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 elementspecific 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 diffrao tion 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.

The K-shell excitation spectra of the hydrides water, ammonia, and methane have been measured in photoabsorption experiments using synchrotron radiation in combination with a high-resolution monochromator. For the case of methane, in particular, a wealth of spectral detail is observed which was not accessible in previous studies. The measured excitation energies and relative intensities compare well with values calculated using a complete second-order approximation for the polarization propagator. In order to determine the extent of admixing of valence excitations (i.e., transitions into virtual 0 orbitals) to the Rydberg manifolds, the X-H bond lengths have been varied in the calculations. In the case of H20, the two lowest-energy bands are due to the 0 1s-4a&/3s and 0 1s-2b2/3p transitions and have strong valence character; their width indicates that both excitations are dissociative. The NH3 and ND3 spectra are also broad which is not only due to possible dissociation but also to unresolved vibrational fine structure (v2 mode) and a Jahn-Teller instability. Valence character is concentrated in the lowest excited state in the Rydberg ns manifold, but is distributed more uniformly over the np(e) manifold. The weak dipole-forbidden C 1s -3s ( a & ) transition in CH4 and CD4 is accompanied by vibrational structure due to the v4 mode, indicating that it derives its intensity from vibronic coupling with the C 1s-3p(t2) transition. The structure on the latter band is extremely complicated due to Jahn-Teller coupling and cannot be assigned at present, as is the case for the Rydberg transitions at higher energies. The higher np Rydberg excitations contain considerable valence character. PACS number(s): 33.20.Rm 35.20. -i I. INTRQDUCTIDNE-shell excitation of the CH4, NH3, and H20 molecules has been studied in the past using both electronenergy-loss spectroscopy (EELS) [1 -4] and x-ray absorption spectroscopy [5 -9). Although the spectral resolution obtained with the latter technique has generally been rather modest and could not match that of EELS, recent advances in grazing-incidence monochromator design [10 -14] have resulted in a dramatic improvement of resolution with the "optical" approach. This has been shown in a number of high-resolution K-shell excitation spectra of small molecules reported recently (e.g. , ). In the present paper we describe measurements of the photoabsorption spectrum of gas phase H20, NH3, and CH4 in the near-E-edge region at a resolution sufficiently high to reveal directly the spectral line shape. To substantiate the assignments we have also calculated excitation energies and oscillator strengths using a polarization propagator method [20].In the virtual orbital spectrum of these hydrides and other saturated molecules one expects antibonding valence-type (cr ) orbitals which are the counterparts of the occupied bonding X-H orbitals. An important question is to what extent this antibonding valence character appears in the excitation spectrum. In most of the earlier E-shell studies on the h...

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