[en] The saturation levels of small nickel clusters Ni_n (n=2-20) with CO are determined. Modeling of the CO-covered clusters shows that in almost all cases saturation is governed by the geometrical sizes of the nickel clusters and the van der Waals size of the CO molecules. While electron counting rules, along with an assumed cluster structure, can predict saturation levels, the predictions generally overestimate the levels based on geometry. In the case of the smallest clusters (n≤13), CO adsorption appears to cause changes in structure to more open ones that result in increased adsorption, giving coverages close to the electron counting rule predictions for the bare cluster geometries. Larger clusters, with internal metal atoms, seem to be more resistant to structural changes. Comparisons with earlier studies of nickel cluster positive and negative ions are made and the significance of a short reaction time scale on the nature of the reaction products is discussed. (c) 2000 American Institute of Physics

[en] An energy transfer probability distribution function, P(E,E), for the collisional relaxation of a highly vibrationally excited donor molecule (C₆F₆, pyrazine) is constructed for the first time from experimental data on the bath (CO₂) energy gain. A prescription for mapping bath quantum state resolved data onto P(E,E) is described in detail. Analysis of earlier experimental data allows a calculation of the high ΔE=E-E region (-7000 cm^-1< E-E<-1500 cm^-1) of P(E,E) for the above systems. Comparison of the P(E,E) functions reveals that C₆F₆ is a more efficient donor molecule than pyrazine, in agreement with previous experiments and trajectory calculations. In addition, resonance like structures in the P(E,E) functions arising from long range force mediated, V endash V excitation of the carbon dioxide ν₃ mode are discussed. These results indicate that accurate P(E,E) functions can be determined from experiments involving probes of the bath energy gain. This technique can be expected to provide stringent tests of current energy transfer theory and can, in principle, be used in conjunction with measurements of thermal kinetics to obtain energy dependent unimolecular rate constants, k_E. copyright 1997 American Institute of Physics

[en] Infrared spectroscopy has been utilized to examine the structure and vibrational decay dynamics of CH₄-OH complexes that have been stabilized in the entrance channel to the CH₄+OH hydrogen abstraction reaction. Rotationally resolved infrared spectra of the CH₄-OH complexes have been obtained in the OH fundamental and overtone regions using an IR-UV (infrared-ultraviolet) double-resonance technique. Pure OH stretching bands have been identified at 3563.45(5) and 6961.98(4) cm-1 (origins), along with combination bands involving the simultaneous excitation of OH stretching and intermolecular bending motions. The infrared spectra exhibit extensive homogeneous broadening arising from the rapid decay of vibrationally activated CH₄-OH complexes due to vibrational relaxation and/or reaction. Lifetimes of 38(5) and 25(3) ps for CH₄-OH prepared with one and two quanta of OH excitation, respectively, have been extracted from the infrared spectra. The nascent distribution of the OH products from vibrational predissociation has been evaluated by ultraviolet probe laser-induced fluorescence measurements. The dominant inelastic decay channel involves the transfer of one quantum of OH stretch to the pentad of CH₄ vibrational states with energies near 3000 cm-1. The experimental findings are compared with full collision studies of vibrationally excited OH with CH₄. In addition, ab initio electronic structure calculations have been carried out to elucidate the minimum energy configuration of the CH₄-OH complex. The calculations predict a C_3v geometry with the hydrogen of OH pointing toward one of four equivalent faces of the CH₄ tetrahedron, consistent with the analysis of the experimental infrared spectra. (c) 2000 American Institute of Physics