Electron recollisional excitation of OCS${}^{+}$ in phase-locked $\omega +2\omega$ intense laser fields

Abstract

Photoelectron-photoion coincidence momentum imaging has been performed to investigate excitation processes on dissociative ionization of OCS, OCS $\to$ OCS${}^{+}$ + e${}^{-}$ $\to$ OC + S${}^{+}$ + e${}^{-}$, in phase-locked $\omega +2\omega$ intense laser fields. The electron kinetic energy spectra depend on coincidentally produced ion species, OCS${}^{+}$ or S${}^{+}$. The observed electron momentum distribution shows clear asymmetry along the laser polarization direction with a 2$\pi$-oscillation period as a function of the phase difference between the $\omega$ and $2\omega$ laser fields. The asymmetry of electron emission in the OCS${}^{+}$ channel flips at the electron kinetic energy of 8.2 eV where the dominant scattering direction switches from forward to backward. In the S${}^{+}$ channel, the asymmetry flips at the lower kinetic energy of 4.2 eV. In comparison 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.