Experimental and theoretical studies of hydrogen -terminated silicon and bare carbon clusters

The physical properties of small clusters typically differ from those of the free atoms or molecules that make up the cluster, as well as from the properties of the respective bulk solids. For example, gas-phase clusters of silicon and carbon have been shown to adopt geometries that are specific to the cluster size, and that are different from the geometry of any fragment of an equal size excised out of the bulk solid.;The synthesis of SinHx+ (n = 10–30) clusters has been performed using a novel gas-phase photolysis source. The produced clusters have been studied as a function of temperature using mass spectrometry. For SinHx+ (n = 10–22) clusters, near stoichiometric compositions (x ≅ n) persists over a distinct temperature range. The stoichiometric clusters probably have Si-H units arranged in cage-like geometries. Free energy calculations using density functional theory normalized energies for a variety of SinHx compositions are in qualitative agreement with the experimental results. Comparisons between experiment and theory require consideration of free energies of the Si nHx species, not simply the calculation of the electronic energies at 0 K. For clusters with n ≥ 23, hydrogen-depleted structures with formulas between SinHx+ and Si nH(n−5)+ are found at temperatures greater than 673 K. These clusters may have structures consisting of hydrogen-depleted Si-H cages with one or more internal silicon atoms.;The surface plasmon polariton-enhanced Raman spectra for size-selected C14, C16, C18, and C20 clusters in nitrogen matrices have been measured. Spectra recorded at six laser excitation frequencies show strong resonance enhancement as evidenced by large variations in peak intensity for closely spaced excitation wavelengths. Calculated vibrational frequencies for the ring and linear chain isomers of the clusters are presented. The observed spectra appear to most closely agree with those calculated for linear chains, which are probably formed in the ion neutralization step. In addition, C14 and C18 clusters display strong fluorescence and vibronic structure. The origins of the observed fluorescence have been investigated using the ZINDO/S, CIS and TD-B3LYP methods.