Time-dependent ab__initio molecular-orbital decomposition for high-harmonic generation spectroscopy

TL;DR

This paper introduces a real-time ab initio method to decompose high-harmonic generation spectra into contributions from individual molecular orbitals, providing detailed insights into electron dynamics in molecules under strong laser fields.

Contribution

The authors develop a novel time-dependent ab initio approach using a configuration-interaction-singles ansatz to analyze MO contributions to HHG spectra, accounting for ionization and continuum states.

Findings

Successfully decomposed HHG spectra for CO2 and H2O molecules.

Reproduced experimental and theoretical MO contributions to HHG.

Provided new insights into the role of HOMO, HOMO-1, and HOMO-2 in H2O under different polarizations.

Abstract

We propose a real-time time-dependent ab__initio approach within a configuration-interaction-singles ansatz to decompose the high-harmonic generation (HHG) signal of molecules in terms of individual molecular-orbital (MO) contributions. Calculations have been performed by propagating the time-dependent Schr{\"o}dinger equation with complex energies, in order to account for ionization of the system, and by using tailored Gaussian basis sets for high-energy and continuum states. We have studied the strong-field electron dynamics and the HHG spectra in aligned CO2 and H2O molecules. Contribution from MOs in the strong-field dynamics depends on the interplay between the MO ionization energy and the coupling between the MO and the laser-pulse symmetries. Such contributions characterize different portions of the HHG spectrum, indicating that the orbital decomposition encodes nontrivial information on the modulation of the strong-field dynamics. Our results correctly reproduce the MO contributions to HHG for CO2 as described in the literature experimental and theoretical data and lead to an original analysis of the role of the highest occupied molecular orbitals HOMO, HOMO-1, and HOMO-2 of H2O according to the polarization direction of the laser pulse.