Pulsed field ionization-photoelectron bands for CO 2 + (A 2 Π u and B 2 Σ u + ) in the energy range of 17.2-19.0 eV: An experimental and theoretical study High resolution pulsed field ionization-photoelectron study of CO 2 + (X 2 Π g ) in the energy range of 13.6-14.7 eV J.Vacuum ultraviolet pulsed field ionization-photoelectron ͑PFI-PE͒ spectra for OCS have been obtained in the energy range 15.0-19.0 eV, covering the vibronic bands of OCS ϩ ͑A 2 ⌸, B 2 ⌺ ϩ , and C 2 ⌺ ϩ ͒. The ionization energies for the formation of the ground vibrational levels of OCS ϩ (A 2 ⌸ 3/2 , A 2 ⌸ 1/2 , B 2 ⌺ ϩ , and C 2 ⌺ ϩ ͒ from the ground OCS(X 1 ⌺ ϩ ) state have been determined as 15.0759Ϯ0.0005 eV, 15.0901Ϯ0.0005 eV, 16.0403Ϯ0.0005 eV, and 17.9552Ϯ0.0005 eV, respectively. We have also generated the theoretical adiabatic three dimensional potential energy functions ͑PEFs͒ for OCS ϩ (A 2 ⌸) 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 ϩ (A 2 ⌸) are calculated variationally. These calculations have made possible the identification of many PFI-PE vibronic bands for OCS ϩ (A 2 ⌸), which are originated from vibronic and Fermi resonance interactions. Owing to the different equilibrium geometries between the OCS ϩ (A 2 ⌸) and OCS(X 1 ⌺ ϩ ) states, the PFI-PE spectrum for OCS ϩ (A 2 ⌸) exhibits a long vibronic progression extending well above the OCS ϩ (B 2 ⌺ ϩ ) state. On the contrary, the PFI-PE spectra for OCS ϩ ͑B 2 ⌺ ϩ and C 2 ⌺ ϩ ͒ are overwhelmingly dominated by the ground ͑0,0,0͒ bands, exhibiting only weak vibrational progressions.