N-2 ON TUNGSTEN CLUSTERS - MOLECULAR AND DISSOCIATIVE ADSORPTION

We have studied the size-dependent reactivity of W-10-W-60 with N-2 un der single-collision-like conditions by using a laser-vaporization sou rce, a low-pressure reaction cell and a laser-ionization time-of-fligh t mass spectrometer. The reaction probability with the first and secon d Na molecule was measured at two different cluster-source temperature s: room temperature (RT) and liquid-nitrogen temperature (LNT). For th e RT clusters, a strong size dependence in the reaction probability wa s observed in the size range similar to 10-26 atoms, With distinct loc al maxima at W-16, W-22, and W-23. Upon cooling of the cluster source, the reaction probability increased significantly overall, and the rel ative variations with size decreased, but persisted. To get an indicat ion of the bond Strength of N-2 onW(n), we heated the cluster products after reaction through irradiation with 4.02 eV photons from a XeCl e xcimer laser and checked for consequent desorption of adsorbate atoms or molecules. For the LNT clusters, heating with laser light caused a substantial decrease in the abundance of reaction products with nitrog en, whereas no significant change in the abundance of WnN2 was observe d for the RT clusters. This indicates that a proportion of the N-2 is relatively weakly bound to the LNT clusters, whereas on W-n produced a t RT, only the strongly bound state/states of N-2 exist. Based on comp arisons with the N-2-adsorption on W bulk surfaces, we conclude that t he weakly and strongly bound states represent molecularly and dissocia tively bound N-2, respectively. (C) 1998 American Institute of Physics . [S0021-9606(98)70232-6]