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

TL;DR

The paper develops a real-time ab initio RT-TD-CIS framework with a MO decomposition to analyze how individual molecular orbitals contribute to high-harmonic generation in molecules under strong laser fields. It computes HHG spectra using complex-energy propagation and Gaussian continuum bases, and supports both HF-CIS and KS-TDDFT-TDA (LC-$\omega$PBE) excited-state formulations. The approach reveals that MO symmetry and ionization energetics govern MO contributions and interferences, successfully reproducing the CO$_2$ dynamic minimum around the 23rd harmonic and highlighting multi-MO dynamics in H$_2$O under different polarizations. This orbital-resolved view provides detailed insights into strong-field processes and is scalable to larger systems, making it a valuable tool for interpreting HHG tomography and designing experiments.

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{ö}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.

Figures (10)

• Figure 1: HF occupied orbitals of CO$_{2}$.
• Figure 2: Total HHG spectra for CO$_{2}$, with laser-pulse polarization along $y$ axis (light purple line) and along $z$ axis (light green line), at the RT-TD-CIS level of theory. The arrow represents the minimum for the $z$-polarized case.
• Figure 3: Top: MO decomposition of the HHG spectrum of CO$_2$, with laser-pulse polarization along the $z$ axis, at the RT-TD-CIS level of theory. The inlet plot is a zoom on the minimum region (H21-H25). Bottom: Interference contributions between different ionization channels with laser-pulse polarization along the $z$ axis, at the RT-TD-CIS level of theory.
• Figure 4: Ground-excited (G-E, top) and excited-excited (E-E, bottom) contributions to the HHG spectrum for the ${\bf X}_a$ channel of CO$_2$, with laser-pulse polarization along the $z$ axis, at the RT-TD-CIS level of theory.
• Figure 5: Top: MO decomposition of the HHG spectrum of CO$_2$, with laser-pulse polarization along the $y$ axis, at the RT-TD-CIS level of theory. The inlet plot is a zoom on the region H21-H25. Bottom: Interference contributions between different ionization channels with laser-pulse polarization along the $y$ direction, at the RT-TD-CIS level of theory.