We present high-resolution valence band and core level spectra of silver for photoelectron kinetic energies up to 8 keV. At these kinetic energies we estimate a surface contribution of less than 3%. Taking advantage of the favourable sp/d relative cross-sections, a comparison with the calculated density of states is presented. We observe an increasing photoemission intensity when approaching the Fermi level, which we assign to a free-electron-like character in the 5p-band, whereas the principal s-like contribution is located at the bottom of the d-band. The difference between measured and calculated values of the sp/d cross-section ratio is discussed.
We have determined the effective attenuation length of photoelectrons over the range of kinetic energies from 4 to 6 keV in Co, Cu, Ge, and Gd2O3. The intensity of the substrate (Si) and overlayer core level peaks was measured as a function of the thickness of the wedge-shaped overlayers. Experimental values vary between 45-50 angstrom at 4 keV and 60-65 angstrom at 6 keV in Co, Cu, and Ge. Smaller values (30 angstrom to 50 angstrom, respectively) are found in Gd2O3. Our results confirm that, for different classes of materials, high energy photoemission spectroscopy has the necessary depth sensitivity to go beyond surface analysis, yielding important information on the electronic properties of the bulk and of buried layers and interfaces
The instrument VOLPE (volume photoemission from solids) is an experimental setup dedicated to high energy photoemission (PE) experiments. The instrument is equipped with an electrostatic hemispherical spectrometer especially designed to analyze high energy electrons (up to 10 keV) with high resolving power. In order to attain an energy resolution of a few tens of millielectron volts, we designed and constructed a dedicated input lens system, high stability power supplies, and a low dark-count detector and readout electronics. The system has been tested and is now operational on the ID16 beamline at European Synchrotron Radiation Facility, where an optical layout has been developed to perform high energy, high resolution PE experiments. First results show an overall energy resolution (electron + photon) of 71+/-7 meV at 5934 eV. The effective attenuation length of the photoelectrons is estimated to be 5+/-0.5 nm at a kinetic energy of 5 keV
X-ray free electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths.
A detailed comparison of the surface sensitivity of x-ray photoemission spectroscopy for hard and soft x rays is presented and discussed. Electron scattering parameters and their energy dependence are given for Si and two Si spectra are analyzed: a Mg K alpha (h nu=1253.6 eV) excited spectrum of the Si 2p and 2s lines and a hard x-ray excited spectrum (h nu=5925 eV) of the Si 1s line. The differential inelastic scattering characteristics for Si are extracted from reflection electron energy loss spectra taken at energies of 1500 and 4000 eV. Using these scattering characteristics and electron mean free paths from the literature, simulated spectra are compared with experiment. The experimental spectra are deconvoluted to give the true intrinsic line shape corresponding to the theoretical collision statistics when interference effects between intrinsic and extrinsic scattering are neglected. The magnitude of interference effects cannot be assessed by our analysis. Within the (unknown) uncertainty introduced by neglecting interference effects, it is possible to determine the relative intensity of intrinsic and extrinsic excitations. In this way, it is found that in the case of the soft x-ray excited photoelectron spectrum of the shallower electronic shells (2p and 2s), intrinsic plasmon creation is rather weak, and the apparent asymmetric line shape of the spectrum might be interpreted as the fact that electron-hole pair creation dominates the intrinsic loss spectrum, while an alternative explanation in terms of surface core level shifted components is also proposed. For the deeper core electronic shell, probed with hard x rays, the opposite situation is observed: while intrinsic electron-hole pair creation was not observed, a strong contribution of intrinsic plasmon losses of about 30% was seen
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