ALOISA Beamline Research Articles

ANCHOR-SUNDYN: A novel endstation for time resolved spectroscopy at the ALOISA beamline

In this report we illustrate the instrumental developments involving the branchline of the ALOISA beamline at the Elettra synchrotron, with the setup of the ANCHOR-SUNDYN endstation. The ANCHOR experimental chamber is designed for the study of in-situ prepared organo-metallic hybrid interfaces, by means of synchrotron based soft x-ray spectroscopic techniques. SUNDYN is a femtosecond pulse width laser setup, optimized for the optical pump of organic systems. It has been integrated in the ANCHOR endstation in order to perform time resolved spectroscopies at MegaHertz repetition rate. By combining resonant photoemission spectroscopy and pump-probe X-ray spectroscopies in the same chamber, we can study the dynamics of the electronic structure of organo-metallic interfaces from the femtosecond to the nanosecond timescales. We provide here a detailed description of this setup and discuss some case study measurements obtained during the commissioning of the apparatus.

OK-edge x-ray absorption study of ultrathinNiOepilayers depositedin situonAg(001)

In a recent contribution [Phys. Rev. Lett. 91, 046101 (2003)] we used polarization-dependent, $\mathrm{Ni}$ $K$ edge, x-ray absorption spectroscopy (XAS) to probe the structure of ultrathin $\mathrm{NiO}$ epilayers deposited on $\mathrm{Ag}(001)$. In that experiment samples were measured ex-situ and a $5\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$-thick $\mathrm{MgO}$ cap was used to avoid the hydroxylation of the $\mathrm{NiO}$ film. In the present paper we report complementary $\mathrm{O}$ $K$ edge XAS data on the same system; $\mathrm{NiO}$ epilayers, in the $3--50\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ thickness range, were grown in situ in the end station of the ALOISA beamline of the ELETTRA facility. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the film is rock-salt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain as a function of the film thickness $T$. An in-plane compressive strain, due to lattice mismatch with the $\mathrm{Ag}$ substrate, is clearly present for the $3\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ film, being the in- and out-of-plane nearest neighbor distances equal to ${r}_{\ensuremath{\Vert}}=2.048\ifmmode\pm\else\textpm\fi{}0.016\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and ${r}_{\ensuremath{\perp}}=2.116\ifmmode\pm\else\textpm\fi{}0.018\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. These values are in agreement with the expected behavior for a tetragonal distortion of the unit cell. The growth-induced strain is gradually released with increasing $T$: it is still appreciable for $10\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ but is completely relaxed at $50\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$. Any significant intermixing with the $\mathrm{Ag}$ substrate has been ruled out. Combining $\mathrm{O}$ and $\mathrm{Ni}$ $K$ edge results we can conclude that $\mathrm{NiO}$ films grow on $\mathrm{Ag}(001)$ in the O-on-$\mathrm{Ag}$ configuration, with an interface distance $d=2.28\ifmmode\pm\else\textpm\fi{}0.08\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. This expansion of the interplanar distance is in agreement with recent ab initio simulations. A comparison with the similar $\mathrm{MgO}∕\mathrm{Ag}(001)$ system is also performed.

Study of the isotropic contribution to the analysis of photoelectron diffraction experiments at the ALOISA beamline

The angular distribution of the intensity in photoemission experiments is affected by electron diffraction patterns and by a smoothly varying contribution originated by both intrumental details and physical properties of the samples. The peculiar variety of scattering configurations available at the ALOISA beamline experimental station in Trieste stimulated the developement of an analytical description for the smooth angular dependence sustaining the diffraction features. We present here the basic formulae and their application to experimental data taken on the Fe/Cu 3Au(001) system in order to highlight the role of the various parameters included in the distribution function. A specific model for the surface illumination has been developed, and the overlayer thickness and surface roughness have been considered.

Structure modulated LMDAD effects in BCC-Fe vs. RCP-Fe

The atomic-like behavior of the photoionization cross-section of core levels in solids is remarkably displayed in the observation of linear magnetic dichroism in the angular distribution of photoelectrons. Structure-related effects are clearly visible as modulations of the dichroism signal, induced by photoelectron diffraction, a fact that can be exploited to obtain information from magnetic surfaces. A systematic investigation of such effects has been carried out for Fe 2p and 3p core levels on the ALOISA beamline at ELETTRA. We present results for Fe(0 0 1) with a reference calibration for amorphous RCP-Fe grown on Fe 30Ni 50B 20, which provides the atomic-like standard, free from diffraction effects.

The ALOISA end station at Elettra:: a novel multicoincidence spectrometer for angle resolved APECS

Coincidence measurements have been extensively performed in atomic and molecular physics since early 1970s. To apply this methodology to solids and surfaces has been a major target since early days, but the long average time needed to complete a coincidence experiment has hampered its attainment. In particular the coincidence technique has not been yet applied in an angle resolved way such for studying the momentum correlation in the ejection of electron pairs from solid surfaces. The experimental chamber at the ALOISA beamline at Elettra, by means of a set of seven homemade electron analyzers, is the first apparatus able to perform Angle Resolved – Auger Photoelectron Coincidence Spectroscopy (AR-APECS) from solid surfaces. In the typical setup ten different pairs of coincident electrons can be measured simultaneously, so reducing the acquisition time by one order of magnitude.

Combined photoelectron and X-ray diffraction from ultrathin Fe films on Cu 3Au(001)

X-ray specular reflectivity, in-plane grazing incidence X-ray diffraction (GIXRD) and photoelectron diffraction (PED) have been employed to study the Fe/Cu 3Au(001) system at the ALOISA beamline (Trieste Synchrotron, Italy). In-plane GIXRD has been used to determine the epitaxial strain of the film lattice by (H00) radial scans around the Cu 3Au(200) diffraction peak. PED has been employed in forward scattering conditions to determine the vertical spacing of the topmost layers and to look at atomic exchange processes. At very low coverage, Fe grows pseudomorphically on the fcc substrate; the bulk-like Fe bcc structure has been observed for an 18-Å thick Fe film. The PED analysis suggests that, at 1 ML Fe coverage, a full Au layer is segregated onto the surface with the Au atoms sitting in their fcc sites. A consistent fraction of a monolayer of Au is still present on top of the 18-Å Fe bcc film.

DETERMINATION OF TiO2(110) SURFACE RELAXATION BY VARIABLE POLARIZATION PHOTOELECTRON DIFFRACTION

A new method to determine the surface relaxation by means 0 of photoelectron diffraction (PED) from an s-core level has been applied to the (1 × 1)- TiO 2(110) surface. Complete 2π solid angle PED patterns for both oxygen 1s and titanium 3s core level peak intensities have been measured at the ALOISA beamline (Elettra Synchrotron, Italy) in two different photon polarization/electron detection geometrical configurations: one with the photon polarization vector parallel to the surface normal (TM polarization) and the other with the photon polarization vector perpendicular to the surface normal (TE polarization). In the latter case the PED sensitivity to the vertical spacing is enhanced. Because forward scattering is suppressed on the node of the primary wave along the surface normal, the relative weight of the distance-sensitive higher order interference fringes is enhanced. The opposite occurs for the TM polarization, which enhances the PED sensitivity to lateral spacing at grazing emission angles. The experimental PED patterns are compared with the theoretical simulations calculated for the relaxed and unrelaxed TiO 2(110) surface. The results prove the feasibility of the method and the adequacy of the PED technique to the studies of relaxation phenomena at crystal surfaces.

PHOTOELECTRON DIFFRACTION STUDY OF THE (3×3)-Sn/Ge(111) STRUCTURE

The photoemission spectra of the Sn 4d electrons from the (3×3)-Sn/Ge(111) surface present two components which are attributed to inequivalent Sn atoms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra storage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energies and vice versa. The bond angle between Sn and its nearest neighbor atoms in the first Ge layer ( Sn–Ge 1) has been measured by taking polar scans along the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as obtained by studying the dependence on the photoelectron kinetic energy with the photon polarization and the collection direction parallel to the Sn–Ge 1 bond orientation (bond emission). A clear difference between the two bonding sites is observed when studying the energy dependence at normal emission, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. The (3×3)-Sn/Ge(111) is thus characterized by a structure where the Sn atom and its three nearest neighbor Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.

The high-energy monochromator for the ALOISA beamline at Elettra

At the ALOISA beamline the photon energy range 0.25 to 8 keV will be provided by switching between a plane mirror/grating monochromator and a crystal monochromator sharing the same beamline mirror optics. All dispersing elements are used in a collimated beam, which is focused downstream by means of vertically mounted mirrors. It is shown that the variations in the geometry of the light spot at the sample remain within acceptable limits over the requested energy range even if a simple nonfixed-exit ‘‘channel-cut’’ crystal is used. The crystal will have an inclined cut which allows to spread the power load delivered by the wiggler source sufficiently over the first reflecting crystal surface to allow for simple back-side cooling. In addition a new type of power filter has been designed based on a rotating, water-cooled Be disk. This design reduces the effective absorbed power density in the filter by two orders of magnitude with respect to commonly used static multifoil solutions, resulting in a cool, clean, UHV-compatible device.