Theoretical Study of Molecular Electronic and Rotational Coherences by High-Harmonic Generation

TL;DR

This paper develops a detailed theoretical framework for understanding how high-harmonic generation spectroscopy can be used to detect coupled electronic and rotational wavepackets in molecules, advancing attosecond science.

Contribution

It introduces a generalized density matrix approach and semiclassical rescattering model for analyzing molecular wavepackets via HHG spectroscopy, including polarization effects and density matrix decomposition.

Findings

Good quantitative agreement between theory and experiment

Analysis of Raman scattering contributions to wavepacket creation

Interpretation of results through irreducible density matrix components

Abstract

The detection of electron motion and electronic wavepacket dynamics is one of the core goals of attosecond science. Recently, choosing the nitric oxide (NO) molecule as an example, we have introduced and demonstrated a new experimental approach to measure coupled valence electronic and rotational wavepackets using high-harmonic generation (HHG) spectroscopy [Kraus et al., Phys. Rev. Lett. 111, 243005 (2013)]. A short outline of the theory to describe the combination of the pump and HHG probe process was published together with an extensive discussion of experimental results [Baykusheva et al., Faraday Discuss 171, 113 (2014)]. The comparison of theory and experiment showed good agreement on a quantitative level. Here, we present the generalized theory in detail, which is based on a generalized density matrix approach that describes the pump process and the subsequent probing of the wavepackets by a semiclassical quantitative rescattering approach. An in-depth analysis of the different Raman scattering contributions to the creation of the coupled rotational and electronic spin-orbit wavepackets is made. We present results for parallel and perpendicular linear polarizations of the pump and probe laser pulses. Furthermore, an analysis of the combined rotational-electronic density matrix in terms of irreducible components is presented, that facilitates interpretation of the results.