Understanding molecular harmonic emission at relatively long intense laser pulses: Beyond the Born-Oppenheimer approximation

TL;DR

This study investigates the complex high-order harmonic generation in molecules under long laser pulses by solving the full electronic Schrödinger equation beyond the Born-Oppenheimer approximation, revealing non-adiabatic and asymmetric emission effects.

Contribution

It provides a detailed numerical analysis of molecular HHG beyond the Born-Oppenheimer approximation, highlighting the roles of non-adiabatic response and asymmetric emission.

Findings

Complex HHG patterns mainly due to non-adiabatic response

Asymmetric emission along polarization contributes to HHG

Electron localization effect was not observed in HHG spectrum

Abstract

The underlying physics behind the molecular harmonic emission in relatively long sin$^2$-like laser pulses is investigated. We numerically solved the full-dimensional electronic time-dependent Schr\"{o}dinger equation beyond the Born-Oppenheimer approximation for simple molecular ion H$_2^+$. The occurrence and the effect of electron localization, non-adiabatic redshift and spatially asymmetric emission are evaluated to understand better complex patterns appearing in the high-order harmonic generation (HHG) spectrum. Results show that the complex patterns in the HHG spectrum originate mainly from a non-adiabatic response of the molecule to the rapidly changing laser field and also from a spatially asymmetric emission along the polarization direction. The effect of electron localization on the HHG spectrum was not observed as opposed to what is reported in the literature.