R. Nyholm

Soft-x-ray polarimeter with multilayer optics: complete analysis of the polarization state of light

The design of a versatile high-precision eight-axis ultrahigh-vacuum-compatible polarimeter is presented. This multipurpose instrument can be used as a self-calibrating polarization detector for linearly and circularly polarized UV and soft-x-ray light. It can also be used for the characterization of reflection or transmission properties (reflectometer) or polarizing and phase-retarding properties (ellipsometer) of any optical element. The polarization properties of Mo/Si, Cr/C, Cr/Sc, and Ni/Ti multilayers used in this polarimeter as polarizers in transmission and as analyzers in reflection have been investigated theoretically and experimentally. In the soft-x-ray range, close to the p edges of Sc, Ti, and Cr, resonantly enhanced phase retardation of the transmission polarizers of as much as 18 degrees has been measured. With these newly developed optical elements the complete polarization analysis of soft-x-ray synchrotron radiation can be extended to the water-window range from 300 to 600 eV.

Beam line I411 at MAX II - Performance and first results

We report on the characteristics and first results from the soft X-ray beam line I411, based on an undulator at the third generation synchrotron facility MAX II, Sweden. The beam line is designed for high-resolution, angle-resolved electron spectroscopy on gases, liquids and solids. Main components are the modified SX700 monochromator and the end station, both of which were previously used at beam line 51 at MAX I. The end station is equipped with a rotatable SES-200 hemispherical electron-analyser. Before the end station, a one-metre section is reserved for exchangeable experimental set-ups. The usable photon energy range is 50-1500 eV and the photon flux is two orders of magnitudes higher compared to beam line 51. At 400 eV a resolving power of about 5700 in the first order of the monochromator grating could be obtained. In gas phase, a total electron energy resolution of 16 meV has been achieved. Detailed results on the undulator performance, flux, photon and electron energy resolution as well as some technical details are presented here. As an example of the capabilities of the beam line I411, we present the fully vibrationally resolved Auger resonant Raman electron spectrum of gas-phase N 2.

Beamline I311 at MAX-LAB: A VUV/soft X-ray undulator beamline for high resolution electron spectroscopy

We describe a beamline based on a plane-grating monochromator and an end-station designed for high resolution photoemission and photoabsorption spectroscopy on surfaces and interfaces using undulator radiation in the energy range 30 1500 eV. The general design of the beamline is based on a horizontally focusing pre-mirror, an SX-700 type of plane-grating monochromator and re-focusing optics of Kirkpatrick-Baez design.

Beamline I511 at MAX II, capabilities and performance

The new undulator beamline I511 at MAX-lab, now under commissioning, has been optimized for X-ray emission and photoelectron spectroscopies. Using an SX-700 high flux monochromator the accessible photon energy range is from 90 eV to about 1500 eV. The per

On the adsorption sites for CO on the Rh(111) single crystal surface

Abstract High resolution photoemission applied to the C 1s and Rh 3d core levels has been used to investigate the adsorption sites at low temperature of CO on the Rh(111) single crystal surface as a function of CO coverage. Two different sites are found to be occupied by the CO molecules. At coverages up to 0.5 monolayers the majority of the molecules are found to adsorb in on-top sites whereas at higher coverages three-fold hollow sites become increasingly populated. The different C 1s binding energy of the CO molecules in these two sites makes it possible to measure diffraction induced intensity variations versus photon energy in a site specific manner. The saturation (2 × 2)-3CO structure formed at a coverage of 0.75 monolayers is argued to contain one on-top and two three-fold hollow molecules per unit cell.

Alkali metal adsorption on Al(111)

The submonolayer adsorption of Na, K, Rb, and Cs on the Al(111) surface at 100 K and at room temperature is investigated by high resolution core level spectroscopy and low energy electron diffraction. It is found that the first alkali atoms on the surface adsorb at surface defects. At higher coverages, up to approximately one third of the maximum submonolayer coverage, alkali atoms adsorbed at defects coexist with a dispersed phase. At higher coverages island formation is found to occur for the majority of the systems. It is argued that all of the ordered structures formed at room temperature involve a disruption of the Al(111) surface in contrast to the situation at 100 K where the alkali atoms adsorb as adatoms.

Vibrational fine structure in the C 1s core level photoemission of chemisorbed molecules : Ethylene and ethylidyne on Rh(111)

The origin of fine structure in the core-level photoemission spectra of the C2H4, C2D4, C2H3 and C2D3 molecules chemisorbed on Rh(111) is probed in a careful high-resolution study showing that this structure arises from internal molecular vibrations rather than from other chemically-shifted carbon atoms. It is shown by comparison of the adsorbate and gas-phase spectra that the underlying features are the same despite differences arising from adsorption. This new approach to the investigation of adsorbed molecules may prove to be useful in further studies of other systems and the possibility that such effects may exist could lead to the reinterpretation of other adsorbate systems.

Coverage- and temperature-dependent site occupancy of carbon monoxide on Rh(111) studied by high-resolution core-level photoemission

Abstract High-resolution core-level photoemission is used to study structural aspects for the molecular adsorption of CO on the Rh(111) single-crystal surface, and in particular to derive the adsorption sites. The site sensitivity of the core-level binding energy and the fact that the core level photoemission signal is proportional to the adsorbate coverage make it possible to study quantitatively how the occupation of different sites changes with temperature and/or CO coverage. For the CO Rh (111) adsorption system we find two sites (on-top and three-fold hollow) to be occupied by the CO molecules. At coverages up to 0.33 ML only on-top sites are occupied, whereas at higher coverages a mixture of three-fold hollow and on-top sites are found. The distribution between these two sites is found to depend strongly on temperature. Quantitative studies of these reversible, temperature-dependent site changes have been carried out for a number of CO coverages. For coverages between 0.33 and ∼0.54 ML, increasing the temperature results in part of the molecules moving from on-top to three-fold hollow sites. This change is strongest for a (4 × 4) structure formed at 0.5 ML where an order-disorder transition is observed at a temperature of 120 K. For coverages above ∼0.54 ML, increasing the temperature leads instead to a decrease of the relative occupation of the three-fold hollow sites. For coverages below 0.33 ML, the molecules occupy on-top sites at all temperatures.

Photoabsorption and the unoccupied partial density of states of chemisorbed molecules

The 1s to 2π* resonances have been studied for CO strongly chemisorbed on Ni(100) and weakly chemisorbed on Cu(100) using high resolution X-ray absorption spectroscopy. We show that the spectra can be consistently described in terms of the local density of unoccupied 2π* electron states with the Fermi level obtained from corresponding X-ray photoelectron spectra. The core hole is demonstrated to induce large shifts in the unoccupied 2π* density of states.

Design and performance of a spherical grating monochromator used at MAX I

We report on the performance of a grazing incidence spherical grating monochromator installed at MAX I and designed to cover the photon energy range from about 15-200 eV with high resolving power. It is intended mainly for angle-resolved photoemission work. Therefore, both refocusing optics, to obtain a small spot size at the sample, and a higher order light suppressor, to reduce the content of higher orders, have been incorporated in the design. The theoretically calculated energy resolution is presented and compared to the resolution obtained in photoabsorption measurements of gas-phase He, Ne, Kr, Xe, and N2. The possibility to reduce the influence of higher orders is illustrated by photoemission data collected on Si (100). The experimental results show that the monochromator fulfills the expected design goals. (Less)