Precursors of copper nitride films: laser photoionization of Cu(NH)n clusters in a supersonic beam

The results of spectroscopic studies of Cu(NH3)n clusters formed in a supersonic beam from the laser ablated metal in gaseous ammonia are reported here. The ionization potentials (IP) of clusters with n=2-9 have been measured. The observed IPs are shifted to lower energies with respect to the bare Cu atom IP. They appear to decrease monotonically with increasing cluster size. This trend is consistent with a model in which the unpaired valence electron of the ground electronic state is delocalized in a well bound surface state. The data are confirmed by energetics and structure calculations performed by the density functional method.