IR STUDIES OF CHEMISORPTION ON MOLECULAR SURFACES: AMMONIA ON SILVER CLUSTERS

Abstract

Metal clusters are proposed as models to develop a molecular perspective on surface processes and to increase our understanding in important areas such as heterogeneous catalysis. This goal requires the characterization of small metal-cluster-ligand complexes in as great detail as possible. One avenue is to apply IR spectroscopy to cluster-ligand complexes. Problems of number density and mass selectivity prohibit the use of direct optical absorption methods. We report on progress towards applying infrared multiphoton dissociation, IRMPD, to obtain spectra by direct photodepletion. In this regard the silver-cluster-ammonia system is promising. The action spectrum for IRMPD of Ag 2 NH 3 , monitored via the Ag 2 product in a fast-flow reactor, reveals an absorption peak at 1065 cm −1 which is assigned to the −NH 3 umbrella mode of the complex. Molecular structure calculations using density-functional theory predict an end-on geometry and are in good agreement with this value. Preliminary results on complexes of larger Ag clusters studied by monitoring the IRMPD depletion of a Ag n NH 3 molecular beam by time-of-flight mass spectrometry are presented. The promise of these systems as models for NH 3 bound to Ag surfaces is discussed.