Adsorption and coadsorption with oxygen of xenon on the (110) and (100) planes of tungsten

Abstract

The adsorption of Xe on clean and oxygen covered (110) and (100) tungsten surfaces has been investigated. On clean surfaces monolayers of Xe (5.5 × 10 14atoms/cm 2) decrease work function by 0.45 eV on (110) and by 1.1 eV on (100). Binding energies E ad are estimated to be 5.3 kcal/mole on (110) and 6.2 kcal/mole on (100). Preadsorbed oxygen decreases the work function change, slightly on (110), and drastically on (100). On (110) a precoverage of O W = 0.5 increases E ad slightly after anneal at 400–800 K. On (100) O W = 0.5 increases E ad by 4–7 kcal/mole for 50% of the Xe layer; the effect decreases with increasing annealing temperature. For full oxygen monolayers only a slight effect is seen. If Xe is preadsorbed, O 2 can reach the W(100) surface at 20 K through 1 Xe layer, to become chemisorbed, and at 33 K by diffusion through 2 or 3 Xe layers. For Xe preadsorption on (100) even very high oxygen coverages increase E ad profoundly, suggesting that oxygen is “steered” to appropriate sites by Xe. It is argued that the XeO interaction is chemical in nature and possible only when O atoms occupy sites which make available to Xe oxygen 2p orbitals in favorable orientations.