Eric D. Tober

Chemisorption geometry of formate on Ti2(110) by photoelectron diffraction

Abstract We have combined photoelectron diffraction in both scanned-angle and scanned-energy modes with Hartree-Fock calculations to investigate the interface structure of HCOO− on TiO2(110). Formate anions bind through the oxygens to Ti cation rows along [001] with a 2× periodicity and an OCO bond angle of 126 ± 4°. Each anion binds to two Ti cations with an adsorbate-substrate bond distance of 2.0 ± 0.1 A.

DIFFRACTION AND HOLOGRAPHY WITH PHOTOELECTRONS AND FLUORESCENT X-RAYS

Abstract We consider studies of the atomic and magnetic structure near surfaces by photoelectron diffraction and by the holographic inversion of both photoelectron diffraction data and diffraction data involving the emission of fluorescent x-rays. The current status of photoelectron diffraction studies of surfaces, interfaces, and other nanostructures is first briefly reviewed, and then several recent developments and proposals for future areas of application are discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low energy electron diffraction and scanning tunneling microscopy, to the epitaxial growth of oxides and metals is considered. Several new avenues that are being opened up by third-generation synchrotron radiation sources are also discussed. These include site-resolved photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements of surface reactions with chemical-state resolution, and circular dichroism in photoelectron angular distributions from both non-magnetic and magnetic systems. The addition of spin to the photoelectron diffraction measurement is also considered as a method for studying short-range magnetic order, including the measurement of surface magnetic phase transitions. This spin sensitivity can be achieved through either core-level multiplet splittings or circular-polarized excitation of spin-orbit-split levels. The direct imaging of short-range atomic structure by both photoelectron holography and two distinct types of x-ray holography involving fluorescent emission is also discussed. Both photoelectron and x-ray holography have demonstrated the ability to directly determine at least approximate atomic structures in three dimensions. Photoelectron holography with spin resolution may make it possible also to study short-range magnetic order in a holographic fashion. Although much more recent in its first experimental demonstrations, x-ray fluorescence holography should permit deriving more accurate atomic images for a variety of materials, including both surface and bulk regions.

Photoelectron diffraction and holography: Present status and future prospects

Abstract Photoelectron diffraction and photoelectron holography, a newly developed variant of it, can provide a rich range of information concerning surface structure. These methods are sensitive to atomic type, chemical state, and spin state. The theoretical prediction of diffraction patterns is also well developed at both the single scattering and multiple scattering levels, and quantitative fits of experiment to theory can lead to structures with accuracies in the ±0.03 A range. Direct structural information can also be derived from forward scattering in scanned-angle measurements at higher energies, from path length differences contained in scanned-energy data at lower energies, and from holographic inversions of data sets spanning some region in angle and energy space. Diffraction can also affect average photoelectron emission depths. Circular dichroism in core-level emission can be fruitfully interpreted in terms of photoelectron diffraction theory, as can measurements with spin-resolved core-spectra, and studies of surface magnetic structures and phase transitions should be possible with these methods. Synchrotron radiation is a key element of fully utilizing these techniques.

Kinetics and atomic structure of O adsorption on W(110) from Time-and State-Resolved Photoelectron Spectroscopy and Full-Solid-Angle Photoelectron Diffraction

We have studied the kinetics of the low-pressure adsorption of oxygen on W(110) via time- and chemical-state-resolved photoelectron spectroscopy (PS) and diffraction (PD). Using high-brightness third-generation synchrotron radiation from the Advanced Light Source, together with a new photoelectron spectrometer/diffractometer system, we are able to resolve four distinct chemical states in W 4f spectra (clean surface, bulk, W bound to two O atoms=O2, and W bound to three O atoms=O3) and to measure such spectra in about 20 s each so as to follow the kinetics of oxygen adsorption at 3×10−9 Torr from the clean surface to near saturation. The time-dependent transformations from one state of the surface W atoms to another have been determined at three temperatures of 298, 360, and 593 K. We also find that, for this adsorption pressure on our surface, no long-range-ordered structures are observable in LEED, even though the previously observed ordered structures of p(2×1), p(2×2), and (1×1)×12 are formed at higher pressures of approximately 10−6 Torr. The room-temperature state-resolved PS data are modelled using a simple Monte Carlo approach which assumes no mobility after molecular dissociation, and these calculations are found to describe the experimental data very well. Combining experiment and theory also permits deriving the sticking coefficient as a function of time, yielding results which agree with prior work. Full-solid-angle PD patterns have also been determined at the end of 298 K oxygen exposure for the O2 and O3 W atoms, and these have been analyzed using multiple scattering theory and R-factor analysis. The final local pseudo-threefold hollow geometries for oxygen are found to be very similar to those for a saturated one-monolayer structure of O on W(110) (the (1×1)×12 structure), including a lateral shift of O away from the position corresponding to three equal bond distances, but with some contraction of the OW vertical separation in going from O2 to O3 suggested. This study indicates considerable potential of such time- and state-resolved PS and diffraction for investigating surface reaction kinetics and structure, particularly for the large number of systems that do not exhibit long-range order and in view of future instrumentation improvements that should lead to much shorter data accumulation times and/or higher ambient pressures of measurement.

PHOTOELECTRON DIFFRACTION: SPACE, TIME, AND SPIN DEPENDENCE OF SURFACE STRUCTURES

The current status of photoelectron-diffraction studies of surface structures is briefly reviewed, and several recent developments and proposals for future areas of application are then discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low-energy electron-diffraction and scanning-tunneling microscopy, to epitaxial growth is considered. Several new avenues that are being opened up by third-generation synchrotron-radiation sources are also considered. These include photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements, and circular dichroism in photoelectron angular distributions. The addition of spin to the photoelectron-diffraction measurement is also considered, and can be achieved either through core-level multiplet splittings or by circular-polarized excitation of spin–orbit-split levels. This last development should make it possible to study short-range magnetic order, perhaps even in a holographic fashion.

Circular dichroism in core photoelectron emission from (1×1) oxygen on W(110): experiment and multiple-scattering theory

Abstract We have measured the circular dichroism in angular distributions (CDAD) of W 4f emission from a stable one-monolayer structure of oxygen on W(110) that is locally (1×1) in character. Photoelectrons were excited at kinetic energies of approximately 250 eV from the two chemically-shifted peaks present in these spectra (for bulk W and ‘oxide’ W bound to three O atoms) using right and left circularly polarized radiation (RCP and LCP, respectively), as well as linearly polarized radiation (LP) to provide a reference data set. Measurements were made over a large fraction of the solid angle above the surface, using a new beamline and experimental station at the Advanced Light Source. In switching between RCP and LCP, the data are found to show large CDAD effects of up to ±50%, as well as both peak ‘rotation’ effects (as first discussed by Daimon et al. in measurements on Si(100)), and more complex changes that are not so simply described. Multiple scattering diffraction calculations are found to very well reproduce all of the effects seen experimentally, and to represent a more accurate way of dealing with such effects than a prior peak rotation model. The LP data is also found to closely resemble the sum of the RCP and LCP intensities, and this is quantitatively explained. Such CDAD effects are thus expected to be generally seen in all work in solid materials, and important to allow for in studies of magnetic materials, with multiple scattering theory expected to yield a quantitative description of them.

Full solid angle photoelectron diffraction from bulk and surface atoms of clean W(110)

The authors have studied the atomic structure of the clean W(110) surface by means of site-resolved scanned-angle W4f{sub 7/2} photoelectron diffraction (PD) data obtained over nearly the full 2{pi} solid angle above the surface. Prior to the availability of high-brightness sources such as the Advanced Light Source, such large high-resolution data sets were prohibitively time consuming to obtain. The well characterized W(110) system was used as a reference case to check the accuracy of structure determinations from such scanned-angle data via R-factor comparisons of experiment with theoretical multiple scattering calculations. The photoelectron kinetic energy of {approximately}40 eV used was also lower than in many prior PD studies, providing further challenges to theory. The influence of various non-structural theoretical input parameters (e.g., scattering phase shifts, electron inelastic attenuation length, and inner potential) was thus also assessed. A final optimized structure is presented, together with comments on the future applications of this method.

Asymmetry of adatoms on Ge(111) observed by scanning tunneling microscopy: the clean c(2 × 8) and Sn-induced (7 × 7) structures

Abstract Scanning tunneling microscopy (STM) is used to study two types of structures formed on Ge(111): clean c(2 × 8) and the Sn-induced (7 × 7). For the clean annealed Ge(111) surface, we have found for the first time co-existent symmetric and asymmetric c(2 × 8) structures in separate domains with typical sizes of 400 A. Exposure to submonolayer amounts of Sn at high temperatures is found to form both (7 × 7) and (5 × 5) structures with domain sizes of 300 A. Empty-state images of the (7 × 7) reveal strong asymmetry among the adatoms in the dimer-adatom-stacking fault (DAS) model: the inner adatoms tend to form twelve-member rings around the corner holes, suggesting preferential Sn adsorption in these rings. The relationship of these observations to previous structural and spectroscopic studies of these surfaces is discussed.

Study of the oxidation of W(110) by full-solid-angle photoelectron diffraction with chemical state and time resolution

The brightness of third-generation synchrotron radiation from beamline 9.3.2 at the Advanced Light Source has been combined with the high-intensities and energy resolutions possible with its advanced photoelectron spectrometer/diffractometer experimental station in order to study the time dependence of the oxidation of the W(110) surface. This has been done via chemical-state-resolved core-level photoelectron spectroscopy and diffraction. This system has been studied previously by other methods such as LEED and STM, but several questions remain as to the basic kinetics of oxidation and the precise adsorption structures involved. By studying the decay and growth with time of various peaks in the W 4f{sub 7/2} photoelectron spectra, it should be possible to draw quantitative conclusions concerning the reaction kinetics involved. The authors have also measured full-solid-angle photoelectron diffraction patterns for the two oxygen-induced W states, and these should permit fully defining the different structures involved in this oxidation process.