A powerful method to calculate high-order harmonic generation from aligned or randomly distributed molecules

We extend our recently developed semiclassical strong-field Herman–Kluk (SFHK) propagator method to calculate high-order harmonic generation (HHG) in diatomic molecules driven by few-cycle intense laser fields. For N₂ and H₂, we show that the high level of accuracy previously demonstrated for atomic systems, both in HHG yield and phase when compared with exact numerical solutions of the time-dependent Schrödinger equation, is preserved for molecules. Furthermore, we present a detailed analysis of the angular dependence of high-order harmonics. To compare with experimental data, HHG calculations require averaging over molecular orientations, which typically involve multidimensional integrations for polyatomic molecules. Here we demonstrate that the SFHK method offers a distinct and powerful advantage, as its intrinsic Monte Carlo sampling enables efficient evaluations of combined multidimensional integrals, together with the integrals over the initial momentum distributions of electron wave-packet right after the tunnel exit.

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award Number DE-SC0023192.