We report on the successful ‘‘soft landing’’ of size selected silver dimers and trimers in solid krypton matrices. Silver cluster cations, produced by sputtering, were mass selected in a quadrupole mass filter and then codeposited with krypton on a cooled sapphire or CaF2 window in the presence of low energy electrons. Neutralized cluster samples were interrogated in situ by excitation and fluorescence spectroscopy. Deposition of slow (≤20 eV) silver dimer cations gave rise to strong excitation bands (centered at λ=275 and 390 nm) from Ag2 plus the characteristic triplet signal of the atom. The spectra imply that fewer than 25% of the dimers were fragmented during the neutralization and deposition steps. In similar experiments with Ag3 we were able to assign trimer absorption bands at 331, 364, 402, 421, 458, and 514 nm and identify characteristic emission features at 381, 560, and 626 nm.

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transition for pyrene on silica gel may also reflect the importance of an asymmetric interaction on the molecule, part of which is in an environment approaching the vapor or inert solvent. This aspect of the photophysics of adsorbed species invites theoretical attention. ConclusionThe picture that emerges from these results can be summarized as follows. The distribution of pyrene molecules on the surfaces in question is not random: there appear to be preferred sites. The result is an inhomogeneous distribution which yields a multicomponent decay. Strong evidence exists for the formation of a very weakly bound ground-state bimolecular association product, particularly on silica gel, and the data suggest associating this phenomenon with particular sites at which an interaction stabilizes this bimolecular association. The data also indicate that this phenomenon may be related to some peculiarity of the hydrogen-bonding interaction with the x system of the aromatic hydrocarbon. It is suggested that the static and highly asymmetric interaction may cause pyrene to behave in a manner different from that in hydroxylic solvents where the interactions are rapidly averaged and, on the time scale of the lifetime of pyrene, the interactions are symmetric over the whole of the x system.If the silanol groups are blocked by a long-chain alcohol and polyalcohol, the pyrene molecules appear forced to be adsorbed in areas of weaker interactions where the tendency to form the bimolecular ground-state arrangement is diminished. The mobility during the lifetime also appears to be greatly enhanced, and dynamic excimer formation is then possible.

The relative stabilities of alkali-like metal clusters, Mn and M+n with 2≤n≤9, are calculated within the framework of the simple Hückel model. With the aid of graph theory, the binding energies for all possible Hückel structures are determined. With the exception of M+5 and M+6 , the Hückel model gives minimum energy structures which are the same as those predicted by recent local-spin-density and configuration interaction calculations. Since the Hückel method is independent of the mechanical details of the bonding, a close connection is inferred between a cluster’s stability and its topology. In the paper following this one, the Hückel results are extended to include absolute atomization energies and ionization potentials. In addition, it is shown that cluster energies may be quantitatively extrapolated to the bulk phase.

The absorption and Raman spectra of hafnium dimers in an argon matrix have been measured. Four weak dimer absorption bands were found between 300 and 700 nm. Resonance Raman spectra (obtained by exciting into a band centered at 620 nm) give w:' = 176.2 (26) cm-l with w a x , < 1 cm-l. Our results represent the first experimental or theoretical study of the dihafnium molecule.