SEXAFS and NISXW studies of adsorbates on transition metal surfaces

The synchrotron radiation based techniques of Surface Extended X Ray Absorption Fine Structure (SEXAFS) and Normal Incidence Standing X Ray Waves (NISXW) have been applied to Four adsorption systems. These involved atomic and molecular chemisorption on the low index planes of single crystal C'u and Ni surfaces. For the Cu(lll)(\/3 X v3) R30 system the NISXW technique has found a layer spacing of 1.81 ± 0.05A, this was consistent with an earlier result found by SEXAFS. For the Ni(100)c(2 X 2)Hg system the combined use of NISXW and SEXAFS was attempted. The SEXAFS study, however, was unsuccessful due to the difficulties of obtaining SEXAFS from core states of high angular momentum. These difficulties were attributed to delayed onset of photoemission from the mercury Mr edge and strong variations of the pure atomic X ray absorption coefficient in the EXAFS energy range. NISXW studies were more successful and yielded an adsorbate - substrate layer spacing of 0.60 ± 0.1A. This layer spacing is consistent with adsorption into the two fold bridge site at the Ni(100) surface rather than the more highly co-ordinated four fold site which might have been expected for a metallic bond. There are, however, some ambiguities in this assignment due to the lack of corroboration of this result by SEXAFS. For the remaining two systems the combined use of SEXAFS and NISXW was successful. The C'u( 111) C'H3S system was typical of most molecular chemisorption systems in that it possessed no long range order. For this system both the NISXW and SEXAFS techniques gave results indicative of reconstructive chemisorption. A corroborative interpretation of results for both techniques invlolved the adsorption of the CH3S group's sulphur atom into distorted three fold hollow sites at the Cu(lll) surface.Intra-inolecular scattering was also observed in this system. This scattering was between the carbon and sulphur atoms in the mercaptide (C'H3S) group, and SEXAFS analysis found a C-S bondlength of 1.88.4. This appears to be the first recorded case of intra- molecular^scattering in SEXAFS data. Simple chemisorption is also ruled out for the (v7 A v 7) R19.I -S on Cu(lll) surface. SEXAFS data for this system, like that for mercaptide, was dominated by a single shell and analysis of the polarization dependence of the SEXAFS amplitude for this shell gave an adsorbate-substrate bond angle, [3, of 67 ± 5°. This value is well outside the range expected for simple non reconstructive chemisorption of S into the high or low symmetry adsorption sites of the C'u( 111) surface. The NISXW data for this system was suggestive of a mixture of at least two distinct adsorbate- substrate layer spacings. One of which was well outside the range of values expected for simple chemisorption. The data from both experimental techniques can only be explained in terms of reconstructive chemisorption which forms a surface sulphide involving a mixed layer of C'u and S. This sulphide is believed to be of distorted tetrahedral co-ordination. This model can explain the two distinct adsorbate-substrate for S as well as the observed polarization dependence of the Cu-S bondlength.