Key ingredients of the alkali atom - metal surface interaction: Chemical bonding versus spectral properties

The interaction of alkali atoms with metal surfaces is reviewed. The peculiar electronic configuration of such atoms, with only one valence electron participating in the bond formation, suggested simple pictures to describe their interaction with a metal surface. But it was early evident that the adsorption properties depend on many aspects, related to the electronic structure of constituents, leading, for example, to different degrees of ionicity/covalency of the alkali atom-metal bond. Sophisticated theoretical modeling tried to shed light on this aspect. The spectral properties are the ultimate features in determining how the systems interact with each other. In this review the electronic and spectral properties are discussed focusing on different theoretical representations of the physical system and on their consequences. Surface projected energy gaps of the substrate as well as the substrate continuous spectrum are key aspects in determining the nature of the interac- tion and bonding with alkali adsorbates.