CO AND O-2 REACTIVITY OF TUNGSTEN CLUSTERS

The reactivity of tungsten clusters, W-n, towards CO and O-2 has been studied and absolute sticking probabilities (S) of the first molecule of CO and O-2 were determined for clusters containing 8-40 atoms. The sticking probability of O-2 shows a rather smooth dependence on cluste r size. For W-8 S is low, similar to 0.2, but increases to similar to 0.6 for W-9. For larger cluster sizes S increases slowly with n and le vels off at an almost constant value of similar to 0.8 for clusters la rger than n approximate to 15. The sticking probability of CO is overa ll somewhat lower than that of O-2, 0.6-0.8 for the larger clusters, a nd differs less between the smallest and the largest cluster sizes. Fo r CO also the sticking probability of the second molecule was determin ed and for all cluster sizes found to be lower than S of the first mol ecule.