Surface structure determinations using X-ray absorption spectroscopy

Abstract

Structure determinations using surface extended X-ray absorption fine structure (SEXAFS) are reported which give insight into two quite different fields of surface-science research. These are alkali-metal adsorption on Al(111) and the adsorption of diatomic low Z molecules on Ni surfaces. SEXAFS data for the ordered low- and room-temperature Na phases on Al(111) are presented which led to the discovery of the reconstruction of the Al surface by room-temperature Na adsorption and confirmed the formation of a binary surface alloy at higher Na coverages between otherwise immiscible Na and Al atoms. These experimental results contradict the traditional view of alkali-metal adsorption on close-packed metal surfaces. They are compared, where possible, with previous corresponding experimental and theoretical results. The SEXAFS data presented for CO and NO adsorption on Ni(110) and Ni(111) show that the complete adsorption geometry including the intramolecular bond length can be determined with high accuracy and reliability and also focus on one particular aspect of the so-called surface-cluster analogy: the adsorption-site assignment on the basis of vibrational data. The SEXAFS experiments demonstrate that the adsorption site in the system Ni(111)–NO and even in the prototypical surface-science system Ni(111)–CO had been incorrectly assumed on the basis of measured stretching frequencies for the adsorbed diatomic molecules. The SEXAFS results are compared with those of low-energy electron diffraction and photoelectron diffraction analyses of the same systems.