sigma* resonances in electron impact-induced vibrational excitation of n-propane, cyclopropane, ethylene oxide, cyclopentane, and cyclohexane

Electron-energy-loss spectra in the range of vibrational excitation, and excitation functions for selected vibrational peaks, were measured for the title compounds. Angular distributions of the vibrationally inelastic peaks were measured for n-propane and cyclopropane. The results in n-propane are similar to the published results in ethane, only one very broad band is observed in all channels, with gradual onset at about 3 eV and a maximum around 8 eV. In contrast, narrower. resonances emerge in all cyclic compounds. The effect is most pronounced in cyclopropane, where two resonances appear, at 2.6 and 5.5 eV. The latter is exceptional in several respects. It is narrow and thus relatively long lived for a shape resonance of this energy. It causes ring stretch excitation with very high selectivity and pronounced angular distribution, which is reproduced very well by the theory of Read and Andrick, revealing dominance of a partial wave with an unusually high angular momentum, l=3, m=3, and unambiguously identifying the resonance as a(2)'. The resonances in ethylene oxide are similar, but somewhat broader and shifted to 3 and 4.8 eV, respectively. Resonances in cyclopentane and cyclohexane resemble loosely the cyclopropane case. It is concluded that the major cause of the dramatic differences in spectral appearance between linear and cyclic alkanes are not major changes of resonant energies, but decrease of their (lifetime-determined) width, caused by higher symmetry, rigidity, and consequently larger contribution of partial waves with high I to the scattering. This implies that the vertical electron affinity of linear alkanes is not around -8 eV, as could be assumed from the position of the peak in the attachment spectra,but higher, around -3 eV. (C) 1996 American Institute of Physics.