D. K. Saldin

SEXAFS without X-rays

Abstract Surface extended X-ray absorption fine structure provides a means of finding the structure of disordered adsorbate layers on a clean substrate, low energy electron diffraction techniques having been confined to well-ordered systems. Here we show how the diffuse patterns characteristic of disordered adsorbates can be analysed to yield structural information. The theory contains terms which are entirely analogous to SEXAFS theory, showing that all the information contained in a SEXAFS experiment is present in a diffuse scattering measurement. Not only that, but the amount and detail of information available for diffuse scattering is much greater than for SEXAFS. The interpretation of diffuse scattering is no more complex than for LEED from the clean well-ordered substrate.

A near-edge X-ray absorption fine structure and photoelectron spectroscopic study of the structure of acetylene on Pd(111) at low temperature

Abstract The structure of acetylene adsorbed on Pd(111) below ∼ 200 K is probed using near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet and X-ray photoelectron spectroscopy. The NEXAFS spectra are calculated using a one-electron cluster formalism corrected by an energy-dependent exchange contribution. Experimental spectra agree well with results calculated for an acetylene molecule adsorbed in a three-fold hollow site with a CCH angle of ∼ 117°, a C-C bond length of ∼ 1.3 A and with its H-C-C-H plane tilted slightly at ∼ 22° from normal to the Pd(111) surface. The thermal removal of carbon from the surface is measured using X-ray photoelectron spectroscopy by monitoring the C 1s signal intensity. These results indicate that ∼ 35% of the surface carbon is removed by heating to 600 K. This is the temperature range over which benzene desorption is detected in thermal desorption spectroscopy implying that ∼ 35% of the acetylene initially adsorbed at ∼ 90 K converts to benzene. In addition, a shift in the C 1s peak position on heating to between 200 and 300 K implies the conversion of the adsorbed acetylenic molecule into another surface species.

Adsorption and reaction of CO2 on Ni{110}: X-ray photoemission, near-edge X-ray absorption fine-structure and diffuse leed studies

Using three different techniques X-ray photoemission (XPS), near-edge X-ray absorption spectroscopy (NEXAFS) and diffuse LEED we have studied the adsorption and reaction of CO, on Ni(ll0). In agreement with previous angle-resolved photoemission (ARUPS) and vibrational electron energy loss (EELS) data both a linear, physisorbed molecule and a bent, chemisorbed species CO,Sare found. An evaluation of the XPS line intensities shows that the stoichiometry of the chemisorbed species is 1 to 2 in carbon and oxygen. The polarisation dependence of the NEXAFS indicates that the physisorbed molecule lies with its axis parallel to the surface and that the molecular plane of the CO;species is perpendicular to the surface. There is no clear preferential azimuthal orientation. This is in agreement with a diffuse LEED analysis where equal numbers of bent molecules adsorbed on atop sites and oriented along the (100) and (110) directions gave the best fit to the data.

Crystallography without crystals. I. The common- line method for assembling a three-dimensional diffraction volume from single-particle scattering

It is demonstrated that a common-line method can assemble a three-dimensional oversampled diffracted intensity distribution suitable for high-resolution structure solution from a set of measured two-dimensional diffraction patterns, as proposed in experiments with an X-ray free-electron laser (XFEL) [Neutze et al. (2000). Nature (London), 406, 752-757]. Even for a flat Ewald sphere, it is shown how the ambiguities due to Friedels law may be overcome. The method breaks down for photon counts below about 10 per detector pixel, almost three orders of magnitude higher than expected for scattering by a 500 kDa protein with an XFEL beam focused to a 0.1 microm diameter spot. Even if 10(3) orientationally similar diffraction patterns could be identified and added to reach the requisite photon count per pixel, the need for about 10(6) orientational classes for high-resolution structure determination suggests that about 10(9) diffraction patterns must be recorded. Assuming pulse and readout rates of approximately 100 Hz, such measurements would require approximately 10(7) s, i.e. several months of continuous beam time.

Adsorbate induced reconstruction phase p(2 × 2)O/Ni(100)

Abstract The adsorption system of p(2 × 2)O/Ni(100) was reinvestigated by a detailed LEED analysis, whereby the purity of the p(2 × 2) phase was controlled by HREELS. Full dynamical calculations were performed allowing for both substrate reconstruction and relaxation which both prove to be essential to yield a satisfying theory-experiment fit. Oxygen resides in the fourfold Symmetrie hollow site at 0.80 A height above the first substrate layer whose distance from the second layer is expanded by 2.5% compared to the bulk distance. The second substrate layer is considerably buckled by as much as 0.10 A. The determined adsorption height as well as the surface relaxation are in agreement with the results of semi-empirical total energy calculations and are consistent with a recent structure determination of the c(2 × 2) phase.

The theory of the excitation of atomic inner-shells in crystals by fast electrons

Abstract We review the theory of the excitation of electrons from atomic inner shells in crystals as relevant to microanalysis in the transmission electron microscope and suggest several improvements and generalizations. The orthodox theory, originating with Kainuma, represents the fast electrons as Bloch waves and the core and ejected electrons as tight-binding wavefunctions. For several reasons we consider it more satisfactory to represent the core electron as a purely atomic wavefunction and the ejected electron as a sum of spherical waves centred on the atom in question. This formulation is ideally suited for generalization to calculate fine-structure effects such as those observed in extended electron energy-loss fine structure and electron energy-loss near-edge structure. We show that the apparent lack of momentum conservation in the excitation process, commented on by Maslen and Rossouw (1983) and Meekison and Whelan (1983) has a simple explanation based on the character of the core wavefunction.

On the mean inner potential in high- and low-energy electron diffraction

Abstract New electron microscopies spanning the energy range from millivolts to kilovolts image different beam-energy-dependent potentials. We review the theories of the average value of the mean inner potential of a solid in order to understand its role in image interpretation and its relationship to charge density and atom positions in crystals. Three common definitions of this quantity are compared, that from high-volatge transmission electron interferometry, the theory of surface dipole layers and the work function, and that from low-energy electron diffraction (LEED). Energy-dependent corrections due to exchange and virtual inelastic scattering are proposed for LEED, photoelectron holography (PEH), the low-energy electron microscope (LEEM) and the point-projection microscope (PPM) operating in the sub-kilovolt range. A calculation of the mean free path of low-energy electrons as a function of their energy is also reproduced.

NEXAFS identification of a catalytic reaction intermediate: C4H4 on Pd(111)

Abstract Polarisation-dependent NEXAFS data are presented for the reaction intermediate C 4 H 4 chemisorbed on Pd(111). Comparison of these data with calculated spectra indicates that this species is a metallocycle with slightly expanded ( ~ 0.05 A) C-C bonds. Dependence of the resonance intensities on photon incidence angle shows that the molecular plane is tilted at ~ 35° with respect to the metal surface. The assigned chemical identity and absorption geometry are fully consistent with all the reactive behaviour of C 4 H 4 .

Structure of a single particle from scattering by many particles randomly oriented about an axis: toward structure solution without crystallization?

In this paper is demonstrated a complete algorithm for determining the electron density of an individual particle from diffraction patterns of many particles, randomly oriented about a single axis. The algorithm operates on angular correlations among the measured intensity distributions. We also demonstrate the ability to recover the angular correlation functions of a single particle from measured diffraction patterns.

Three-dimensional single-particle imaging using angular correlations from X-ray laser data

Femtosecond X-ray pulses from X-ray free-electron laser sources make it feasible to conduct room-temperature solution scattering experiments far below molecular rotational diffusion timescales. Owing to the ultra-short duration of each snapshot in these fluctuation scattering experiments, the particles are effectively frozen in space during the X-ray exposure. In contrast to standard small-angle scattering experiments, the resulting scattering patterns are anisotropic. The intensity fluctuations observed in the diffraction images can be used to obtain structural information embedded in the average angular correlation of the Fourier transform of the scattering species, of which standard small-angle scattering data are a subset. The additional information contained in the data of these fluctuation scattering experiments can be used to determine the structure of macromolecules in solution without imposing symmetry or spatial restraints during model reconstruction, reducing ambiguities normally observed in solution scattering studies. In this communication, a method that utilizes fluctuation X-ray scattering data to determine low-resolution solution structures is presented. The method is validated with theoretical data calculated from several representative molecules and applied to the reconstruction of nanoparticles from experimental data collected at the Linac Coherent Light Source.