Time-resolved four-wave-mixing spectroscopy for inner-valence transitions

TL;DR

This paper introduces a novel time-resolved four-wave mixing spectroscopy technique using femtosecond NIR and attosecond XUV pulses to observe inner-valence electronic couplings in molecules, demonstrated on neon.

Contribution

It presents the first experimental observation of correlations between inner-valence states in the XUV range using a combined NIR-XUV FWM approach, supported by ab initio calculations.

Findings

Revealed coupling dynamics between inner-valence states of neon.

Achieved good agreement between experiment and ab initio models.

Demonstrated potential for site-specific electronic process probing.

Abstract

Non-collinear four-wave mixing (FWM) techniques at near-infrared (NIR), visible, and ultraviolet frequencies have been widely used to map vibrational and electronic couplings, typically in complex molecules. However, correlations between spatially localized inner-valence transitions among different sites of a molecule in the extreme ultraviolet (XUV) spectral range have not been observed yet. As an experimental step towards this goal we perform time-resolved FWM spectroscopy with femtosecond NIR and attosecond XUV pulses. The first two pulses (XUV-NIR) coincide in time and act as coherent excitation fields, while the third pulse (NIR) acts as a probe. As a first application we show how coupling dynamics between odd- and even-parity inner-valence excited states of neon can be revealed using a two-dimensional spectral representation. Experimentally obtained results are found to be in good agreement with ab initio time-dependent R-matrix calculations providing the full description of multi-electron interactions, as well as few-level model simulations. Future applications of this method also include site-specific probing of electronic processes in molecules.