A b initio calculations have been performed to study the structures and energies of intermediate-sized silicon clusters (Sin, n=7–10). All geometries have been optimized at the Hartree–Fock (HF) level of theory with the polarized 6-31G* basis set. The harmonic vibrational frequencies have been evaluated at the HF/6-31G* level of theory. Electron correlation effects have been included by means of fourth order Mo/ller–Plesset perturbation theory. The most stable structure for Si7 is a pentagonal bipyramid and the lowest energy calculated structures for Si8–Si10 correspond to capped octahedral or prismatic geometrical arrangements. The evolution of the cluster geometries with increasing size is discussed. Clusters containing four, six, seven, and ten atoms have been identified as ‘‘magic numbers’’ for small silicon clusters, both theoretically and experimentally. The hybridization and bonding in small silicon clusters is discussed. Our results are used to interpret the recent photoelectron spectra of negative silicon cluster ions.

The spectroscopy and photodissociation dynamics of the vinoxy ͑CH 2 CHO͒ radical B(2 AЉ) ←X(2 AЉ) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: ͑1͒ CH 3 ϩCO, and ͑2͒ HϩCH 2 CO, with a branching ratio of Ϸ1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH 3 ϩCO, and a considerably smaller barrier for HϩCH 2 CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the Ã(2 AЈ) or C(2 AЈ) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, which satisfactorily reproduces the translational energy distributions.

The structure and bonding of aluminum oxide clusters, AlxOy (x=1–2, y=1–5), are studied with anion photoelectron spectroscopy (PES) and are compared with preliminary ab initio calculations. The spectra were obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The 6.42 eV spectrum for AlO− reveals the X 2Σ+ ground state and two excited states of AlO. The 6.42 eV spectrum for AlO2− also shows three states for AlO2: X 2Πg ground state and the A 2Πu and BΣg+2 excited states. The spectra for Al2Oy− clusters show vibrationally resolved ground states which come from Al sp-type orbitals and also high binding energy excited states, which are mainly of oxygen 2p character. Al2O2, which has a D2h rhombus structure, has an electron affinity (EA) of 1.88 eV and its singlet–triplet excitation energy is measured to be 0.49 eV. Much higher EAs are measured for the larger Al2Oy clusters. The PES spectra of Al2O3−, Al2O4−, and Al2O5− show very similar electronic and vibrational structure. Furthermore, the ground state vibrational frequencies of these three molecules are also similar. These observations lead us to suggest that these molecules all have a rhombuslike structure, similar to Al2O2, with the oxygen atoms sequentially attaching to the terminal aluminum atoms. The spectra are consistent with an ionic bonding view of these clusters and the vibrational frequencies are in good agreement with the theoretical results. Significant information about the structure and bonding of these small aluminum oxide clusters is obtained and discussed.

The geometric structures and relative stabilities of small sulfur clusters (S2–S12) are explored by means of ab initio quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. Open forms are more stable for the small clusters up to S4 whereas ring structures are favored for the larger clusters. The stereochemical aspects of cyclo-S5 and cycle-S6 exhibit features very similar to that of the valence-isoelectronic hydrocarbons cyclopentane and cyclohexane. The calculated geometries of S6–S8 and S10–S12 are all in excellent agreement with the experimentally determined structures. Vibrational frequencies have been computed and are compared with experimental data. The relative stabilities of the different sulfur rings are compared to each other.

Accurate ab initio calculations have been performed to investigate the structures and energies of the negative ions of Si2–Si10. The effects of polarization functions, diffuse functions, and electron correlation have been included in these calculations. In most cases, there is a good correspondence between the ground state structures of the negative ions and those of the corresponding neutral species. Adiabatic electron affinities are computed and compared with recent experimental measurements. Si3, Si5, Si8, and Si9 are found to have electron affinities which are larger than their neighbors. This result is interpreted using our previous calculations on the low-lying states of the corresponding neutral species.