Surface Reflectance Core Spectroscopy of Halogens, Rare Gases, Alkalis and Carbon-Monoxide on Metal Surfaces
Cunningham, John Edward, Jr.
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https://hdl.handle.net/2142/77185
Description
Title
Surface Reflectance Core Spectroscopy of Halogens, Rare Gases, Alkalis and Carbon-Monoxide on Metal Surfaces
Author(s)
Cunningham, John Edward, Jr.
Issue Date
1980
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
Reflectivity spectra for the halogens, rare gases, alkalis and the CO molecule adsorbed on metal and insulating surfaces are described. This research employed synchrotron radiation in the energy range from 4 to 20 eV, using both s and p polarized light. Results for adsorbates in three neighboring columns of the Periodic Table principally reveal charge transfer processes and properties of the impurity surface complex.
The excitation spectra for various halogen coverages on the surfaces of magnesium and aluminum have been investigated. The halogen ion is detected at dilution. A transition to the neutral molecular configuration occurs at higher coverages. The observed ionic excitation thresholds occur near the theoretically predicted values for a model in which the halogen ion exists stably bound to the metal surface. Excitation profiles reveal Fano type processes which may be associated with large charge transfer integrals coupling the adsorbate to the substrate.
A thorough investigation was made of rare gases adsorbed on four chemically dissimilar substrates. The spectra depend strongly on the particular rare gas-substrate combination. Processes specifically related to the final state configuration of the excited rare gas atom either open or close the optical charge transfer channel to the substrate. Distinctly different profiles characterize the two cases. A simple model is proposed which consistently describes the experimental results. Excitations of rare gases lacking charge transfer processes reveal weak coupling to the substrate.
Core spectra of cesium on the surfaces of magnesium and aluminum are examined. Transitions from the Cs('+) ionic configuration to a neutral atomic configuration on magnesium are found to occur with increasing coverage. The second Cs monolayer has a spectrum that resembles that of bulk Cs. Modern theories of deep level core responses are not compatible with the observed profiles. Results are qualitatively explained by the local environment alone.
Spectra for the CO molecule on the surfaces of gold, aluminum and aluminum oxide are reported. Spectra obtained for metal surfaces are inconsistent with classical models relating fractional reflectivity changes to adlayer optical constants. Line shapes are different for s and p photon polarizations. Orbital symmetry and the molecular orientation properties can be used to explain these observations.
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