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

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.

💡 Research Summary

In this work the authors investigate electron‑recollision‑induced excitation of carbonyl sulfide (OCS) molecules in intense, phase‑locked two‑color laser fields composed of a fundamental (ω) and its second harmonic (2ω). Using a photoelectron‑photoion coincidence (PEPICO) momentum imaging apparatus, they record three‑dimensional momentum vectors of electrons and ions generated by the laser pulse, allowing them to correlate the electron kinetic energy spectra with the specific ionic fragment produced, either the parent ion OCS⁺ or the fragment S⁺.

The experimental setup employs 800 nm, 70 fs, 1 kHz laser pulses with peak intensities of 5×10¹³ W cm⁻² (ω) and 5×10¹² W cm⁻² (2ω). The two colors are combined co‑linearly, their polarizations are aligned, and the relative phase Δφ between ω and 2ω is actively stabilized using a feedback loop that monitors the interference of second‑harmonic light generated in two β‑BBO crystals. The stabilization keeps the standard deviation of Δφ below 0.06π, ensuring reproducible asymmetric electric‑field waveforms.

The key observation is a pronounced asymmetry of the electron emission along the laser polarization axis that oscillates with a 2π period as Δφ is varied. In the OCS⁺ channel the asymmetry flips sign at an electron kinetic energy of 8.2 eV, whereas in the S⁺ channel the flip occurs at 4.2 eV. Below the flip energy forward‑scattered electrons dominate; above it, backward‑scattered electrons dominate. This energy shift of ≈4 eV between the two channels matches the known excitation energy of OCS⁺ from its ground X ²Π state to the lowest excited A ²Π/B ²Σ⁺ states, indicating that electron recollision excites the parent ion before dissociation.

To interpret the data, the authors perform classical‑trajectory Monte‑Carlo (CTMC) simulations. The initial ionization rate is calculated from weak‑field asymptotic theory using the HOMO of OCS, including the permanent dipole moment. The electron’s initial position is the tunnel exit determined by solving z F(t,Δφ)+V(β,r=0,z)=−I_p, and its transverse velocity follows a Gaussian‑like distribution. The electron propagates under the combined influence of the laser field and a model potential V(β,r,z) for OCS⁺, which is constructed from a density‑functional‑theory (DFT) potential in the inner region and a three‑center Coulomb potential in the outer region.

A boundary is defined where V equals the field‑free ionization potential of OCS⁺ (≈30.3 eV). When the electron crosses this boundary with kinetic energy exceeding the A–X transition threshold (≈4 eV), an inelastic collision is assumed: the electron loses exactly 4 eV, which is transferred to the ion, promoting it to an excited state. Two models for the angular redistribution after the inelastic event are tested: (i) the electron retains its propagation direction, and (ii) its direction is randomized isotropically over 2π. The latter reproduces the experimental asymmetry curves most accurately, suggesting that the recolliding electron is strongly scattered during the excitation process.

The CTMC results reproduce the observed 8.2 eV and 4.2 eV flip energies, the overall shape of the asymmetry versus Δφ, and the relative contributions of forward and backward scattering. This agreement confirms that the S⁺ fragments arise from OCS⁺* states populated by electron recollision, while OCS⁺ ions are produced either directly (no excitation) or after a recollision that does not provide enough energy to dissociate the molecule.

The study demonstrates that phase‑locked ω+2ω fields provide a simple yet powerful knob to control the temporal shape of the electric field, thereby modulating the recollision dynamics. By correlating electron energy and emission direction with specific ionic fragments, the method offers a direct probe of which electronic states are accessed during strong‑field dissociation, even for polyatomic molecules where conventional joint electron‑nuclear energy spectra are difficult to interpret.

In conclusion, the authors have shown that electron recollision can efficiently excite OCS⁺ by ≈4 eV, leading to selective fragmentation, and that combined PEPICO measurements with CTMC modeling constitute an effective approach to unravel ultrafast electron‑driven chemistry in intense laser fields.