EUV ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization and electron energy-loss spectra

TL;DR

This study investigates the ionization processes of helium nanodroplets under EUV radiation, revealing Penning-like ionization below the threshold and inelastic electron collisions at high energies through advanced spectroscopic techniques.

Contribution

It provides new insights into the ionization dynamics of helium nanodroplets, including evidence for Penning-like processes and inelastic collisions, using VMI-PEPICO spectroscopy.

Findings

Detection of Penning-like ionization below autoionization threshold

Observation of inelastic electron collisions at high photon energies

Mass-correlated photoelectron and photoion spectra analysis

Abstract

The ionization dynamics of pure He nanodroplets irradiated by EUV radiation is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence (VMI-PEPICO) spectroscopy. We present photoelectron energy spectra and angular distributions measured in coincidence with the most abundant ions He+, He2+, and He3+. Surprisingly, below the autoionization threshold of He droplets we find indications for multiple excitation and subsequent ionization of the droplets by a Penning-like process. At high photon energies we evidence inelastic collisions of photoelectrons with the surrounding He atoms in the droplets.