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An intense source of positive, negative, and neutral carbon clusters using excimer laser ablation of polyimide is described. The detection probability for large masses using a standard channel plate configuration is shown to increase exponentially with ion velocity. The large clusters are formed via aggregation of atomic carbon or small carbon molecules. The small ‘‘clusters’’ may be molecular fragments from the polymer which rearrange on leaving the surface to form stable structures.

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The solvent viscosity and polarity dependence of the magnetic field effect in polymethylenelinked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene-(CH2)n-N,N-dimethylaniline, has been studied. A stochastic Liouville equation is used, in which the dynamics of the polymethylene chain, the spin Hamiltonian as a function of the varying radical distance (exchange interaction), and a distance-dependent back electron transfer rate are incorporated. The results are compared with predictions made on the basis of the (static) subensemble approximation.

Results of scanning-tunneling-microscopy (STM) and molecular-dynamics (MD) annealing studies based on quantum-mechanically derived interatomic forces using a semiempirical density-functional approach are combined for analyzing diamond surface structures. Experimentally obtained STM images of diamond (100) and (111)faces on polycrystalline films reveal (1 X 1),(&3 X &3) R 30', and possible (2 X 1) structures. The (100) faces show stable (2 X 1) reconstruction with dimer formation. Surface structures with and without adsorbed hydrogen are determined and their stability is obtained by MD simulated annealing techniques. The bulklike and (&3X&3) 830 structures, as they are observed on grown (111) facets, are attributed to the two diff'erent single atomic (111)layers, which support growth mechanisms, in which the two alternating single atomic layers grow in turn and not simultaneously. The equilibrium surface modifications which have been realized are electronically characterized by investigating the local electronic density of states at selected surface atoms. This information is compared and related to the features seen in the STM images.