No abstract

No abstract

We present carbon K emission spectra of diamond excited with high-resolution undulator radiation. The valence-band emission spectra are shown to be strongly dependent on the excitation energy, up to 20-30 eV above the C K edge. It is proposed that the dependence is indicative of the resonant inelastic scattering description of these emission spectra, i.e. , the absorption-emission process should be described as a single scattering event where the momenta of the photoelectron and the valence hole in the final state are related by momentum conservation.PACS numbers: 78.70.g, 71.25.Rk X-ray absorption and the consequent decay through xray emission near an absorption edge are generally assumed to be two independent one-photon processes. With the exception of the x-ray resonant Raman scattering [1-3] and emission from molecules [4,5], there has been no clear experimental evidence questioning the validity of this assumption and thus no need to interpret this type of photon-in-photon-out experiment by the general x-ray inelastic scattering treatment [3]. Recent synchrotron radiation studies showing strong excitation energy dependencies in the x-ray emission spectra [5][6][7][8][9][10] were mostly attributed to multielectron processes such as shakeup satellites within the "absorption followed by emission" picture [11]; i.e. , the absorption and emission processes are independent.In this Letter we report carbon K emission spectra of diamond excited with high-resolution synchrotron radiation. The intensities of valence-band features are shown to be strongly dependent on the energy of the incident photons which excite the C 1s electrons into successive symmetry points in the conduction band, suggesting the presence of correlation between the absorption and emission processes. Furthermore it appears that this correlation can be related to the momentum of the photoelectron and that of the valence hole in the final state. We propose that the excitation energy dependence can be explained most straightforwardly by treating the absorption-emission process as a single inelastic scattering process with well-defined momentum conservation.In this interpretation, as a result of the negligible momentum transfer from the soft-x-ray photon, the final state of the solid prefers to have equal momentum for the photoelectron and the valence hole. This enhances emission from symmetry points where the photoelectron has been excited to. Since, with the availability of the next generation synchrotron radiation sources, emission experiments excited with high-resolution photons will be performed with increasing frequency, our results have important implica-tions for interpreting these experiments. Finally, the use of the momentum-resolved inelastic scattering as a new band mapping technique will be discussed briefly.Diamond was chosen for this study because it has a very simple and broad band structure that has been well studied.For this system of delocalized sp electrons, correlation eA'ects, which have been found to be very important in the prev...

The x-ray fluorescence and absorption of highly oriented pyrolytic graphite have been measured using monochromatic synchrotron radiation. The spectra can be separated into contributions from~and oband components by measuring at different angles of incidence and at different emission angles. The shape of the x-ray fluorescence spectra varies dramatically with excitation energy near the C E edge.This dependence on excitation energy can be interpreted within a resonant-inelastic-scattering formalism. The results are compared with previously published band-structure calculations and photoemission results, and demonstrate the potential for using x-ray fluorescence to obtain symmetry-resolved band information.

No abstract

Resonant soft x-ray emission spectroscopy has been applied to study the issue of symmetry breaking upon core-hole excitation in molecular oxygen. The results provide direct evidence that the inversion symmetry is not broken in the core-excited states. Furthermore, the experiments themselves demonstrate a new experimental technique of broad applicability for studies of electronic structure and excitation dynamics in free atoms and molecules.

No abstract

Resonantly excited x-ray fluorescence spectra of the benzene molecule are presented and analyzed in terms of symmetry-selective resonant scattering processes. The sharp frequency dependency that is observed can be understood from strict parity and symmetry-selection rules operating in the full 06h point group symmetry. The experimental results prove that the electronic symmetry is not broken for the lowest unoccupied level involved in the resonant x-ray emission process.