IR STUDIES OF CHEMISORPTION ON MOLECULAR-SURFACES - AMMONIA ON SILVERCLUSTERS

Metal clusters are proposed as models to develop a molecular perspecti ve on surface processes and to increase our understanding in important areas such as heterogeneous catalysis. This goal requires the charact erization of small metal-cluster-ligand complexes in as great detail a s possible. One avenue is to apply IR spectroscopy to cluster-ligand c omplexes. Problems of number density and mass selectivity prohibit the use of direct optical absorption methods. We report on progress towar ds applying infrared multiphoton dissociation, IRMPD, to obtain spectr a by direct photodepletion. In this regard the silver-cluster-ammonia system is promising. The action spectrum for IRMPD of Ag2NH3, monitore d via the Ag-2 product in a fast-flow reactor, reveals an absorption p eak at 1065 cm(-1) which is assigned to the -NH3 umbrella mode of the complex. Molecular structure calculations using density-functional the ory predict an end-on geometry and are in good agreement with this val ue. Preliminary results on complexes of larger Ag clusters studied by monitoring the IRMPD depletion of a AgnNH3 molecular beam by time-of-f light mass spectrometry are presented. The promise of these systems as models for NH3 bound to Ag surfaces is discussed.