No abstract

Near edge x-ray absorption spectra and decay electron spectra for resonant and nonresonant excitations in the C 1s and N 1s regions have been measured for condensed furan (C4H4O) and pyrrole (C4H4NH) layers. The decay spectra after resonant excitation into the lowest unoccupied orbital (π*4) are pure autoionization spectra. The participant channels show strong resonant enhancements of the photoelectron peaks, in particular after excitation of the C 1s core electrons. Overall however, the spectra are dominated by the spectator channels. The spectator spectra resemble the normal Auger spectra shifted by spectator shifts of ≊4 eV for furan and ≊2.5 eV for pyrrole. For furan the participant spectra for excitation at the two different carbon atoms show significant differences that can be qualitatively understood in terms of a simple one-electron picture by considering the localization of the corresponding orbitals on the core hole. The decay spectra after excitation into the π*5 orbital show negligible enhancement of the participant channel, a strongly reduced spectator shift, and less-pronounced structures than at higher photon energies. The latter is attributed to a competition of spectator decay and normal Auger decay after internal photoionization into Rydberg derived bands in the condensate. For excitations into the σ*-resonances pure Auger spectra are observed, indicating that these resonances fully ionize before core decay takes place.

A surface sensitive purely optical technique is demonstrated for the investigation of rapid dephasing processes of excited electronic surface states. The combination of degenerate four‐wave‐mixing (DFWM) with second‐harmonic generation (SHG) can be employed to measure the diffracted second‐harmonic signal from a transient population grating of Si(111)7×7 dangling‐bond states. By this five‐wave‐mixing process, which is described by a χ(4)‐tensor and is thus dipole‐forbidden in bulk silicon, we deduce decoherence times of ∼5 fs.