Contribution of resonance excitation on ionization of OCS molecules in strong laser fields

Photoelectron images from single ionization of OCS molecules are measured at 800 nm or 400 nm linearly polarized laser pulses (∼50 fs) using velocity-map imaging techniques. The resonant ionization of the molecules has been studied by tracing the electron energy shift in the photoelectron energy spectra at different laser intensities, and the contributions of three excited states at certain laser intensities and wavelengths are identified. During the ionization of OCS molecules, two excited states (1Δ and 1∏) are shifted into resonance at different 800 nm laser intensities, and the Rydberg state 4p ∏ is involved in the resonance for a 400 nm laser. The demonstration of resonant excitation contribution experimentally will help further the understanding of OCS ionization in strong laser fields.