The reaction of iron clusters with ammonia. I. Compositions of the ammoniated products and their implications for cluster structure

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Abstract
Studies are described of the chemisorption of ammonia on isolated neutral iron clusters Fen for 2≤n≤165. Clusters are generated by laser vaporization in a continuous-flow-tube reactor, and reaction products are detected by laser-ionization time-of-flight mass spectrometry. Ammonia is found to chemisorb nondissociatively on cluster surfaces on the 1 ms time scale of these experiments. Measurements of ammonia uptake provide information on adsorption kinetics and on the number and nature of the binding sites. The ammonia binding energy is found to decrease with increasing cluster coverage. For chemically saturated clusters, the ratio of adsorbed NH3 molecules to surface iron atoms is found to decrease with increasing cluster size, going from >1/3 for small clusters to 100. Ammonia chemisorption is accompanied by a large decrease in cluster ionization potentials, as much as 2 eV for saturated clusters. At sufficiently high exposure the beginning of the formation of a second, physisorbed layer of molecules is seen. Detailed measurements of product composition under different exposure conditions give evidence for numerous changes in cluster structure throughout the growth sequence from small to large clusters. Often these structural changes involve particularly stable reaction products. Evidence for the existence of metastable structures is presented. Several possibilities for cluster structure are suggested.