Carbon monoxide (CO) is known for its ability to populate different adsorption sites, depending on the substrate, CO coverage, temperature, and influence from coadsorbate species. In this work, the researchers compared the electronic bond structure of CO adsorbed onto a nickel substrate in three different geometries. The objective was to study how the electronic structure of the CO changes when it is brought into direct contact with one, two, and four atoms at the surface (i.e., with increasing coordination with the substrate atoms).

Schematic of adsorbed CO in contact with one ("top"), two ("bridge"), and four ("hollow") nickel atoms.

The electronic structure of the CO-nickel complex results from the mixing of the CO electron orbitals with those of the nickel substrate. This hybridization creates new orbitals that can be characterized as having either p or s symmetry, depending on the shape of the resulting orbitals' electon distribution. A study of how electrons are shared and redistributed upon adsorption of a molecule at different sites requires an experimental method that allows the determination of atom- and symmetry-specific electronic structures. Furthermore, the information from the small number of adsorbed molecules must be separated from the information from the huge number of substrate atoms.

A Deeper Understanding of Surfaces

Surface science is where the rubber hits the road, so to speak. All materials have surfaces, and like a tire that grips or slips depending on its surface properties, a material's surface properties (mechanical, optical, magnetic, electronic, and chemical) determine how it interacts with the outside world. Chemical surface behavior in particular is central to a wide variety of technologies, including the catalytic converters used in cars to reduce CO emissions. Such applications depend on CO adsorption (where the CO molecule attaches to a surface but does not penetrate). In surface science, CO is often used as a prototypical adsorbate because it displays a wide variety of surface behaviors and is relatively easy to study by various methods. In this work, the researchers observe how the electronic orbitals of CO change depending on the adsoption site's geometry, an interaction that is of fundamental importance for understanding many surface phenomena.

At ALS Beamline 8.0.1, atom-specific valence electrons can be selectively probed using x-ray emission spectroscopy: only the valence electrons in proximity to the core hole localized on either the carbon or oxygen atom participate in the x-ray decay process. Also, by varying the detection angle of the x rays emitted upon decay, the researchers can distinguish between electron states of p and s symmetry.

From the total charge density (gray envelope), valence electrons with p angular momentum (contour lines) decay into the O 1s core hole. Because of the core-hole localization, the outgoing x-ray emission gives a measure of the atom-specific projection of the electron density with p angular momentum. By selective excitation, this projection can be performed around the oxygen or carbon atom.

The resulting carbon and oxygen K-edge spectra show, most notably, a strong adsorption-induced band (d_p) and different CO molecular states, both with significant intensity variations. To compare the experimental results with theory, orbital contour plots were generated based on ab initio electron density calculations (using density functional theory). Analysis shows that the s interaction is repulsive while the p interaction is attractive and also weakens the internal CO bond. Both the p and s interactions increase with higher coordination, but the two contributions partly compensate for each other.

C and O K-edge spectra of CO adsorbed on Ni(100). Energy scale is relative to the Fermi level.

Contour plots for p (left) and s (right) orbitals from ab initio calculations.

With this information, we can better understand the rich chemistry of CO adsorbed on metals and the variety of behaviors arising from the different possible adsorption sites. The small differences in adsorption energies previously observed for the different sites had been interpreted as an indication of rather similar bonding. In addition, it was known from vibrational spectroscopy that the CO stretch frequency decreases with increasing coordination to the substrate. Based on the findings reported here, such phenomena can be understood in terms of the interplay between the p and s interactions. The p interaction weakens the internal CO bond, decreasing the CO stretch frequency as coordination increases, and the balance between p bonding and s repulsion leads to small differences in adsorption energies despite very large differences in electronic structure.

Research conducted by A. Föhlisch, J. Hasselström, O. Karis, and A. Nilsson (Uppsala University, Sweden); and M. Nyberg and L.G.M. Pettersson (University of Stockholm, Sweden).

Research funding: Swedish Natural Science Research Council (NFR) and the Göran Gustafsson Foundation for Research in Natural Sciences and Medicine. Operation of the ALS is supported by the Office of Basic Energy Sciences, U.S. Department of Energy.

Publication about this research: A. Föhlisch, M. Nyberg, J. Hasselström, O. Karis, L.G.M. Pettersson, A. Nilsson, "How Carbon Monoxide Adsorbs in Different Sites," Phys. Rev. Lett. 85(15), 3309 (2000).

ALSNews Vol. 176, May 9, 2001