Noble-gas hydride molecules with the general formula HNgY (Ng denotes noble-gas atom and Y denotes electronegative fragment) are usually prepared in solid noble gases. In many cases, the matrix-isolated HNgY molecules show a characteristic structure of the H-Ng stretching absorption: A close doublet as the main spectral feature and a weaker satellite at higher energy. This characteristic band structure is studied here for matrix-isolated HXeBr and HKrCl molecules. Based on the experimental and theoretical results, we suggest a model explaining the common features of the band structure of the HNgY molecules in noble-gas matrices. In this model, the main doublet bands are attributed to matrix sites where the splitting is caused by specific interactions of the embedded molecule with noble-gas matrix atoms in certain local morphology. The weaker blueshifted band is probably a fingerprint of hindered rotation (libration) of the embedded molecule in the lattice. This librational band has a mirror counterpart at lower energies appearing at higher matrix temperatures. Our present ab initio calculations for the one-to-one Xe...HXeBr complexes and the simulation of hindered rotation in a matrix support this image.

The solvation of a proton by up to six rare-gas atoms He, Ne, Ar, Kr, and Xe is investigated by B3LYP density functional theory with large basis sets, forming the Ðrst systematic study of all rare gases, He through Xe, on the same high level of theory. The solvation energy for regular two-fold, trigonal planar, tetrahedral and octahedral coordination shows, as known previously, that the protonated rare gas dimer is the most stable conÐguration in every case. Solvation of a point charge by hard polarizable spheres yields the same preference for two-fold coordination. Two rare gas atoms shield the proton efficiently, and additional rare gas atoms may be coordinated in an equatorial plane or along the axis of the central protonated rare gas dimer, with binding energies and bond lengths comparable to those of the corresponding rare gas solids. The inÑuence of additional solvent atoms on the harmonic stretching frequencies is minor and cannot explain the large shift observed in low temperature matrices. Proton di †usion is examined by calculating the transition state for isomerization of species, which yields barrier heights of 8.8, 11.5, 29.7, 32.3, and 35.5 kJ mol~1 for He, Rg 3 HǸ e, Ar, Kr, and Xe, respectively. Geometries, harmonic frequencies, bond dissociation energies and partial charges of mixed protonated rare gas dimers reveal a consistently smooth trend of these properties with size and polarizability of the rare gas atoms. Based on these Ðndings, the assignment of spectral lines attributed to the mixed ArH`Kr, ArH`Xe, and KrH`Xe species is questioned. The stabilization of positive charge centers in solid Xe in the presence of hydrogen atoms is also discussed.

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In spite of the negative electron affinity of Ne atoms, appreciable concentrations of electrons can be trapped in solid neon layers formed by depositing the gas on a cold substrate with concurrent electron irradiation. These are trapped at defect sites, and can be promoted into the conduction band in an annealing experiment. They can then recombine with positive charges producing vacuum ultraviolet "thermoluminescence," but can also be extracted from the solid, and detected as an "exoelectron" current. The thermally stimulated exoelectron emission profiles of the electron current versus temperature reveal two broad features near 7.5 and 10 K. These are shown to correspond to two distributions of electron trapping sites with slightly differing activation energies. For the narrower, higher temperature maximum, an average activation energy of about 23 meV is deduced, in good agreement with predictions based on the theory of electronic defect formation.