Acetaldehyde has been studied by the technique of low-energy variable-angle electron energyloss spectroscopy. With this method the low-lying spin-forbidden transitions have been located via the behavior of the relative differential cross sections, providing the first identification by this technique of such states in acetaldehyde. High-lying states were also investigated and some assignments of dipole symmetry-forbidden/quadrupole symmetry-allowed excitations were made on the basis of characteristic angular behavior, evident for the asymmetric molecule acetaldehyde just as for the symmetric molecules formaldehyde and acetone. Through a comparison of the acetaldehyde results with those for formaldehyde and acetone the trends in the allowed and forbidden transition energies were examined as a function of methyl substitution and found to be relatively linear.

The spectra ofthe diketone compounds biacetyl, acetylacetone, acetonylacetone, 1,2-cyclohexanedione, and 1,4-cyclohexanedione have been investigated by the technique oflowenergy variable-angle electron energy-loss spectroscopy. With this method low-lying, spinforbidden transitions have been observed. The energy difference between the lowest spin-allowed and spin-forbidden n-+ 1r* excitations in the acyclic diketones is found to be 0.35 eV, on average, which is nearly the same as that of comparable acyclic monoketone compounds; in 1,2-cyclohexanedione, however, this energy difference is 0.84 eV, more than twice as large. This discrepancy in the magnitude of then-+ 1r* singlet-triplet splittings may be attributed to differing amounts of overlap between the initial and final orbitals.

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The electron energy-loss spectra of Cr(CO)6, Mo(CO)6, and W(CO)6 were measured at impact energies of 25, 50, and 100 eV and at scattering angles from 0° to 90°. The differential cross sections (DCS’s) were obtained for several features in the 3–7 eV energy-loss region. The symmetry-forbidden nature of the 1A1g→1A1g,2t2g (π)→3t2g(π*) transition in these compounds was confirmed. Several low energy excitations were assigned to ligand field transitions on the basis of the energy and angular behavior of their associated DCS’s. No transitions which could clearly be assigned to singlet→triplet excitations involving metal orbitals were located in these molecules. In addition, a number of states lying above the first ionization potential were observed for the first time. Several of these excitations seem to correspond quite well to some of the transitions observed in free CO.

Angular distributions of Rh atoms desorbed by energetic ion bombardment of an oxygen covered Rh{111} surface are measured accurately using a multiphoton resonance ionization (MPRI) detection technique. The results, in conjunction with molecular dynamics calculations of the ion impact event show that these distributions reflect the near-surface crystal structure. The molecular dynamics calculations were performed using a many-body embedded-atom potential to describe the dynamics of the Rh atoms and a pair-wise additive potential to describe the oxygen–Rh interactions. Several oxygen overlayer structures were considered for molecular dynamics modeling of the desorption process, including p(2×2) overlayers with a coverage of 0.25 monolayer (ML), and p(2×1) overlayers with a coverage of 0.50 ML, both of which are consistent with low energy electron diffraction (LEED) data. Three different adsorption sites were tested: threefold symmetric sites over second layer Rh atoms, threefold symmetric sites over third layer Rh atoms, and atop sites. The calculated azimuthal angular distributions of desorbed Rh atoms for each of these cases are unique, matching the experimental data best in the case of a p(2×1) overlayer with oxygen atoms adsorbed in threefold symmetric sites over third layer Rh atoms. The calculated Rh atom desorption yield (ejected atoms per incident ion) is sensitive to the oxygen coverage in the range 0.25–0.50 ML. These calculations are important in developing a surface bonding site and coverage consistent with LEED and our experiments. The peak in energy distribution of ejected Rh atoms from the oxygen covered surface is at a lower energy value than that of the clean metal. This indicates that collisional energy loss processes contribute to determining the peak position as well as the well known binding energy effect.