A. M. Bradshaw

Adsorbate structure determination on surfaces using photoelectron diffraction

Photoelectron diffraction is the name given to the phenomenon resulting from the coherent interference of the directly emitted component of an electron wavefield, emerging from an atom as a result of core level photoemission, with other components elastically scattered by surrounding atoms. Experimental characterization of this effect provides information which can be used to provide quantitative determinations of the structure of surfaces, and particularly of adsorbed species on surfaces, in an element-specific fashion. Since the initial. Demonstration of the phenomenon in the late 1970s, an extensive methodology for surface structure determination has been developed. In this review the background physics of the process, and the development of the technique is described. A brief discussion of the high energy forward scattering version of the technique (X-ray photoelectron diffraction-XPD), which utilizes zero-order diffraction effects, is included, but the most of the review is concerned with the lower energy backscattering method more relevant to the determination of detailed adsorption sites on surfaces. In addition to the general theoretical, experimental and methodology background, a number of the more recent developments are described including use of direct inversion methods for (approximate) structure determination, including a survey of photoelectron holography, and the realization of chemical shift photoelectron diffraction to allow structure determinations of surfaces including atoms of one element in more than one inequivalent site. All of the developments are illustrated with specific examples, mainly of molecular and atomic adsorbates on metal surfaces.

Reaction of molecular oxygen with C60: spectroscopic studies

Using both freshly sublimed powders and evaporated thin films, the reactivity of C60 towards molecular oxygen has been demonstrated using the techniques of X-ray diffraction, X-ray absorption, photoemission and infrared spectroscopy. At ambient temperature a molecular intercalation compound is formed, characterised by oxygen-rich surface and sub-surface regions. At higher temperatures oxidation to CO2 is preceded by the formation of various carbon suboxide intermediates, as reported in the literature.

On the complex structure of reaction-diffusion fronts observed during CO oxidation on Pt{100}

Abstract In the catalytic oxidation of CO on Pt{100}, we have been able to identify, using small-area LEED, the different adsorbate-covered regions which constitute the reaction-diffusion fronts observed in photoelectron emission microscopy. The resulting concentration profile is simulated in one dimension with the help of a simple two-variable model derived from the Langmuir-Hinshelwood mechanism. The characteristic features of the experiment arc reproduced if it is assumed that the CO diffusion coefficient is strongly enhanced (by a factor of 7) at coverages corresponding to the (√2 × √2)R45°-CO phase and reduced by surface defects.

Determination of the local adsorption structure of acetylene on Ni(111)

Abstract Scanned-energy mode C is photoelectron diffraction has been applied to the adsorption system of acetylene on Ni(111) to determine the local adsorption structure which is compared with the results of a similar study of acetylene on Cu(111). The adsorption site in both cases is with the molecule essentially parallel to the surface and in a crossed bridge site such that the two C atoms of the molecule occupy inequivalent hollow sites. A substantial, essentially identical stretching of the Cue5f8C bond relative to the gas phase is found for both surfaces (consistent with the interpretation of vibrational spectroscopic data), despite the marked difference in chemical reactivity of Cu(111) and Ni(111) towards hydrocarbons.

Experimental tests of new direct methods for adsorbate structure determination using photoelectron diffraction

Two recently proposed methods for approximate but direct adsorbate site determination using scanned‐energy mode photoelectron diffraction data have been tested on experimental data from three different adsorption systems. The three systems studied, O, CO, and C2H4 on Ni(111) involve three different local adsorption sites (single hollow, mixed hollows and near‐atop) and have all been subjected to separate full quantitative structure determinations. Both the simple Fourier transform approach of Fritsche and Woodruff, and the projection method of Hofmann and Schindler are found to correctly identify all the adsorption sites. The precision of these approximate structure determinations, and the size of the data bases used, are compared with each other and with other proposed methods.

Photoelectron diffraction investigation of the adsorption site and local structure for potassium on Ni(111)

Abstract A recent LEED study of the Ni(111)(2 × 2)-K structure indicates that this is one of a small number of examples of alkali metal adsorption on close-packed metal surfaces now believed to involve atop adsorption. In order to provide an independent test of this surprising result we have conducted scanned energy mode photoelectron diffraction experiments on the K2s and 2p emission at a range of polar emission angles. A simple Fourier transform mapping of the nearest neighbour contribution to these spectra provides direct evidence for the atop adsorption site, and subsequent theory-experiment comparisons using multiple scattering calculations confirm this site and give a nearest neighbour bondlength of 2.86 0.03 A, essentially identical to the LEED result (2.82 ± 0.04 A). The new results also indicate a significant contraction (0.17 ± 0.06 A) of the outermost Ni atom layer, but find essentially none of the “rumpling” obtained in the LEED study.

Structure Determination of Molecular Adsorbates Using Photoelectron Diffraction

This chapter is concerned with photoelectron diffraction in both its scanned-energy and scanned-angle forms. The phenomenon itself and its possible use as a structural tool was first described by Liebsch in 1974 in a theoretical study [4.1]. The first experimental studies were performed independently by three groups in 1978 [4.2-4]. As in Surface Extended Absorption Fine Structure (SEXAFS) the technique is based on the interference of photoemitted electron waves and does not have the requirement of long-range order, but there are distinct differences between the two experiments, as will be discussed below. The scanned energy form (referred to here as PhD) also requires the use of synchrotron radiation, whereas the scanned angle variant at higher photoelectron kinetic energies can be performed with laboratory sources. The latter is often referred to as XPD (X-ray Photoelectron Diffraction), but in the case of adsorbed molecules is largely restricted to the determination of bond directions. The scanned energy form of the method, on the other hand, has the potential of delivering much more quantitative geometric information and, in some cases, even providing an almost complete structural characterisation [4.5, 6]. The present chapter therefore concentrates on the scanned-energy experiment and the reader is referred to two recent review articles for more exhaustive accounts of XPD [4.7, 8].

The potassium-induced reconstruction of Cu{110}: the K atom adsorption site

Abstract Using scanned energy mode photoelectron diffraction we show that in the potassium-induced reconstruction of the Cu{110} surface the K atoms occupy four-fold hollow sites formed by second layer Cu atoms. This same site, which is the key to the reconstruction, is observed both at low coverages, where the reconstruction is local in nature, and at coverages corresponding to the (1 × 3) and(1 × 2) missing row structures.

The adsorption site of pyridine on Ni{111} determined by low-energy photoelectron diffraction

Abstract The adsorption site of pyridine (C 5 H 5 N) on Ni{111} has been investigated by scanned-energy mode photoelectron diffraction using the N 1s core level. It was found that the nitrogen atom adsorbs close to the atop position with an angle of 18° ( + 2° − 4°) between the Nue5f8Ni bond and the surface normal. The Nue5f8Ni bond length is 1.97 ± 0.03 A. Since recent X-ray absorption studies show that the π system of the pyridine is also tilted by about 20° away from the surface normal, we conclude that the Nue5f8Ni bond and the ring plane are co-planar.

Direct methods for adsorbate structure determination using photoelectron diffraction

Abstract We describe the application of two direct methods for the transformation of scanned-energy mode photoelectron diffraction spectra which are capable of providing useful adsorption site identifications, and which form part of an integrated methodology for quantitative adsorbate structure determination. We draw some comparisons between the principles and proven success of this approach and that of ‘photoelectron holography’. Specific examples given include the adsorption sites of intact and partially reacted ammonia-derived species on Cu(110), the methoxy species, CH 3 O-, on Ni(111), and the growth on Fe on Cu(111).