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.

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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.

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The electronic spectrum of the methyl radical CH3 was investigated by the technique of variable-angle electron energy-loss spectroscopy. By means of pyrolytic decomposition three possible sources of this radical were tried (tetramethyl tin, ethyl nitrite, and di-t-butyl-peroxide). The spectra of these precursors were obtained. Using di-t-butyl-peroxide, relative differential cross sections for the lowest allowed A″2 3s Rydberg transition in CH3 (5.73 eV) were determined at incident energies of 50 and 25 eV. The behavior of the differential cross section for this band is analogous to that of a spin-allowed transition in a closed shell system and, as expected, in the vicinity of this band no transition of a spin-forbidden nature is detected.