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 (root 3 x root 3)R30(0)-Xe a nd the close-packed (3 x 3)-Kr structures. The Xe atoms sit on-top, wi th unexpectedly short Xe-Ru distance. In the Kr structure the adatom l ayer locks into threefold sites and the other sites are variable; howe ver, most Kr atoms sit close to on-top, and these possess the shortest Kr-Ru distances. We argue that these site selections and distance var iations can be taken as evidence for a chemical contribution in rare g as-metal bonding, in addition to the Van der Waals attraction. Some co nsequences and difficulties of this concept are discussed.