Rubidium atoms have been deposited on helium nanodroplets and optical excitation and emission spectra of the 5 2P–5 2S transition have been measured and interpreted. After laser excitation of the 5 2P3/2 state, fluorescence from a Rb*He exciplex is observed while pumping the D1 (5 2P1/2–5 2S) line yields only emission from free rubidium atoms. This observation is in agreement with the predictions from a recent model by Reho et al. [J. Chem. Phys. 113, 9694 (2000)] about the extraction of an alkali–helium exciplex from the doped helium nanodroplet surface. A high barrier along the Hen–He–Rb* axis of the 1 2Π1/2 potential prevents desorption of Rb*He within the excited state lifetime, whereas the 1 2Π3/2 potential permits the exciplex extraction. The excitation spectrum, on the other hand, reflects the structure of the excited states 1 2Π1/2, 1 2Π3/2, and 2 2Σ1/2 of the HenRb complex whose potential surfaces will depend strongly on the alkali–He interaction dynamics near the droplet surface. For a heavy surface dopant like Rb or Cs the droplet surface will be strongly distorted upon vibrational excitation of the dopant. Some of the consequences for the potentials are discussed for the example of the 1 2Σ1/2 ground state.

Due to its large surface area and strongly attractive potential, a bundle of carbon nanotubes is an ideal substrate material for gas storage. In addition, adsorption in nanotubes can be exploited in order to separate the components of a mixture. In this paper, we investigate the preferential adsorption of D 2 versus H 2 (isotope selectivity) and of ortho versus para (spin selectivity) molecules confined in the onedimensional grooves and interstitial channels of carbon nanotube bundles. We perform selectivity calculations in the low coverage regime, neglecting interactions between adsorbate molecules. We find substantial spin selectivity for a range of temperatures up to 100 K, and even greater isotope selectivity for an extended range of temperatures, up to 300 K. This isotope selectivity is consistent with recent experimental data, which exhibit a large difference between the isosteric heats of D 2 and H 2 adsorbed in these bundles.

We explore the thermodynamic behavior of gases adsorbed within a nanopore. The theoretical description employs a simple lattice gas model, with two species of site, expected to describe various regimes of adsorption and condensation behavior. The model includes four hypothetical phases: a cylindrical shell phase (S), in which the sites close to the cylindrical wall are occupied, an axial phase (A), in which sites along the cylinder's axis are occupied, a full phase (F), in which all sites are occupied, and an empty phase (E). We obtain exact results at T=0 for the phase behavior, which is a function of the interactions present in any specific problem. We obtain the corresponding results at finite T from mean field theory. Finally, we examine the model's predicted phase behavior of some real gases adsorbed in nanopores.

An interaction potential energy between an adsorbate (Xe and Ar) and the 10-fold Al-Ni-Co quasicrystal is computed by summing over all adsorbate-substrate interatomic interactions. The quasicrystal atoms' coordinates are obtained from LEED experiments and the Lennard-Jones parameters of Xe-Al, Xe-Ni and Xe-Co are found using semiempirical combining rules. The resulting potential energy function of position is highly corrugated. Monolayer adsorption of Xe and Ar on the quasicrystal surface is investigated in two cases: 1) in the limit of low coverage (Henry's law regime), and 2) at somewhat larger coverage, when interactions between adatoms are considered through the second virial coefficient, C AAS . A comparison with adsorption on a flat surface indicates that the corrugation enhances the effect of the Xe-Xe (Ar-Ar) interactions. The theoretical results for the low coverage adsorption regime are compared to experimental (LEED isobar) data.

We present Monte Carlo simulation results for strongly coupled dipolar fluids, such as ferrofluids, confined to a narrow slit pore accommodating only a few layers of particles. Our results show that the ferromagnetic ordering observed in dense bulk systems and in thick fluid films persists down to nanoscopic wall separations where the system consists of only 3 monolayers. The ferromagnetic transition density in these systems approaches experimentally accessible values. For even smaller wall separations, we observe stripelike defects and finally the breakdown of orientational ordering for systems close to the two-dimensional limit. Our results for the liquid phase are supported by simulations starting from quasicrystalline soft-sphere configurations.