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[sup +] (A ²Π, B ²Σ[sup +], and C ²Σ[sup +]). The ionization energies for the formation of the ground vibrational levels of OCS[sup +] (A ²Π[sub &frac32;], A ²Π[sub ½, B ²Σ[sup +], and C ²Σ[sup +]) from the ground OCS(X ¹Σ[sup +]) 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[sup +](A ²Π) 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[sup +](A ²Π) are calculated variationally. These calculations have made possible the identification of many PFI-PE vibronic bands for OCS[sup +](A ²Π), which are originated from vibronic and Fermi resonance interactions. Owing to the different equilibrium geometries between the OCS[sup +](A ²Π) and OCS(X ¹&Sigma[sup +]) states, the PFI-PE spectrum for OCS[sup +](A ²Π) exhibits a long vibronic progression extending well above the OCS[sup +](B ²&Sigma[sup +]) state. On the contrary, the PFI-PE spectra for OCS[sup +] (B ²&sSigma[sup +] and C ²&Sigma[sup +]) are overwhelmingly dominated by the ground (0,0,0) bands, exhibiting only weak vibrational progressions. [ABSTRACT FROM AUTHOR]