Collision of cesium atoms on helium nanodroplets: Unraveling mechanisms for surface capture at experimental velocities

The collision of cesium atoms on the surface of helium nanodroplets (HNDs) containing 1000 atoms is described by the ZPAD-mPL approach, a zero-point averaged dynamics (ZPAD) method based on a He-He pseudopotential adjusted to better reproduce the total energy of He1000. Four types of collisional patterns were identified depending on the initial projectile speed v0 and impact parameter b. At the lowest speeds (v0 ≲ 250 m s-1), Cs atoms are softly captured by the HND surface, while at the highest ones (v0 ≳ 500-600 m s-1), Cs atoms can travel through the droplet and move away. In between these two extreme cases, Cs atoms can be temporarily submerged in the HND before being expelled to the surface if b = 0 or cross the HND before being captured on its surface. The possibility for Cs capture at experimental velocities and droplet piercings at the highest ones contrasts with time-dependent density functional theory calculations, which predict Cs capture for velocities lower than 75 m s-1, and ring-polymer molecular dynamics (RPMD) or former ZPAD-like methods, which predict soft Cs capture up to 500 m s-1. ZPAD-mPL results are attributed to the liquid but non-superfluid nature of the droplet, which favors energy exchanges with the helium environment, and to low He-He binding energy and HND surface tension, which can stimulate helium ejections, especially at high projectile speeds. Despite the use of a pseudopotential to model He-He interactions, the heliophobicity of Cs atoms is maintained as demonstrated by their ability to remain localized on the HND surface or to be expelled to the HND surface after transient submersion in helium.