Ainsworth, Simon (1985) Surface EXAFS (extended X-ray absorption fine structure) studies of adsorbates on surfaces. PhD thesis, University of Warwick.

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Abstract

This thesis describes detailed SEXAFS studies of iodine adsorption onto Ni(100), and some preliminary chlorine-Cu (111) studies. The main conclusions axe as follows:-

The standard analysis technique (i.e. the method of deducing atomic structural information from measures of X-ray absorption coefficients) - the Fourier filtering approach - has potential problems. For short range spectra, a second shell can 'frequency' modulate the main oscillation giving incorrect phase shifts and bond lengths. Large ΔEₒ's compensating for erroneous phase shifts produce Δr errors up to 0.1 Ǻ. A bond length derivation is not the weighted mean of two contributing bond lengths. A Fourier transform modulus is only a 'frequency' spectrum; discretion is needed in relating this to a radial distribution function and ‘Ramsauer-Townsend effects' which should rarely be observed. A multi-shell modelling approach is useful for SEXAFS analyses. A surface NiI₂ structure is confirmed as being a slice of bulk Nil₂ on Ni(100); for θ = ½ I-Ni (100), adatoms lie in hollow sites; for θ = 3/8 and 1/3, they lie in hollow, bridge, or some mixture of such sites. The I-Ni bond length remains 2.78 Ǻ. The accuracy limitation of higher shell distances set by the single ΔEₒ optimisation is not problematic, since generally the nearest shell dominates. There is no difference in I-Ni chemical bonding throughout the coverages; charge transfer after excitation causes large ΔEₒ' s.

Experimental problems limited the quality of Cl-Cu SEXAFS spectra, but some results could be obtained: For θ = 1/3, Cl adatoms lie in hollows, Cl-Cu = 2.39 (± 0.03) Ǻ; for θ = 0.45, adatoms lie in mixed co-ordination sites, bond length 2.33 (± 0.05) Ǻ, similar to the 2.34 Ǻ in bulk CuCl. A spline is a better estimation of the atomic absorption background, μo ; polynomials can produce significant bond length errors.

Finally, the deduced structures are compared with present understandings of surface chemical bonding.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Surface extended X-ray absorption fine structure, Absorption spectra, Iodine -- Analysis
Official Date:	February 1985
Dates:	Date Event February 1985 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Woodruff, Phil
Sponsors:	Science and Engineering Research Council (Great Britain) ; British Petroleum Company
Extent:	xx, 278 leaves : illustrations
Language:	eng

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