Surface location of alkaline-earth atom impurities on helium nanodroplets

TL;DR

This study investigates the surface localization of alkaline-earth atoms like Mg, Ca, Sr, and Ba in helium nanodroplets by analyzing ionization yields, revealing they predominantly reside near the droplet surface.

Contribution

The paper provides experimental evidence on the surface location of alkaline-earth atoms in helium nanodroplets using electron energy dependence of ionization yields.

Findings

Mg, Ca, Sr, and Ba atoms are located at or near the droplet surface.

Surface and interior solvation states show distinct ionization yield profiles.

Penning ionization plays a significant role for surface-residing atoms.

Abstract

There has been notable uncertainty regarding the degree of solvation of alkaline-earth atoms, especially Mg, in free He-4 nanodroplets. We have measured the electron energy dependence of the ionization yield of picked-up atoms. There is a qualitative shape difference between the yield curves of species solvated in the middle of the droplet and species located in the surface region; this difference arises from the enhanced role played by the Penning ionization process in the latter case. The measurements demonstrate that Mg, Ca, Sr and Ba all reside at or near the droplet surface.