Ionization and expansion dynamics of atomic clusters irradiated with short intense VUV pulses

TL;DR

This study models the ionization and expansion of xenon clusters under short, intense VUV pulses using kinetic Boltzmann equations, revealing size-dependent shell formation, Coulomb explosion, and recombination effects consistent with experimental data.

Contribution

It introduces a unified kinetic model that accurately predicts cluster dynamics, including shell formation and expansion mechanisms, across a wide range of cluster sizes and pulse fluences.

Findings

Small clusters undergo Coulomb explosion driven by charged outer shells.

Large clusters expand hydrodynamically after outer shell explosion.

Significant neutral atom formation occurs within the cluster core due to recombination.

Abstract

Kinetic Boltzmann equations are used to model the ionization and expansion dynamics of xenon clusters irradiated with short intense VUV pulses. This unified model includes predominant interactions that contribute to the cluster dynamics induced by this radiation. The dependence of the evolution dynamics on cluster size, $N_{atoms}=20-90000$, and pulse fluence, $F=0.05-1.5$ J/cm$^2$, corresponding to intensities in the range, $10^{12}-10^{14}$ W/cm$^2$ and irradiation times, $\leq 50$ fs, is investigated. The predictions obtained with our model are found to be in good agreement with the experimental data. We find that during the exposure the cluster forms a shell structure consisting of a positively charged outer shell and a core of net charge equal to zero. The width of these shells depends on the cluster size. The charged outer shell is large within small clusters ($N_{atoms}=20,70$), and its Coulomb explosion drives the expansion of these clusters. Within the large clusters ($N_{atoms}=2500,90000$) the neutral core is large, and after the Coulomb explosion of the outer shell it expands hydrodynamically. Highly charged ions within the core recombine efficiently with electrons. As a result, we observe a large fraction of neutral atoms created within the core, its magnitude depending on the cluster size.