# Simulated XUV Photoelectron Spectra of THz-pumped Liquid Water

## TL;DR

This study explores the use of XUV photoelectron spectroscopy to detect ultrafast structural changes in water induced by intense THz pulses, revealing spectral shifts linked to heating but limited in temperature sensitivity.

## Contribution

It introduces ab initio calculations of photoelectron spectra for water clusters under THz-induced conditions, providing insights into spectral shifts and their relation to structural dynamics.

## Key findings

- Valence photoelectron peaks shift up to 0.4 eV after THz excitation
- Spectral broadening observed in photoelectron spectra
- Shifts are related to structural changes but not sensitive enough for precise temperature measurement

## Abstract

Highly intense, sub-picosecond terahertz (THz) pulses can be used to induce ultrafast temperature jumps (T-jumps) in liquid water. A supercritical state of gas-like water with liquid density is established, and the accompanying structural changes are expected to give rise to time-dependent chemical shifts. We investigate the possibility of using extreme ultraviolet (XUV) photoelectron spectroscopy as a probe for ultrafast dynamics induced by sub-picosecond THz pulses of varying intensities and frequencies. To this end, we use ab initio methods to calculate photoionization cross sections and photoelectron energies of (H2O)$_{20}$ clusters embedded in an aqueous environment represented by point charges. The cluster geometries are sampled from ab initio molecular dynamics simulations modeling the THz-water interactions. We find that the peaks in the valence photoelectron spectrum are shifted by up to 0.4 eV after the pump pulse, and that they are broadened with respect to unheated water. The shifts can be connected to structural changes caused by the heating, but due to saturation effects they are not sensitive enough to serve as a thermometer for T-jumped water.