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Resonant photoemission has been studied above the carbon 1s ionization thresholds in gas-phase benzene and ethylene. The experimental data for both molecules include relative partial cross section and asymmetry-parameter measurements for the C 1s main line and asymmetry-parameter measurements for one C 1s shake-up satellite in each system. Resonances above the C K edge have been analyzed on the basis of their decay to either the C 1s main line or valence-hole states, and have been tentatively assigned as either shape resonances or doubly excited states according to their observed one-electron or many-electron decay, respectively. The importance of determining the resonant behavior of all available photoemission channels in the proximity of a resonance is thus illustrated. [ABSTRACT FROM AUTHOR]

A discussion is presented on the general nature of shape resonances in small molecules and how they may relate to molecular bond lengths. Criteria for assigning photoabsorption features as shape resonances are described, and the usefulness of photoemission experiments to such assignments is highlighted. Based on these criteria, all unambiguously identified K-shell shape-resonance features in molecules containing B, C, N, O, and F are examined in an attempt to identify empirically a shape-resonance energy/bond length relationship. Although the available data are insufficient to establish a quantitative correlation, they indicate the influence of other variables besides the bond length to the energy position of a shape resonance. [ABSTRACT FROM AUTHOR]

The relationship between shape-resonance energies and bond lengths within agiven molecule is explored further.It is contended that some shape-resonance assignments presented by Stohr and Hitchcock (ref.1) are not convincing.(AIP) [ABSTRACT FROM AUTHOR]