E. Coccia

Bosonic helium droplets with cationic impurities : Onset of electrostriction and snowball effects from quantum calculations

Variational Monte Carlo and diffusion Monte Carlo calculations have been carried out for cations such as Li(+), Na(+), and K(+) as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms. The interaction has been modeled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab initio calculations over a broad range of interatomic distances. This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are discussed here in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the cluster features.

Nanoscopic phase changes in doped 4He droplets

A novel analysis of final walkers distributions from quantum Diffusion Monte Carlo calculations is presented for the smaller 4He clusters containing Li+, Na+ and K+. The results are shown to provide a clear pictorial view of electrostriction effects leading to the formation of solid-like snowballs that can be surrounded by a liquid-like environment of additional solvent atoms. Transitions from one nanophase to the other are clearly shown by the stochastic results as a function of cluster size and of the chemical nature of the specific dopant ion.

Binding He atoms to hydrogen moieties: quantum features from ultraweak interactions

The possible existence of bound states in small helium clusters containing atomic and molecular hydrogen has been investigated by means of three completely independent sets of calculations. The first set employs the Jacobi–Davidson filtering procedure recently implemented in the Distributed Gaussian Functions (DGF) approach, the second one is based on the solution of the Faddeev equation by means of the hyperspherical adiabatic expansion method and the third follows the Quantum Monte Carlo approach. The partners within each complex are taken to interact via two-body potentials chosen among the most accurate existing in the literature. All the present quantum treatments show that no bound states are found for H(4He)2 and for D(4He)2 while only one very diffuse bound state is present in T(4He)2, where H, D and T are the three isotopes of hydrogen. The substitution of the atomic dopant with its molecular counterpart, H2, also gives rise to only one bound state whose spatial attributes are reported and analysed, underlining again the ultraweak nature of the interaction forces and the need to implement highly specific tools for the investigation of the ensuing bound states.

Quantum structuring of4He atoms around ionic dopants: Energetics of Li+, Na+ and K+ from stochastic calculations

Quantum Variational and Diffusion Monte Carlo (VMC, DMC) calculations are carried out using accurate, ab-initio interaction potentials for alkali metal ions doping small 4He clusters. The results for Li+, Na+ and K+ with clusters up to n=16 reveal an interesting interplay between ionic forces and He-He interactions when driving the spatial delocalization of the solvating helium adatoms which form the initial snowball structures within the larger droplets.