Electron recollisional excitation of OCS⁺ in phase-locked ω + 2ω intense laser fields

Photoelectron-photoion coincidence momentum imaging has been performed to investigate excitation mechanisms on dissociative ionization of OCS, OCS → OCS⁺ + e^- → OC + S⁺ + e^-, in phase-locked ω + 2ω intense laser fields. The electron kinetic energy spectra depend on the ion species, OCS⁺ or S⁺, produced in coincidence. The observed electron momentum distribution shows clear asymmetry along the laser polarization direction with a 2π-oscillation period as a function of the phase difference between the ω and 2ω laser fields. The asymmetry of electron emission in the OCS⁺ channel flips at an electron kinetic energy of 8.2 eV, below which forward-scattered electrons dominate and above which backward-scattered electrons dominate. In the S⁺ channel, the asymmetry flips at a lower kinetic energy of 4.2 eV. By comparing with a classical trajectory Monte Carlo simulation, it has been clarified that this energy shift between the OCS⁺ and S⁺ channels corresponds to the excitation energy of the parent ion and that electron recollisional excitation takes place to form the fragment ion in intense laser fields.