[en] There are various effects which determine the line shape of a core-level electron spectrum. These are due to the finite life-time of the core hole, inelastic scattering of the outgoing photoelectron, electronic shake-up and shake-off processes and vibrational excitations. For free atoms and molecules the different contributions to the observed line shapes can often be well separated. For solids, surfaces and adsorbates the line shapes are in general much broader and it has in the past been assumed that no separation of the various contributions can be made. In the present report the authors will show that this is indeed not the case. Surprisingly, the vibrational fine structure of CO adsorbed on Ni(100) can be resolved in the C 1s and O 1s electron spectra. This was achieved by the combination of highly monochromatized soft X-rays from B18.0 with a high resolution Scienta 200 mm photoelectron spectrometer. X-ray photoelectron spectroscopy (XPS) with tunable excitation energy yields as a core level spectroscopy atomic and site-specific information. The presented measurements allow for a determination of internuclear distances and potential energy curves in corehole ionized adsorbed molecules. The authors analysis of the c(2x2) phase CO/Ni(100) on open-quotes topclose quotes yielded a vibrational splitting of 217 +/- 2 meV for C 1s ionization. For O 1s ionization a splitting of 173 +/- 8 meV was found

[en] Highlights: • Laser induced space-charge acceleration of electrons (up to 1 keV) is investigated. • A simple scaling behavior of the time-dependence and all-over energy shift is found. • Measured absolute energy shifts agree well with the model results. • Thus, image charges and a realistic angular hot-electron-distribution are important. • Results do not depend on Auger vs. photo electron-production mechanism. - Abstract: The so-called vacuum space-charge acceleration of emitted electrons triggered by absorption of soft X-rays (a few 100 eV) under intense near-infrared laser excitation is studied experimentally and theoretically. The influence of high excitation densities on the properties of Auger and photo electrons liberated by a probe X-ray beam is investigated for grazing-incidence photons interacting with an atomically clean Cu (111) surface as a model system. Transient electron spectra have been taken in a pump–probe setup at the BESSY II storage ring using a newly developed compact electrostatic retarding Bessel-box spectrometer. Strong electron-energy shifts have been found and assigned to space-charge acceleration. Model calculations based on experimental input parameters are in good agreement with experimentally obtained energy-gain values indicating ultimate limits of ultrafast X-ray experiments with photo or Auger electrons as a probe.

[en] We have investigated the resonant charge transfer from an adsorbed Ar atom in an electronically excited 2p_3/2⁵4s¹ state into different metal substrates, i.e., Ag(1 1 1), Ni(1 1 1) and Cu(1 1 1). Using the so-called core hole clock method we have been able to determine the resonant charge transfer times for the three systems. They differ roughly by a factor of two with the fastest transfer time being observed for Ni(1 1 1) and the slowest for Ag(1 1 1). Furthermore we observe strong energy dependences of the charge transfer times across the resonances which are opposite to the expectations from a simple tunneling model, or from a detuning induced shortening of the interaction time. Possible reasons for the observed behavior are discussed

[en] The size-dependent magnetic properties of small iron clusters deposited on ultrathin Ni/Cu(100) films have been studied with circularly polarized synchrotron radiation. For x-ray magnetic circular dichroism studies, the magnetic moments of size-selected clusters were aligned perpendicular to the sample surface. Exchange coupling of the clusters to the ultrathin Ni/Cu(100) film determines the orientation of their magnetic moments. All clusters are coupled ferromagnetically to the underlayer. With the use of sum rules, orbital and spin magnetic moments as well as their ratios have been extracted from x-ray magnetic circular dichroism spectra. The ratio of orbital to spin magnetic moments varies considerably as a function of cluster size, reflecting the dependence of magnetic properties on cluster size and geometry. These variations can be explained in terms of a strongly size-dependent orbital moment. Both orbital and spin magnetic moments are significantly enhanced in small clusters as compared to bulk iron, although this effect is more pronounced for the orbital moment. Magnetic properties of deposited clusters are governed by the interplay of cluster-specific properties on the one hand and cluster-substrate interactions on the other hand. Size dependent variations of magnetic moments are modified upon contact with the substrate

[en] X-ray scattering spectra of Cu and Ni metals have been measured using monochromatic synchrotron radiation tuned from far above to more than 10 eV below threshold. Energy conservation in the scattering process is found to be sufficient to explain the modulation of the spectral shape, neglecting momentum conservation and channel interference. At excitation energies close to and above threshold, the emission spectra map the occupied local partial density of states. For the subthreshold excitations, the high-energy flank of the inelastic scattering exhibits a Raman-type linear dispersion, and an asymmetric low-energy tail develops. For excitation far below threshold the emission spectra are proportional to a convolution of the occupied and unoccupied local partial densities of states

[en] Valence photoionization of N₂ has been performed with high-brilliance vacuum ultraviolet radiation at hν=38.5±0.2 eV from the free-electron laser at Hamburg (FLASH). As a function of fluence, saturation of photoionization occurs due to the depletion of the ground-state molecular target within each femtosecond radiation pulse. With increasing fluence, the vibrational envelope of the valence photoionization spectral distribution experiences radiation-field-induced changes

[en] A new spectrometer, utilizing a reflection zone plate based grating, for the Mn L fluorescence line was recently designed, manufactured and tested at Helmholtz Zentrum Berlin. The angular acceptance of the grating is ∼0.011 rad² the absolute efficiency at 640 eV is 16%, and the energy resolution, for a detector slit size of 120 μm and in simultaneous spectra registration mode, is about λ/Δλ ∼ 100 FWHM.

[en] Using high resolution S 2p and O 1s x-ray photoelectron spectroscopies, the adsorption of SO₂ and its surface bound reaction products on Ru(0001) have been investigated simultaneously while dosing SO₂ and while heating the adsorbed species. SO₂ is found to adsorb on Ru(0001) at 100 K molecularly in two variants as well as dissociatively and to react to SO₃, SO₄, SO, and S with increasing coverage. After the monolayer has been saturated, SO₂ adsorbs molecularly in multilayers. When heating adsorbed SO₂ from 100 K, SO, SO₂, and SO₄ decompose in a wide temperature range up to 305 K. In contrast SO₃ is found to be stable bound to Ru(0001) up to 300 K and to disappear from the surface to below 325 K. At 550 K the surface remains with a saturated atomic sulfur and oxygen layer and some sulfur species in a second layer. Our quantitative analysis of the sulfur amount bound to the surface supports a simple desorption process only for SO₄. All other species mainly or partly decompose on the surface

[en] An extreme ultraviolet split and femtosecond delay unit based on grazing incidence Mach-Zehnder geometry has been designed and implemented on the plane grating monochromator beamline PG2 at FLASH, the Free Electron Laser at DESY. This device splits the FLASH radiation into two beams, which can independently be steered, filtered and temporally delayed between -5.1 and +5.1 ps with uncertainty in the temporal accuracy of 210 as. To demonstrate the performance of this device, we have performed longitudinal coherence studies of FLASH radiation as well as measured the pulse length by nonlinear two-photon double-ionization in helium.

[en] Vibrational fine structure has been observed in the C1s and O1s photoelectron lines of CO adsorbed on Ni(100). This allows a detailed separation of vibrational and electronic contributions to the line profile. The main line is found to represent the fully screened final state. The width of the photoelectron lines is dominated by the intramolecular stretch mode, which strongly depends on the chemical state of the adsorbate. The identification of the adiabatic transition makes the determination of chemical shifts more precise. The equivalent core approximation is found to be highly accurate and electronic and geometric equilibrium properties of the core-ionized adsorbate are extracted. copyright 1998 The American Physical Society