The absolute photoabsorption cross section of benzene (C6H6), encompassing the C 1s−1 π*e2u resonance, the C 1s threshold, the satellite thresholds, and extending up to 800 eV, has been measured using synchrotron radiation. Measurements of the discrete absorption structure from below the C 1s ionization threshold have been performed at high resolution. In order to unambiguously assign all structure present in the photoabsorption cross section, C 1s photoelectron spectra were measured from the C 1s threshold region up to 350 eV along with satellite spectra. The C 1s−1 single-hole and the satellite cross sections have been derived in absolute units, and their angular distributions have been determined. Resonant and normal Auger spectra were taken on the main features of the photoabsorption and single-hole cross sections. From the best resolved photoelectron spectra the underlying structure in the asymmetric benzene photoelectron peak can be partly disentangled. The experimental data show that at least two vibrational modes play a role in the C 1s photoelectron spectrum. The behavior of the investigated shake-up structure closely resembles that of ethene and ethyne, where the satellite bands due to π→π* excitations gain intensity towards threshold, an observation which may be attributed to conjugate shake-up processes. These processes lead to a significant contribution of the satellite intensity to the production of the absorption features traditionally assigned to the carbon shape resonances in benzene. An EXAFS analysis of the wide range oscillations present on the photoabsorption cross section has been performed, and reveals the C–C nearest-neighbor distance.

The C 1s partial photoionization cross section and photoelectron angular distribution of methane ͑CH 4 ͒ have been measured with high-energy resolution between threshold and 385 eV photon energy. From the analysis of the vibrational fine structure on the C 1s Ϫ1 photoelectron line a vibrational energy of 396Ϯ2 meV and an equilibrium bond length of 1.039͑Ϯ0.001͒ Å for the CH 4 ϩ ion have been determined. The lifetime broadening was found to be 83͑Ϯ10͒ meV. The weak feature in the photoabsorption cross section just above threshold does not influence the vibrational fine structure in a way typical for a shape resonance. We therefore suggest that it is due to doubly excited states of the type C ͑1s͒ Ϫ1 ͑Val͒ Ϫ1 ͑Ryd͒ a 1 ͑Ryd͒ b 1 , an assignment which is supported by recent Auger decay studies. Measurements of the shakeup structure revealed six satellite lines, one of which increases strongly in intensity at threshold, thus pointing to the existence of a conjugate shakeup process.

Steps in inequivalent crystallographic directions are usually assumed to have different atomic structures. However, this Letter demonstrates that bilayer steps in the two principal crystallographic directions (͗ 1 12͘ and ͗112 ͘) of the Br-terminated Si(111)-͑1 3 1͒ surface have the same atomic edge structure, due to the introduction of stacking faults along the ͗112 ͘ step edges. Similar results are also observed for Cl-and I-terminated Si (111) surfaces. This strong preference for a particular step edge structure determines the surface morphology of steps, islands, and etch pits, and has profound ramifications for dry etching and chemical vapor deposition growth for this system. [S0031-9007(96)02002-9]

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