Structure and chemistry of atomic clusters from supersonic beams

A tandem time-of-flight (TOF) apparatus was designed to study the structure and chemistry of cold transition metal cluster ions from supersonic beams. By means of a photodissociation laser fluence dependence technique, binding energies of Nb$\sb{\rm x}\sp{+}$ (x = 2 $-$ 20), Co$\sb{\rm x}\sp{+}$ (x = 4 $-$ 20) and etc. were found to generally increase with cluster size. The desorption energies of Nb$\sb{\rm x}$N$\sb2\sp{+}$ (x = 2 $-$ 17) and Nb$\sb{\rm x}$CO$\sp{+}$ (x = 2 $-$ 10) also increase with cluster size with some oscillations similar to the size dependent reactivities of these clusters. Photodetachment studies revealed that electron affinities of copper clusters increase with cluster size with a sharp even/odd alternation. Unlike other noble metals, Ag$\sb{\rm x}\sp{-}$ clusters display two competing processes: photodissociation and photodetachment. Relative reactivities of cluster ions of Nb, Co, Ag, and etc. have been measured using a fast flow cluster reactor, displaying a similar function of cluster size to that of the neutrals. In addition, preliminary photoelectron experiments have been performed on Cu$\sb{\rm x}\sp{-}$ and Nb$\sb{\rm x}\sp{-}$. A magnetic Time-of-flight ultraviolet photoelectron spectrometer (MTOFUPS) has been developed to study electronic structures of cold metal and semiconductor cluster anions prepared in supersonic beams. Application of this spectrometer to carbon clusters with a F$\sb2$ laser (7.9 eV) allowed their electron affinities and UPS patterns to be measured,demonstrating a remarkable structural evolution of these clusters: Chains (C$\sb2\sp{-}$-C$\sb9\sp{-}$) - Rings (C$\sb{10}\sp{-}$-C$\sb{29}\sp{-}$) - Cages (C$\sb{38}\sp{-}$-C$\sb{84}\sp{-}$). In particular, the UPS of C$\sb{60}\sp{-}$ is in excellent agreement with the CNDO/S calculation, providing a striking spectral evidence for the highly symmetric icosahedral soccer ball structure--Buckminsterfullerene. For comparison, the UPS of Si$\sb{\rm x}\sp{-}$ and Ge$\sb{\rm x}\sp{-}$ are presented. Unlike carbon clusters which prefer structures of low dimensionality, these clusters tend to take a network structure. In conjunction with calculations, some tentative structural assignments were possible