Interaction of silicon cluster ions with ammonia: The kinetics

Abstract

The chemical reactions of Si+n (n=10–50) with ammonia have been studied using injected ion drift tube techniques at thermal energies (296–414 K), and low energy ion beam techniques, at a center of mass collision energy of ∼0.2 eV. Virtually all of the products arise from the adsorption of one or more ammonia molecules on to the parent cluster ion. In the drift tube experiments all clusters (except those with 11, 13, 14, 19, 22, and 23 atoms) were found to react with ammonia at close to the collision rate at room temperature. The reaction rates decrease with increasing temperature. Thermally activated desorption of ammonia from the products contributes to the negative temperature dependence. This observation suggests that unlike bulk silicon surfaces, which are known to adsorb NH3 dissociatively (and desorb H2 at ∼800 K), the silicon clusters may not be able to dissociate ammonia (at least on the time scale of our experiments). For clusters with 30–50 atoms, total cross sections for adduct formation were measured at collision energies of ∼0.2 eV. The cross sections are close to the hard sphere values and increase slowly with cluster size. In contrast to the results of Smalley and co-workers, obtained using Fourier transform ion cyclotron resonance, we do not find Si+33, Si+39, and Si+45 to be particularly unreactive. Several possible explanations for the large differences in the reactivities of these clusters (as measured by the two different experimental techniques) are discussed.