A new method to study metastable fragmentation of clusters using a reflectron time-of-flight mass spectrometer Rev.We describe a new reflectron time-of-flight mass spectrometer configuration for laser photodissociation of mass-selected ions and the initial performance characteristics observed for this instrument. Ions are produced by laser photoionization within the acceleration region of the instrument or by laser vaporization in an external pulsed-nozzle cluster ion source. Mass selection is accomplished with pulsed deflection plates at the end of an initial drift section. Laser photodissociation of selected ions takes place at the turning point in the ion trajectory in the reflectron. The transit time through a second drift section defines the fragment ion masses. Optimized operating conditions and the role of mass discrimination in this instrument are-discussed.

Tin and lead clusters are produced by laser vaporization in a pulsed nozzle source and studied with laser photoionization mass spectroscopy. ‘‘Magic numbers’’ are observed in both cluster size distributions under a variety of laser wavelength and power conditions which can be understood in terms of ionization thresholds, relative ionization cross sections, and multiphoton-induced fragmentation. After investigation of the photoionization dynamics, relative abundances of different sized clusters are estimated. Abundance patterns of tin and lead clusters are compared to those reported previously for other group IV elements (C, Si, Ge) to investigate the role of periodicity in cluster growth and bonding properties. Especially abundant 10-atom cluster species are observed for both tin and lead, as has been observed previously for both silicon and germanium. Other features not observed for silicon and germanium, such as abundance patterns characteristic of atom closepacking geometries, are observed to a limited degree for tin clusters and are more prominent in lead clusters.

Silver clusters containing up to 40-50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd: YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N= 3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag 3, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via the E state at 249 nm establishes the dissociation energy of Ag] to be < 2.1 eV.

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Binary clusters composed of Group IV and Group V metals (e.g., Sn/Bi) are prepared by laser vaporization of corresponding alloys in a pulsed nozzle molecular beam source. Relative abundances of these clusters are studied with laser photoionization mass spectroscopy at 193 and 157 nm. Mass spectral abundance patterns for all systems studied (Sn/Bi, Pb/Sb, Sn/As) exhibit highly nonstatistical combinations of component elements. Preferential stoichiometries follow common patterns throughout these cluster systems, which can be related to the bonding and stability of condensed phase cluster ions of these same metals. These patterns are predicted by a simple valence electron counting model.