Probing the electronic structure of mono-nitrogen doped aluminum clusters using anion photoelectron spectroscopy

We report a photoelectron spectroscopic investigation of mono-nitrogen doped aluminum cluster anions Al_nN^- (n = 2-22). Well-resolved spectra were obtained at three photon energies (355, 266, and 193 nm), revealing the structural and electronic evolution as the number of aluminum atoms increases in the doped clusters. For small Al_nN (n < 9) clusters, the Al atoms may be viewed to be monovalent, similar to pure aluminum clusters. Even-odd alternation of the electron affinities was observed for Al_nN clusters, suggesting that neutral clusters with odd n are closed shell and those with even n are open shell. The most interesting observation is the similarity between the spectra of Al_nN^- and Al_(n-1)^- for n>12. This observation suggests that these clusters can be described as (AlN)Al_(n-1)^-, i.e., an AlN unit weakly interacting with Al_(n-1)^- clusters. The electronic and atomic structural implications of this observation are discussed.