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

TL;DR

This study investigates how phase-locked $5+52$ laser fields influence electron recollisional excitation in OCS$^+$ ions, revealing energy-dependent asymmetries and ion-specific excitation processes through momentum imaging and simulations.

Contribution

It provides new insights into electron recollision dynamics and excitation mechanisms in OCS molecules under phase-locked laser fields, supported by experimental and classical trajectory simulations.

Findings

Electron emission asymmetry oscillates with phase difference.

Asymmetry flips at specific electron kinetic energies for different ions.

Recollisional excitation energy shifts explain fragment ion formation.

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.