Penning ionization of doped helium nanodroplets following EUV excitation

TL;DR

This study investigates how helium nanodroplets, doped with different atoms, undergo Penning ionization after EUV excitation, revealing rapid excitation migration to the surface especially for alkali metals.

Contribution

It demonstrates that alkali metals are efficiently Penning ionized upon helium droplet excitation, highlighting surface migration effects not previously characterized.

Findings

Alkali metals are efficiently Penning ionized after excitation.

Rapid migration of excitation to droplet surface is observed.

Different ionization behaviors depending on dopant location.

Abstract

Helium nanodroplets are widely used as a cold, weakly interacting matrix for spectroscopy of embedded species. In this work we excite or ionize doped He droplets using synchrotron radiation and study the effect onto the dopant atoms depending on their location inside the droplets (rare gases) or outside at the droplet surface (alkali metals). Using photoelectron-photoion coincidence imaging spectroscopy at variable photon energies (20-25 eV), we compare the rates of charge-transfer to Penning ionization of the dopants in the two cases. The surprising finding is that alkali metals, in contrast to the rare gases, are efficiently Penning ionized upon excitation of the (n=2)-bands of the host droplets. This indicates rapid migration of the excitation to the droplet surface, followed by relaxation, and eventually energy transfer to the alkali dopants.