Francesco Ancilotto

The binding of alkali atoms to the surfaces of liquid helium and hydrogen

Alkali atoms have been shown previously to have only unstable binding states inside liquid4He. We calculate the equilibrium configurations and binding energies of single alkali atoms near the liquid-vapor interface of4He and3He. A simple interface model is used to predict the surface deformation due to the presence of the atoms. A more realistic density functional model yields somewhat higher energies in the case of4He. For all alkali atoms, we find the surface binding energies to be around 10 to 20 K. A similar analysis with atom-H2 interactions finds that alkali atoms tend to submerge into liquid H2, with the exception of Li.

Physics of solvation

Calculations are presented of the energetics of an impurity (atom or ion) interacting with a fluid. Two possible configurations are considered: a surface state and a solvated state. For two distinct model problems which we consider (any classical fluid and superfluid helium) we find a common behaviour: the value of a dimensionless parameter λ determines the relative stability of the surface and solvated states. For λ greater (less) than 1.9, the sovated (surface) state is favored. A more realistic estimate for a classical fluid is λ ∼ 1. Predictions are made of a universal solvation behaviour derived from the law of corresponding states. Results are presented for the solvated fraction as a function of cluster radius and temperature. Quantum corrections and the kinetics of solvation are discussed briefly.

Ab initio potential energy surfaces of He-CO2 and Ne-CO2 van der Waals complexes

We have calculated ab initio the ground-state potential energy surfaces of Ne-CO2 and He-CO2 van der Waals complexes by using the fourth-order Moller-Plesset perturbation theory with a large basis set containing bond functions. The rigid molecule approximation has been used for CO2. Both complexes have the lowest energy at the T-shaped configuration. In addition, a shallow minimum is found for the linear configuration, with a small barrier connecting the two minima on the potential energy surface, whose height is larger for the lighter noble-gas atom. The implications of these potentials for the wetting behavior of adsorbed films are discussed.

Adsorption of benzene on Si(100) from first principles

Adsorption of benzene on the Si(100) surface is studied from first principles. We find that the most stable configuration is a tetra-

Physisorption, Diffusion, and Chemisorption Pathways of H2 Molecule on Graphene and on (2,2) Carbon Nanotube by First Principles Calculations.

\sigma

Freezing of 4He and its liquid-solid interface from density functional theory

-bonded structure characterized by one C-C double bond and four C-Si bonds. A similar structure, obtained by rotating the benzene molecule by 90 degrees, lies slightly higher in energy. However, rather narrow wells on the potential energy surface characterize these adsorption configurations. A benzene molecule impinging on the Si surface is most likely to be adsorbed in one of three different di-

Wetting transitions of 4 He on alkali-metal surfaces from density-functional calculations

\sigma

Van der Waals interactions at surfaces by density functional theory using Wannier functions

-bonded, metastable structures, characterized by two C-Si bonds, and eventually converts into the lowest-energy configurations. These results are consistent with recent experiments.

Spectroscopy on Rydberg states of sodium atoms on the surface of helium nanodroplets

We investigate the interaction of the H2 molecule with a graphene layer and with a small-radius carbon nanotube using ab initio density functional methods. H2 can interact with carbon materials like graphene, graphite, and nanotubes either through physisorption or chemisorption. The physisorption mechanism involves the binding of the hydrogen molecule on the material as a result of weak van der Waals forces, while the chemisorption mechanism involves the dissociation of the hydrogen molecule and the ensuing reaction of both hydrogen atoms with the unsatured C-C bonds to form C-H bonds. In our calculations, we take into account van der Waals interactions using a recently developed method based on the concept of maximally localized Wannier functions. We explore several adsorption sites and orientations of the hydrogen molecule relative to the carbon surface and compute the associated binding energies and adsorption potentials. The most stable physisorbed state on graphene is found to be the hollow site in the center of a carbon hexagon, with a binding energy of -48 meV, in good agreement with experimental results. The analysis of diffusion pathways between different physisorbed states on graphene shows that molecular hydrogen can easily diffuse at room temperature from one configuration to another, which are separated by energy barriers as small as 10 meV. We also compute the potential energy surfaces for the dissociative chemisorption of H2 on highly symmetric sites of graphene, the lowest activation barrier found being 2.67 eV. Much weaker adsorption characterizes instead the physisorption interaction of the H2 molecule with the small radius (2,2) CNT. The barriers for H2 dissociation on the nanotube external surface are significantly lowered with respect to the graphene case, showing the remarkable effect of the substrate curvature in promoting hydrogen dissociation.

The Structure and Energetics of 3He and 4He Nanodroplets Doped with Alkaline Earth Atoms

We show that, at high densities, fully variational solutions of solid-like type can be obtained from a density functional formalism originally designed for liquid 4He. Motivated by this finding, we propose an extension of the method that accurately describes the solid phase and the freezing transition of liquid 4He at zero temperature. The density profile of the interface between liquid and the (0001) surface of the 4He crystal is also investigated, and its surface energy evaluated. The interfacial tension is found to be in semiquantitative agreement with experiments and with other microscopic calculations. This opens the possibility to use unbiased DF methods to study highly non-homogeneous systems, like 4He interacting with strongly attractive impurities/substrates, or the nucleation of the solid phase in the metastable liquid.