X) over tilde (1)Sigma(+) --> (A) over tilde (+) (2)Pi and (X) over tilde 1Sigma(+) --> (B) over tilde (+) (2)Sigma(+) bands between 122 500 and 129 500 cm(-1). The spectrum of the (X) over tilde --> A(+) transition consists of extensive and strongly perturbed vibrational progressions in the CO- and CS-stretching modes v(3) and v(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 (A) over tilde (+) ionic state into account. This analysis provides a fully satisfactory explanation for the strongly perturbed positions of the vibrational levels of the (A) over tilde (+) 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 (A) over tilde (+) state of OCS+. Two closely spaced strong lines have been observed near the origin of the (X) over tilde --> (B) over tilde (+) band. The upper one was assigned to the origin of the (X) over tilde --> (B) over tilde (+) transition by extrapolation of the autoionizing Rydberg series.