Alkali-helium snowball complexes formed on helium nanodroplets
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 (AkHeN)+ when multiply doping the droplets. Fragmentation of clusters AkN upon ionization appears to enhance snowball formation. In the case of Na and Cs we also detect snowballs (Ak2HeN)+ 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 (AkHe41)+. 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.