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 + 2 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 and 2 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.