The highly tilted a 3 state of CO on Fe(OOl) has been studied using combined polar and azimuthal xray photoelectron diffraction. A more accurate structural picture is derived, with the molecule being tilted at an angle of 55° ± 2° with respect to the surface normal along <100> azimuths and probably occupying fourfold hollows. An estimate is also made of the vertical C distance above the Fe surface. The utility of combined 0/0 x-ray photoelectron diffraction measurements for such unusual adsorbate structures is demonstrated.The existence of an "abnormal" or "unusual" type of surface bonding state of CO that involves a tilted or lying-down geometry and a considerably lowered CO stretch frequency has been discussed recently for several different transition-metal surfaces including Fe(OOl), 1 " 3 Cr(llO), 4,5 and Mo(OOl). 6 This state is of considerable interest because it may represent a precursor to dissociation with a weakened CO bond due to additional electronic interactions with the metal substrate. x However, little is known about the exact geometric structure of the CO for any of these cases. For the case of a^ CO on Fe(001) that we will consider here, electron-energy-loss (EELS) measurements show a very low CO stretch frequency of 1210 cm -1 . 1 Near-edge x-ray-absorption fine-structure (NEXAFS) measurements on this state have been used to deduce an elongation of the CO bond by 0.07 ± 0.02 A from a shift in the position of the a resonance. 3 From the relative lack of a change in the absolute and relative intensities of the n and a resonances with a change in radiation polarization, an estimated tilt angle with respect to normal of 45° ± 10° has been deduced. 3 However, the precision of this determination is not high, and it also was not possible in this study to specify whether there is any preferred azimuthal orientation of the tilt. Measurements of electron-stimulateddesorption ion angular distributions (ESDIAD) are consistent with such a high tilt in showing a very weak signal for 0 + , but no bond angle information could be derived, probably due to the strong image forces which deflect and attenuate the desorbing ions. 2 We have thus applied x-ray photoelectron diffraction (XPD) for the first time to this intriguing adsorption state in an attempt to better understand its structure, combining both polar and azimuthal scans of core-level intensity in a way that has not been attempted before for such adsorbate structure studies. This method makes use of the dominance of highly-peaked forward scattering in core-level emission at energies of greater than approximately 500 eV. 7 " 10 As these peaks have typical FWHM values of only about 20°, the precision of bond tilt determinations should be considerably higher than in measurements such as NEXAFS where the dependence on variable polarization goes as a more slowly varying cos 2 function. 3,810 Also, a simple single scattering cluster (SSC) model has been shown in prior studies to model very well this kind of intramolecular scattering, 7 " 10 although useful str...

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We consider the accuracy of holographic inversions of electron emission data at ~10 3 eV to yield atomic positions. Theoretical calculations for small clusters of 2-5 atoms in both single and multiple scattering show that self-interference effects may be present, and that forward-peaked scattering tends both to shift peaks by as much as 0.5-1.0 A from the true positions and to elongate images along forward-scattering directions. However, eliminating forward-peaked intensities before inversion significantly improves image quality.

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Multiple-scattering effects in Auger electron diffraction and photoelectron diffraction are assessed using a new spherical-wave multiple scattering formalism which is based on a separable approximation to the scattering Green's function. Results obtained in both single-scattering and fullyconverged multiple-scattering limits are presented for linear chains of Cu atoms. Simulations of some Auger electron diffraction and photoelectron diffraction experimental results with this new theory are also presented for the first time.