Until now theoretical studies of chemisorbed species have been concentrated around a limited number of systems, the alkali metals, the elements in the second period of the periodic table and some molecules as well. The reason is of course the important role these adsorbates play in many catalytic reactions and also that many of them are simple to handle and that the results gained may be used for qualitative descriptions of more complex systems. However, there are many classes of interesting elements in the periodic table, both for theoretical and applied reasons: metallic and non-metallic, electropositive and electronegative, etc.
The work presented here deals with the elements of group IIA, Be, Mg, Ca and Sr, and two neighbouring elements, Li and B, and is part of a systematic investigation of potential-energy curves, adsorbate-induced dipole moments and density of states and other chemisorption properties.
It is shown that the adsorption of Be and Mg is characterized by the fact that these elements have filled s-subshells which result in weak chemisorption and strong physisorption on a jellium surface. The substrate lattice gives additional effects, such as strong electrostatic stabilization of Be on high-coordination sites. In the case of Mg, geometric considerations play an even stronger role than for Be, giving a weak chemisorption on some close-packed surfaces.
The properties of chemisorbed Ca and Sr differ radically from those of Be and Mg, for instance the chemisorption energy is much larger. The origin of this lies in the fact that Ca and Sr are neighbours to the transition metals and have d-states that are empty in the free atom ground state, but get partially filled upon chemisorption. In the interaction with the substrate lattice, the directional properties of the d-states influence the adsorption site.
Sub-shell closure, early p- and d-mixing and core-orthogonalization are identified as important adsorbate factors for the chemisorption bond. With respect to their chemisorption properties the group IIA elements can be classified as neutral atoms that are apt for polarization and charge transfer due to their low-lying p- and d-orbitals.