Electron recollisional excitation of OCS$^+$ in phase-locked $ω+ 2ω$ 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 $ω+ 2ω$ 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$π$-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 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.

Figures (8)

• Figure 1: Examples of phase-locked $\omega + 2\omega$ laser electric fields for different (a) $\phi_\text{CEP}$ and (b) $\Delta \phi$.
• Figure 2: (a) Schematic of the experimental setup. MM: motorized mount; BBO: $\beta$-barium borate crystal; DM: dielectric mirror; HWP: half-waveplate; WP: wedge plate; OAP: off-axis parabolic mirror; W: window; PSD: position-sensitive detector; L: lens; WW: wedged window; CCD: charge-coupled device camera; P: polarizer; SP: spectrometer. (b) Distributions of phase difference $\Delta \phi$ measured with feedback-loop stabilization (red solid line) and without stabilization (grey broken line).
• Figure 3: Two-dimensional plot of the potential energy $V(\beta = 0, r, z)$ employed in the CTMC simulation at $\beta = 0$. The position of each nucleus is at the equilibrium position in neutral OCS. The carbon atom is at the origin, the oxygen atom is at $z < 0$, and the sulfur atom is at $z > 0$. The laser polarization direction is along the $z$-axis.
• Figure 4: Schematic of potential energy curves of OCS$^+$ along the C--S coordinate ($r_\text{CO} = 1.15\angstrom$) adapted from Ref. Hirst.MolPhys.2006 by digitizing the published data. Solid, dashed, dotted, dash-dotted, and dash-dot-dotted lines represent $^2\Pi$, $^2\Sigma^+$, $^4\Sigma^-$, $^2\Delta$, and $^2\Sigma^-$ states, respectively. Vertical line indicates the equilibrium geometry of OCS.
• Figure 5: Electron kinetic energy $E_\text{kin}$ distribution of (a) the OCS$^+$ channel and (b) the S$^+$ channel obtained by averaging over $\Delta \phi$ from 0 to 2$\pi$. Momentum images of the photoelectron coincidentally detected with (c) OCS$^+$ and (d) S$^+$ in phase-locked $\omega + 2\omega$ laser fields ($I_\omega = 5e13\W\per\cm\squared, I_{2\omega} = 5e12\W\per\cm\squared$) at $\Delta \phi = \pi$. The laser polarization direction is indicated as $\varepsilon$. The shape of the $\omega + 2\omega$ laser electric field at $\Delta \phi = \pi$ is schematically shown at the bottom-right corner.