Penning ionization of acene molecules by He nanodroplets

TL;DR

This study investigates how acene molecules and C60 fullerenes embedded in helium nanodroplets are ionized via Penning ionization using EUV radiation, revealing broad electron spectra altered by electron-He scattering.

Contribution

It provides the first detailed analysis of Penning ionization of embedded molecules in helium nanodroplets and demonstrates the significant impact of electron-He scattering on electron spectra.

Findings

Electron spectra are broad and structureless.

Electron-He scattering significantly alters spectra.

Simulations suggest larger-than-expected helium droplets.

Abstract

Acene molecules (anthracene, tetracene, pentacene) and fullerene (C$_{60}$) are embedded in He nanodroplets (He$_N$) and probed by EUV synchrotron radiation. When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction $\mathrm{He}_N^*+\mathrm{M}\rightarrow\mathrm{He}_N + \mathrm{M}^+ + e^-$. However, the Penning electron spectra are broad and structureless -- showing no resemblance neither with those measured by binary Penning collisions, nor with those measured for dopants bound to the He droplet surface. The similarity of all four spectra indicates that electron spectra of embedded species are substantially altered by electron-He scattering. Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra, but require the assumption of unexpectedly large He droplets.