Single-shot electron imaging of dopant-induced nanoplasmas

Cristian Medina ,Ľudovít Haizer ,Bálint Kiss ,Sz Tóth ,Andreas Heidenreich ,R Moshammer ,D Schomas ,Daniel Uhl ,Mathieu Dumergue ,Ltaief Ben Ltaief ,Thomas Pfeifer ,Sivarama Krishnan ,Zoltán Filus ,Markus Debatin ,Balázs Farkas ,A Ngai ,Nicolas Rendler ,F Stienkemeier ,Balázs Major ,Roland Flender ,Máté Kurucz ,M Mudrich

doi:10.1088/1367-2630/abf7f9

Abstract

We present single-shot electron velocity-map images of nanoplasmas generated from doped helium nanodroplets and neon clusters by intense near-infrared and mid-infrared laser pulses. We report a large variety of signal types, most crucially depending on the cluster size. The common feature is a two-component distribution for each single-cluster event: a bright inner part with nearly circular shape corresponding to electron energies up to a few eV, surrounded by an extended background of more energetic electrons. The total counts and energy of the electrons in the inner part are strongly correlated and follow a simple power-law dependence. Deviations from the circular shape of the inner electrons observed for neon clusters and large helium nanodroplets indicate non-spherical shapes of the neutral clusters. The dependence of the measured electron energies on the extraction voltage of the spectrometer indicates that the evolution of the nanoplasma is significantly affected by the presence of an external electric field. This conjecture is confirmed by molecular dynamics simulations, which reproduce the salient features of the experimental electron spectra.

Highlights

The generation of nanoplasmas in clusters and nanoparticles by intense femtosecond laser pulses is being widely studied, both for exploring fundamental light–matter interactions under extreme conditions, and in view of potential applications
In usual photoionization experiments of atomic or molecular beams, the ionization probability per laser shot is kept well below one to avoid blurring of electron velocity-map images (VMIs) and broadening of TOF peaks by space-charge effects. In this experiment a single laser pulse creates a large number of electrons and ions when the nanoplasma is formed; we may expect that the detected electron distributions are dominated by Coulomb repulsion between the electrons
Electrons directly emitted by outer ionization during the laser pulse produce an extended distribution that is not affected by the space charge

Summary

Introduction

The generation of nanoplasmas in clusters and nanoparticles by intense femtosecond laser pulses is being widely studied, both for exploring fundamental light–matter interactions under extreme conditions, and in view of potential applications. Initiated by tunnel ionization (TI) [11], an ionization avalanche is launched within a few optical cycles mainly by electron-impact ionization, thereby releasing a large fraction of electrons from their parent atoms or ions (inner ionization) [12] These quasi-free electrons remain trapped in the space-charge potential of the cluster and mainly determine the optical response of the resulting nanoplasma. As the nanoplasma heats and charges up, it starts expanding and electrons evaporate out of the collective Coulomb potential of the cluster core. This leads to a thermal distribution of free electrons characterized by an essentially exponential energy dependence [15,16,17,18,19]. Excited neutral and ionic species formed during this phase can decay by correlated electronic decay processes, as recently observed [18,19,20,21]

Methods

Results

Discussion

Conclusion