Photoionization and imaging spectroscopy of rubidium atoms attached to helium nanodroplets.

Highly excited states of rubidium (Rb) atoms attached to helium (He) nanodroplets are studied by two-photon ionization spectroscopy in combination with electron and ion imaging. We find high yields of RbHe and RbHe(2) exciplexes when exciting to the 4D and 6P bands but not at the 6S band, in accord with a direct formation of exciplexes in binding RbHe pair potentials. Photoion spectra and angular distributions are in good agreement with a pseudodiatomic model for the RbHe(N) complex. Repulsive interactions in the excited states entail fast dissociation followed by ionization of free Rb atoms as well as of RbHe and RbHe(2) exciplexes.

Toward a density functional description of liquid pH2.

A finite-temperature density functional approach to describe the properties of parahydrogen in the liquid-vapor coexistence region is presented. The first proposed functional is zero-range, where the density-gradient term is adjusted so as to reproduce the surface tension of the liquid-vapor interface at low temperature. The second functional is finite-range and, while it is fitted to reproduce bulk pH(2) properties only, it is shown to yield surface properties in good agreement with experiments. These functionals are used to study the surface thickness of the liquid-vapor interface, the wetting transition of parahydrogen on a planar Rb model surface, and homogeneous cavitation in bulk liquid pH(2).

Squeezing a helium nanodroplet with a Rydberg electron.

We have investigated, by means of density functional theory, the structure of a "scolium", that is, an electron circulating around a positively charged 4He nanodroplet, temporarily prevented from neutralization by the helium-electron repulsion. The positive ion core resides in the center of the nanodroplet where, as a consequence of electrostriction, a strong increase in the helium density with respect to its bulk value occurs. The electron enveloping the 4He cluster exerts an additional electrostatic pressure which further increases the local 4He density around the ion core. We argue that under such pressure, sufficiently small 4He nanodroplets may turn solid. The stability of a scolium with respect to electron-ion recombination is investigated.

Evidence concerning drying behavior of Ne near a Cs surface.

Using density functional and Monte Carlo methods, we have studied the properties of Ne adsorbed on a Cs surface, focusing on the region at and near saturated vapor pressure (SVP). In the case of Ne/Rb, the experimental data of Hess, Sabatini, and Chan are consistent with the calculations based on an ab initio fluid-substrate potential, while in the Ne/Cs case there is indication that the potential is approximately 9% too deep. In that case, the calculations yield partial drying behavior consistent with the experimental finding of depressed fluid density near the surface, above SVP. However, we find no evidence of a drying transition, a result consistent with the mean-field calculation of Ebner and Saam.

Dissociation of methane into hydrocarbons at extreme (planetary) pressure and temperature.

Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature. Contrary to the current interpretation of shock-wave experiments, the simulations suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascals. The simulation conditions (100 to 300 gigapascals; 4000 to 5000 kelvin) were chosen to follow the isentrope in the middle ice layers of Neptune and Uranus. Implications on the physics of these planets are discussed.