Metallic water: transient state under ultrafast electronic excitation

TL;DR

This study investigates how ultrafast electronic excitation transforms water into a transient metallic state, revealing rapid ion acceleration, band gap collapse, and chemical fragment formation through advanced simulations.

Contribution

It introduces a detailed computational analysis of water's response to ultrafast electronic excitation, highlighting the transient metallic state and associated nonequilibrium phenomena.

Findings

Water becomes electronically conducting after a threshold electronic temperature.

Nonthermal ion acceleration occurs within sub-100 fs timescales.

Various chemically active fragments form depending on excitation dose.

Abstract

The modern means of controlled irradiation by femtosecond lasers or swift heavy ion beams can transiently produce such energy densities in samples that reach collective electronic excitation levels of the warm dense matter state where the potential energy of interaction of the particles is comparable to their kinetic energies (temperatures of a few eV). Such massive electronic excitation severely alters the interatomic potentials, producing unusual nonequilibrium states of matter and different chemistry. We employ density functional theory and tight binding molecular dynamics formalisms to study the response of bulk water to ultrafast excitation of its electrons. After a certain threshold electronic temperature, the water becomes electronically conducting via the collapse of its band gap. At high doses, it is accompanied by nonthermal acceleration of ions to a temperature of a few thousand Kelvins within sub-100 fs timescales. We identify the interplay of this nonthermal mechanism with the electron-ion coupling, enhancing the electron-to-ions energy transfer. Various chemically active fragments are formed from the disintegrating water molecules, depending on the deposited dose.