The adsorption of alkali metals on graphite has been the subject of various studies for the past two decades. Briefly, two main reasons can be offered to justify the persisting interest in these adsorption systems. First, experiments have pointed out intriguing structural phase transitions of the adsorbed species, and, second, in an attempt to explain the experimental results, the more complicated question of the nature of alkali metal-graphite bonding arose. Despite the relative simplicity of the electronic structure of the alkali metals, their interaction with the graphite surface is still the subject of current debate. This review paper presents relevant experimental data and results of selected theoretical calculations that, in time, guided the process of scientific discovery towards the current understanding of the alkali metals/graphite adsorption systems. Contents1. Introduction 996 2. General considerations 997 3. Structural study of single-crystal graphite and highly oriented pyrolytic graphite 999 4. The surface structures formed by alkali metals adsorbed on graphite 1001 4.1. Potassium on graphite 1001 4.2. Rubidium on graphite 1003 4.3. Caesium on graphite 1003 4.4. Lithium on graphite 1006 4.5. Sodium on graphite 1006 5. The determination of the alkali metal-graphite charge transfer. The nature of alkali metal-graphite bonding 1007 5.1. Calculation of alkali metal-graphite charge transfer having experimental data as starting point 1008 5.2. Theoretical calculations of alkali metal-graphite charge transfer 1015

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

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