Nuclear-Motion Effects in Attosecond Transient Absorption Spectroscopy of Molecules

TL;DR

This study explores how nuclear motion influences attosecond transient absorption spectra in molecules, revealing broad features and vibrational effects absent in fixed-nuclei models, and validates a computational approach for complex molecules.

Contribution

The paper demonstrates the impact of nuclear vibrations on molecular spectra and validates a Born-Oppenheimer based method for simulating these effects in molecules.

Findings

Nuclear motion causes broad absorption features in spectra.

Vibrational states significantly influence absorption structures.

Light-induced structures are affected by nuclear dynamics.

Abstract

We investigate the characteristic effects of nuclear motion on attosecond transient absorption spectra in molecules by calculating the spectrum for different model systems. Two models of the hydrogen molecular ion are considered: one where the internuclear separation is fixed, and one where the nuclei are free to vibrate. The spectra for the fixed nuclei model are similar to atomic spectra reported elsewhere, while the spectra obtained in the model including nuclear motion are very different and dominated by extremely broad absorption features. These broad absorption features are analyzed and their relation to molecular dissociation investigated. The study of the hydrogen molecular ion validates an approach based on the Born-Oppenheimer approximation and a finite electronic basis. This latter approach is then used to study the three-dimensional hydrogen molecule including nuclear vibration. The spectrum obtained from H$_2$ is compared to the result of a fixed-nuclei calculation. In the attosecond transient absorption spectra of H$_2$ including nuclear motion we find a rich absorption structure corresponding to population of different vibrational states in the molecule, while the fixed-nuclei spectra again are very similar to atomic spectra. We find that light-induced structures at well-defined energies reported in atomic systems are also present in our fixed nuclei molecular spectra, but suppressed in the ${\text{H}_2}^+$ and H$_2$ spectra with moving nuclei. We show that the signatures of light-induced structures are closely related to the nuclear dynamics of the system through the shapes and relative arrangement of the Born-Oppenheimer potential energy curves.