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Scanned-energy mode photoelectron diffraction (PhD) has been used to determine the adsorption geometries and structural parameters of a selection of molecular adsorption systems. Furan on Pd(111) has been investigated with the support of 0 K-edge near-edge X-ray absorption fine 'structure (NEXAFS) measurements. Adsorption at temperatures below 160 K is molecular, with decomposition of the molecule occurring after heating the surface to 340 K via the reaction: C4H40 -+ CO + C3H3 + H. Low coverage molecular furan phase NEXAFS measurements determined that the plane of the molecule is approximately parallel to the surface, in agreement with previous scanning tunnelling microscopy (STM) measurements. C 1s PhD measurements of C which is bonded to the 0 heteroatom (denoted aC) were used in the identification of several adsorption geometries, all with the aC atoms in off-atop sites. The results show agreement with those of a previous density functional theory (OFT) study. C 1s measurements found that CO bonds via the C atom, equally occupying both the fcc and hcp hollows. The structural parameters determined show good agreement with a previous PhD study of pure CO on Pd(111). For C3H3, PhD analysis found two possible structures within experimental variance. Comparison of the structural parameters revealed that C3H3 adsorbs with the molecular plane parallel to the surface, centred over an hollow site. The adsorption geometry of S02 and S03 on Ni(111) has been investigated. S 2p and 0 1s measurements were obtained for S02 at 145 K, which determined that the S atom adopts an off-atop site, with the 0 atoms situated between atop and hollow sites, in general accord with previous experimental studies. 0 1s PhD measurements for S03, prepared by heating the S02 covered surface to 210 K, identified an adsorption geometry in which one 0 atom is off-atop with two 0 atoms in off-bridge sites and the S atom centred above. The adsorption geometry determined is different to the results of OFT and normal incidence X-ray standing waves (NIXSW) studies, however the geometry found here identifies an alternative fit of the NIXSW data. C 1s PhD measurements were made to determine the adsorption geometry of CCl2 on Ag(111), prepared by heating a CCl4 covered surface to 198 K. The strongest modulations were observed at grazing emission angles suggesting adsorption in the hollow sites. Extensive analysis failed to yield a geometry which showed a good fit between theory and experiment in inequivalent azimuths. Examination of the experimental data revealed that the presence of spectral features, in particular the CI (LMM) Auger peak, led to modulations with unphysical characteristics precluding reliable structural determinations.

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