Modeling protonated helium clusters across the size-resolved to the droplet regimes: Structure and low-energy collision dynamics

Abstract

A many-body polarizable potential is developed to model HenH+ clusters in their electronic ground state across a broad size range, trained to reproduce quantum chemical CCSD(T) calculations with a basis set of quadruple zeta quality. Putative global minima of small clusters containing up to n = 50 helium atoms exhibit a rigid HeH+He trimer core, around which additional atoms arrange into icosahedral-like motifs. The binding energy inferred from path-integral molecular dynamics simulations displays successive changes in its slope at n > 2, n > 6, and n > 13, and vibrational delocalization, as measured from the inherent structure entropy, is found to be significant already at small sizes. In large clusters, the trimer core is preserved and a much softer second solvation shell of ∼20 atoms is identified. The collision dynamics of a proton impinging on a 1000-atom helium droplet was simulated using ring-polymer molecular dynamics for various conditions and moderate energies in the eV range. Soft capture is found to take place for collision energies of ∼1 eV and below, while 10 eV collisions have the proton piercing through the droplet before being possibly captured back after losing sufficient energy. The resulting sticking cross sections generally follow the predictions of a simple Langevin capture model, extended to account for the finite radius of the helium target, except in the piercing regime. In the low collision energy limit, the sinking of an initial hydrogen atom after sudden ionization is found to be very slow once the trimer core is formed within 2 ps.