Rydberg spectroscopy of OCS: New assignments in the 70 000-74 000 cm ^-1 energy region

The first Rydberg states of OCS have been reinvestigated by applying a combination of several spectroscopic methods. The comparison of (2+1)- and (3+1)-multiphoton ionization (MPI) and one photon absorption spectra reveals quite different intensities indicating different symmetries of the involved Rydberg states. Assuming propensity rules for the ionization process and by measuring the ion ground state population with (2+1)-MPI photoelectron spectroscopy, it was possible to find new assignments for the spin-orbit components of the ion core and vibrational structures in the Rydberg states. Characteristic isotopic shifts in the (2+1)-MPI spectra confirm these assignments and deliver additional arguments for the identification of transitions to bending vibrations. In particular, for the bending vibration split by Renner-Teller interaction, ambiguities in the MPI photoelectron (PE) spectra could be solved by photodissoziation spectroscopy of the OCS⁺ ions prepared by (2+1)-MPI using the Rydberg transitions as intermediate resonances.