Theory of high-order harmonic generation from molecules by intense laser pulses

TL;DR

This paper presents a theoretical framework for high-order harmonic generation (HHG) from molecules, demonstrating how accurate photo-recombination cross sections can improve spectral predictions and enable femtosecond chemical imaging.

Contribution

It introduces a method combining wave packets from simple models with accurate molecular cross sections to predict HHG spectra, facilitating time-resolved molecular imaging.

Findings

HHG spectra agree with numerical solutions for H2+

Using accurate PRCS improves spectral predictions

Potential for femtosecond chemical imaging

Abstract

We show that high-order harmonics generated from molecules by intense laser pulses can be expressed as the product of a returning electron wave packet and the photo-recombination cross section (PRCS) where the electron wave packet can be obtained from simple strong-field approximation (SFA) or from a companion atomic target. Using these wave packets but replacing the PRCS obtained from SFA or from the atomic target by the accurate PRCS from molecules, the resulting HHG spectra are shown to agree well with the benchmark results from direct numerical solution of the time-dependent Schr\"odinger equation, for the case of H$_2^+$ in laser fields. The result illustrates that these powerful theoretical tools can be used for obtaining high-order harmonic spectra from molecules. More importantly, the results imply that the PRCS extracted from laser-induced HHG spectra can be used for time-resolved dynamic chemical imaging of transient molecules with temporal resolutions down to a few femtoseconds.