Electron transmission, inner-shell electron energy loss and magnetic circular dichroism spectra have been analyzed in an effort to trace the positions of the * antibonding valence MOs in benzene and its fluorinated derivatives. The correlation of negative-ion resonances in these systems shows clearly that a a* valence level descends with increasing fluorination so as to become the lowest virtual MO in hexafluorobenzene. This is understandable in terms of the perfluoro effect acting upon virtual MOs in a way parallel to that known to occur for occupied MOs. In addition to the low-lying a* negative-ion shape resonances, several negative-ion Feshbach resonances are identified as involving 3s and 3p Rydberg orbitals. The search for low-lying * levels in heavily fluorinated benzenes is extended to their C Is and F Is inner-shell spectra. The carbon K-shell spectra of benzene and its fluorinated derivatives below the respective C Is ionization potentials are dominated by excitations to 1 * and 2 * valence levels. In the C Is spectra of pentafluoro-and hexafluorobenzene, additional low-lying bands are observed and assigned to C ls(C-F) -* a*(C-F) transitions. Spectral stripping indicates the location of the corresponding (C ls(C-F)-1, *) states in the spectra of the other fluorobenzenes. A systematic shift of these * levels to lower energy with increasing fluorination is observed which is consistent with the perfluoro effect. Resonances terminating at ff*(C-C) are found to dominate the C Is near continuum, with dramatic enhancement of these transitions in the more highly fluorinated species. Investigation of hexafluoroand 1,2,4,5-tetrafluorobenzene by vacuum-ultraviolet magnetic circular dichroism in the vapor phase confirms the presence of bands which are not tt -> tt*. Once again, low-lying * MOs are invoked as terminating orbitals.

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The carbon K-shell spectra of gaseous furan, pyrrole, tetrahydrofuran, pyrrolidine, tetrahydropyran, and piperidine have been recorded by electron energy loss spectroscopy (ISEELS) under electric dipole scattering conditions (2.5 keV impact, small angle.) The spectra are dominated by transitions to unoccupied valence states of π and σ symmetry. Features attributed to transitions to π*(CH2) levels are consistently observed below the ionization threshold in the spectra of the saturated species. the positions of continuum features are generally in agreement with a previously documented correlation with bond lengths. Additional weak continuum features are observed in the smaller saturated heterocyclic species which are ascribed to delocalized σ* states.

Optical oscillator strength spectra of formamide (HCONH2), formic acid (HCOOH), and formyl fluoride (HCOF) in the region of K-shell excitation have been derived from electron energy loss spectra recorded under electric dipole dominated scattering conditions (>2.7 keV impact energy, small scattering angle). The observed features are assigned to promotions of 1s electrons to π*(C=O), σ*(HCX), σ*(C–X), σ*(C=O), and Rydberg orbitals. Systematic changes in the term values for the 1s→π*(C=O) transitions are related to the π donor strengths of the X substituents of the carbonyl group. Broad weak features, observed only in the carbonyl C1s and O1s spectra around 7 eV above the IP, are assigned to 1s→σ*(C=O) transitions. The positions of these features are in agreement with a previously documented correlation with bond length, as are the positions of features associated with σ*(C–N) in formamide, σ*(C–O) in formic acid, and σ*(C–F) in formyl fluoride. The oscillator strengths of the 1s→π* features in the various K-shell spectra are compared to HAM/3 calculations and are used to estimate the spatial distributions of the π*(C=O) orbital in the (1s−1, π*) core excited states of these three substituted carbonyl species. We discuss the degree to which these derived orbital maps reflect the spatial distributions of π*(C=O) orbitals in the ground state.

Si and Ge K-edge x-ray absorption fine structure ͑XAFS͒ spectra of strained and relaxed Si x Ge 1Ϫx crystalline alloys grown by molecular beam epitaxy on Si͑001͒ substrates are reported. For alloys with less than 30% Si, fluorescence yield detection is shown to be essential to avoid distortions of the Si K-edge XAFS signal caused by the underlying Ge L-edge XAFS signal from the majority Ge species. The average first shell structure has been deduced using simultaneous fitting of all data for relaxed alloys, while imposing physically reasonable constraints. The Ge-Ge, Ge-Si, and Si-Si first-shell distances are found to vary with composition. The results are compared with other experimental results and theoretical predictions in the literature. Our results are generally consistent with other experimental studies but they differ from recent theoretical predictions based on macroscopic elastic properties in that we observe a different compositional dependence ͓i.e.,