UNSPECIFIED (1997) Alkali-promoted oxidation of Al(111):Rb/O and K/O coadsorption and the role of surface structure. Surface Science, 391 (1-3). pp. 300-314. ISSN 0039-6028

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Abstract

Synchrotron radiation core-level photoemission from the Al 2p, K 3p and Rb 4p states has been used to characterise the role of preadsorbed Rb and K on the interaction of oxygen with an Al(111) surface, Specific precoverages have been used corresponding to two different (root 3 x root 3)R30 surface structures, a metastable low temperature phase involving alkali atoms in atop sites, and a stable higher temperature phase with substitutional alkali atoms. In all cases a significant promotion of both dissociation and oxidation is seen relative to the activity of the clean Al(111) surface. In comparison with earlier results for Na/O coadsorption Rb is found to promote oxidation most strongly and Na least strongly with K being intermediate, the Rb room temperature substitutional phase, in particular, show oxidation at the lowest oxygen exposures and no indication for the Rb-O chemisorption precursor comparable with the Na-O one identified on the Na-covered surface. By contrast the Rb atop and K-atop geometry surfaces do show evidence of some discrete chemisorption states in the Al 2p spectra of the type seen on alkali-free Al(111), but only in the presence of other spectral structure assigned to mixed-coordination geometrics. At low temperatures the effect of both Rb and K on oxidation, but not on initial oxygen adsorption, is generally suppressed, an effect ascribed to the role of bulk diffusion. Measurements of normal incidence X-ray standing wavefield absorption for the Rb/O coadsorption structures at very low oxygen exposure also indicate that no simple single sites are occupied, particularly in the case of the more reactive Rb-substitutional phase at room temperature, although there appears to be a relatively well-defined O-Al layer spacing attributed to small but laterally incommensurate oxide islands. Measured work function changes at low oxygen exposure in the Rb/O and Na/O systems can be reconciled with oxygen penetration of the alkali layer except For the Na-substitutional phase, for which the data are qualitatively consistent with the previously reported atop geometry. (C) 1997 Elsevier Science B.V.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry Q Science > QC Physics
Journal or Publication Title:	Surface Science
Publisher:	ELSEVIER SCIENCE BV
ISSN:	0039-6028
Date:	26 November 1997
Volume:	391
Number:	1-3
Number of Pages:	15
Page Range:	pp. 300-314
Publication Status:	Published
URI:	http://wrap.warwick.ac.uk/id/eprint/16184

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