We report a photoelectron spectroscopic investigation of mono-nitrogen doped aluminum cluster anions
AlnN- (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 AlnN (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 AlnN 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 AlnN- 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.