Picosecond solvation dynamics of alkali cations in superfluid 4He nanodroplets

The dynamics following the photoionization of neutral Rb and Cs atoms residing in a dimple at the surface of a superfluid 4He1000 nanodroplet has been investigated within time-dependent density functional theory, complementing a previous study on Ba. The calculations reveal that structured high density helium solvation layers form around both the Rb+ and Cs+ cation on a picosecond time scale, forming so-called snowballs. In contrast to the Rb+ ion, Cs+ is not solvated by the 4He1000 droplet but rather desorbs from it as a Cs+Hen snowball. This outcome is partially related to the large size of Cs+ cation in relation to the helium droplet as is revealed by calculations performed using a planar helium surface. The large droplet deformations induced by the solvation of the Rb+ cation is found to lead to efficient nucleation of quantized vortex loops or rings.