Rare-gas clusters in intense VUV, XUV and soft x-ray pulses: Signatures of the transition from nanoplasma-driven cluster expansion to Coulomb explosion in ion and electron spectra

TL;DR

This study uses simulations to analyze how rare-gas clusters respond to intense short-wavelength laser pulses, revealing a transition from plasma-driven expansion to Coulomb explosion based on photon energy, with electron spectra serving as key signatures.

Contribution

It demonstrates a wavelength-dependent transition in cluster expansion mechanisms, highlighting electron spectra as more reliable indicators than ion spectra.

Findings

Transition from plasma-driven to Coulomb explosion with increasing photon energy

Electron spectra provide clearer signatures of expansion mechanisms

Time-dependent dynamics correlate with final ion and electron spectra

Abstract

We investigate the wavelength dependent ionization, heating, and expansion dynamics of medium-sized rare-gas clusters (Ar$_{923}$) under intense femtosecond short-wavelength free electron laser pulses by quasi-classical molecular dynamics simulations. A comparison of the interaction dynamics for pulses with $\hbar\omega$=20, 38, and 90\,eV photon energy at fixed total excitation energy indicates a smooth transition from plasma-driven cluster expansion, where predominantly surface ions are expelled by hydrodynamic forces, to quasi-electrostatic behavior with almost pure Coulomb explosion. Corresponding signatures in the time-dependent cluster dynamics as well as in the final ion and electron spectra support that this transition is linked to a crossover in the electron emission processes. The resulting signatures in the electron spectra are shown to be even more reliable for identifying the cluster expansion mechanisms than ion energy spectra itself.