Molecular overlayers on homogeneous and heterogeneous metal surfaces studied by core-level photoemission
Abstract: The main topic of this thesis is the investigation of small molecules adsorbed on metal surfaces by means of core-level photoemission spectroscopy. The thesis consists of three parts; The first group of papers concerns the effects of internal molecular vibrations on the core-level photoemission spectra. The second and third groups of papers concern more basic adsorption properties, like adsorption energies, adsorption sites, diffusion and dissociation on homogeneous and heterogeneous surfaces, respectively. The influence on the C 1s spectra of photoemission induced internal vibrations in some hydrocarbons, i.e. methoxy adsorbed on Cu(100), gas-phase methanol, and ethylidyne adsorbed on Rh(111) and Pd(111), is presented. In the C 1s photoemission spectra from all of these hydrocarbons the C-H stretch vibration is resolved. It is shown that the characteristics of the C-H vibrational progression can serve as a fingerprint of which kind of hydrocarbon group is present on the surface under investigation. Furthermore, the ethylidyne/Rh(111) spectra were measured with sufficient energy resolution to resolve a second vibrational mode, which is believed to be the C-C stretch vibration. The C-O stretch vibration in CO molecules adsorbed on metal surfaces has a vibrational energy which is about half that of the C-H stretch in hydrocarbons. The properties of this vibrational mode in core-ionized CO adsorbed on Rh(111) and Pd(111) are presented. The C-O vibrational properties on the Rh(111) surface are shown to depend on CO adsorption site. The core-ionized vibrational energy was found to decrease with increasing coordination to the surface, in the same manner as for the vibrational energy of the neutral CO molecule. In this CO on Rh(111) study a new C 1s component, not reported before, was found at intermediate CO coverages, with a binding energy in between that of the previously known on-top and three-fold hollow components. The coverage dependent isosteric heat of adsorption (adsorption energy) was derived for CO on Rh(111) by a Clausius-Clapeyron analysis of photoemission data measured for a large range of different CO coverages and sample temperatures. The adsorption energy was found to decrease from just above 1.5 eV/molecule to ~1.3 eV/molecule as the coverage is increased from 0.18 ML to about 0.3 ML. Above 0.3 ML the adsorption energy levels off at a constant value of about 1.3 eV/molecule. In the study of CO adsorbed on Pd(111) the coverage dependent adsorption sites at different sample temperatures were derived. At 120 K it was found that CO adsorbs in three-fold hollow sites at low CO coverages, while at coverages close to saturation both on-top, bridge and three-fold hollow sites are occupied. At 300 K, the three-fold hollow site dominates at low coverages, whereas the bridge site becomes more important at high coverages. At this temperature, no on-top adsorbed CO molecules are found. In the CO/Mo(110) study it was shown that CO molecules with different tilt angles with respect to the surface normal appear as different components in the C 1s spectra. Steric effects are argued to cause the thermally induced dissociation of CO to start at a lower temperature for CO overlayers with low coverage than for the saturation coverage overlayer. The studies of CO adsorption on heterogeneous surfaces, consisting of 0.5 layers of Pd on Rh(111), showed that CO diffuse to the parts of the surface having the largest adsorption energy at a particular CO coverage. It was demonstrated how the diffusion could be manipulated by predosing the surface with oxygen. In addition, the heterogeneous surfaces consisting of 1.x layer of Pd on Mo(110), x = 1, 3, and 5, were studied. It was shown that a temporary mobile state exists at room temperature on the first layer Pd areas of the surface, even though there is no stable adsorption site on these parts. Independent of the size of the second layer islands, the CO adsorption rate is approximately constant up to coverages close to saturation for each overlayer. That is, up to saturation nearly all CO molecules, which impinge on the surface, end up being adsorbed on the second layer Pd islands, either by direct adsorption or by diffusion from the first layer parts.
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