Vacuum ultraviolet pulsed field ionization-photoelectron study of OCS in the energy range of 15-19 eV

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for OCS have been obtained in the energy range 15.0-19.0 eV, covering the vibronic bands of OCS+ (A (2)Pi, B (2)Sigma(+), and C (2)Sigma(+)). The ionization energies for the formation of the ground vibrational levels of OCS+ (A (2)Pi(3/2), A (2)Pi(1/2), B (2)Sigma(+), and C (2)Sigma(+)) from the ground OCS(X (1)Sigma(+)) state have been determined as 15.0759+/-0.0005 eV, 15.0901+/-0.0005 eV, 16.0403+/-0.0005 eV, and 17.9552+/-0.0005 eV, respectively. We have also generated the theoretical adiabatic three dimensional potential energy functions (PEFs) for OCS+(A (2)Pi) by employing the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Using these PEFs, the spectroscopic constants and low-lying rovibronic energy levels for OCS+(A (2)Pi) are calculated variationally. These calculations have made possible the identification of many PFI-PE vibronic bands for OCS+(A (2)Pi), which are originated from vibronic and Fermi resonance interactions. Owing to the different equilibrium geometries between the OCS+(A (2)Pi) and OCS(X (1)Sigma(+)) states, the PFI-PE spectrum for OCS+(A (2)Pi) exhibits a long vibronic progression extending well above the OCS+(B (2)Sigma(+)) state. On the contrary, the PFI-PE spectra for OCS+ (B (2)Sigma(+) and C (2)Sigma(+)) are overwhelmingly dominated by the ground (0,0,0) bands, exhibiting only weak vibrational progressions. (C) 2002 American Institute of Physics.