Alkali-helium snowball complexes formed on helium nanodroplets

TL;DR

This study explores the formation and stability of alkali-helium snowball complexes on helium nanodroplets, revealing species-dependent sizes and shell closures through femtosecond photo-ionization experiments.

Contribution

It provides the first systematic investigation of alkali-helium snowball complexes, highlighting the influence of alkali species and cluster size on snowball formation and stability.

Findings

Snowballs form around alkali ions with species-dependent sizes.

Heavier alkali atoms form larger snowballs, up to at least 41 helium atoms.

Mass spectra show shell closure features consistent with theoretical predictions.

Abstract

We systematically investigate the formation and stability of snowballs formed by femtosecond photo-ionization of small alkali clusters bound to helium nanodroplets. For all studied alkali species Ak=(Na, K, Rb, Cs) we observe the formation of snowballs (AkHe_N)^+ when multiply doping the droplets. Fragmentation of clusters Ak_N upon ionization appears to enhance snowball formation. In the case of Na and Cs we also detect snowballs (Ak_2He_N)^+ formed around Ak dimer ions. While the snowball progression for Na and K is limited to less than 11 helium atoms, the heavier atoms Rb and Cs feature wide distributions at least up to (AkHe_41)^+. Characteristic steps in the mass spectra of Cs-doped helium droplets are found at positions consistent with predictions on the closure of the 1st shell of helium atoms around the Ak^+ ion based on variational Monte Carlo simulations.