THE ADSORPTION OF AG ON CO, O, AND H COVERED W(110) .1. OVERLAYER FORMATION AND STABILITY

The adsorption of Ag mono- and multilayers on O/W(110), CO/W(110), and H/W(110) was investigated, from 90 K to Ag desorption temperatures. Auger measurements indicate that Ag growth occurs layer by layer at 90 K on all substrates. For Ag/O/W(110) dewetting, i.e. partial balling up of Ag occurs from 200 to 400 K. At T greater-than-or-equal-to 500 K segregation sets in, with O forced into p(1 X 1) islands, and Ag pushed into islands which can be several layers thick. This behavior is similar to that seen previously with 0 and Cu and 0 and Pd on W(110). There is also a slight amount of 0 left trapped under the Ag covered surface. In the case of CO partial dewetting also seems to set in at 200-300 K. Ag adsorption does not cause CO desorption. However, with an Ag overlayer there is a slight decrease from 400 to 350 K in the virgin-CO desorption peak, and the ratio of virgin to beta-desorption increases with Ag coverage. This suggests that Ag competes with remaining CO for sites which could be used for dissociative, i.e. beta-adsorption. CO is only very weakly adsorbed on Ag1/W(110); this explains the relatively small effect on v-CO desorption, since there is no driving force for CO-Ag interchange, as in the case of Cu/CO/W(110), where CO is still strongly bound on Cu1/W(110). The effect of Ag on H2 desorption from H/W(110) is quite dramatic, with the appearance of a sharp low temperature peak at 150 K and a second sharp peak at 400 K, also lower than the broad main peak for H/W(110). This behavior is very similar to what was seen previously for Hg/H/W(110) and Pd/H/W(110) and may result from a modification of H binding sites on W(110) by the overlayer metal, even if the latter does not interact directly with H.