Structural evidence for chemical contributions in the bonding of the heavy rare gases on a close-packed transition metal surface: Xe and Kr on Ru(001)

We have determined the geometries of two ordered structures of Xe and Kr on Ru(001) by LEED-IV analysis, of the (√3 × √3)R300-Xe and the close-packed (3 × 3)-Kr structures. The Xe atoms sit on-top, with unexpectedly short XeRu distance. In the Kr structure the adatom layer locks into threefold sites and the other sites are variable; however, most Kr atoms sit close to on-top, and these possess the shortest KrRu distances. We argue that these site selections and distance variations can be taken as evidence for a chemical contribution in rare gas-metal bonding, in addition to the Van der Waals attraction. Some consequences and difficulties of this concept are discussed.