F. Marinetti

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.

IONIC OH AS DOPANT OF HELIUM DROPLETS: AB INITIO POTENTIAL ENERGY SURFACES FOR OH+(3Σ-)-4He, OH-(1Σ-)-4He, AND STABLE STRUCTURES OF THEIR SMALLER CLUSTERS

Accurate, post-Hartree–Fock calculations have been carried out for a broad range of geometries in order to obtain an extended description of the orientational anisotropy and radial strength of the interaction of OH+ and OH- ions with the unit species of a weak quantum solvent as that provided by 4He droplets. The potential energy surfaces (PES) are obtained for the smallest complex system with one 4He atom, and the possible role and importance of many-body effects is also investigated. The energetics and shapes of the OH+-4He and OH--4He internal bound states are obtained and analyzed in detail.

Energetics and structures of charged helium clusters: comparing stabilities of dimer and trimer cationic cores.

We present accurate ab initio calculations of the most stable structures of He(n)(+) clusters in order to determine the more likely ionic core arrangements existing after reaching structural equilibrium of the clusters. Two potential energy surfaces are presented: one for the He(2)(+) and the other with the He(3)(+) linear ion, both interacting with one He atom. The two computed potentials are in turn employed within a classical structure optimization where the overall interaction forces are obtained within the sum-of-potentials approximation described in the main text. Because of the presence of many-body effects within the ionic core, we find that the arrangements with He(3)(+) as a core turn out to be energetically preferred, leading to the formation of He(3)(+)(He)(n-3) stable aggregates. Nanoscopic considerations about the relative stability of clusters with the two different cores are shown to give us new information on the dynamical processes observed in the impact ionization experiments of pure helium clusters and the importance of pre-equilibrium evaporation of the ionic dimers in the ionized clusters.

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.