Femtosecond nonadiabatic confinement of ethylene dication yield.

Ultrashort extreme-ultraviolet (XUV) pulses combined with near-infrared (NIR) probe fields enable real-time tracking of molecular dynamics, including nonadiabatic relaxation in molecular ions relevant to astrophysics [1]. We present a theoretical investigation of the ethylene dication yield driven by an attosecond XUV pulse-train pump and a delayed NIR probe, for which recent measurements have observed a yield peak around 15 fs [2]. The broadband pump ionizes the neutral ethylene into several low-lying cationic states (D0–D4), triggering the nonadiabatic nuclear dynamics. The delayed NIR field induces then the multiphoton ionization of the cation. We employ trajectory surface hopping simulations to obtain the time-dependent D0–D4 populations and geometries [3], and compute the average multiphoton ionization yield by solving the time-dependent Schrödinger equation (TDSE) for each cation in the presence of the NIR pulse. The TDSE propagation is performed using ASTRA, a close-coupling implementation to the molecular multi-channel ionization, designed to reproduce photoionization observables driven by ultrashort laser pulses [4]. Simulations over a range of NIR intensities, matching the experimental conditions, reproduce the observed dication yield maximum at 10–20 fs delays [2]. The main contribution to the yield's peak corresponds to the ionization from the D1 and D2 cations, for which the C=C bond elongation facilitates the appearing of resonantly-enhanced multiphoton ionization paths. At longer time delays the nonadiabatic relaxation depletes the excited cations and the ionization from D0 starts to dominate the dication production, resulting in a much lower yield.

[1] M. Harvé et al., Nat. Phys. 17, 327 (2021).

[2]  C. Marante et al., J. Phys. Chem. Lett. 17, 3 (2026)

[3] L. Fransén et al., J. Phys. Chem. A 128, 1457 (2024).

[4] J. Randazzo et al., Phys. Rev. Res. 5, 043115 (2023).