Unexpected dipole instabilities in small molecules after ultrafast XUV irradiation

TL;DR

This study reveals that ultrafast XUV pulses can cause unexpected occupation inversion and dipole instabilities in small molecules like N₂, leading to observable signatures in photo-electron spectra, challenging prior assumptions about ultrafast electronic dynamics.

Contribution

It demonstrates a novel mechanism of occupation inversion and dipole instability induced by ultrafast XUV irradiation in small molecules, with implications for understanding ultrafast electronic processes.

Findings

Occupation inversion occurs around 50 eV XUV energy in N₂.

Dipole instability causes spontaneous dipole reappearance after pulse.

Low-energy structures in photo-electron spectra indicate the instability.

Abstract

We investigate the depletion of single-electron states in small molecules under the influence of very short XUV pulses. In N$_2$, for a certain window of XUV energies around 50 eV, we observe a marked occupation inversion, i.e. a situation where depletion of the deepest bound valence electron state is much larger than for any other state. This represents a realistic mechanism which is able to cut, almost instantaneously, a hole into a deep lying state, a situation which is often assumed ad hoc in numerous theoretical studies of energetic ultrafast processes. This occupation inversion furthermore drives a dipole instability, i.e. a spontaneous reappearance of the dipole signal long after the laser pulse is over and the dipole signal has died out. The dipole signal that emerges from this instability can be identified as a particular low-energy structure in photo-electron spectra.