Pulsed laser reactive ablation of Al in ammonia atmosphere: photoionization thresholds and structures of Al(NH3) clusters

A photoionization study of the aluminum ammonia clusters produced by the interaction of the laser ablated aluminum vapour with gaseous NH3 is reported. In the near-threshold photoionization spectrum of Al(NH3) the observed structures are assigned to vibrational frequencies of the ionized cluster. Ionization potentials of Al(NH3)n clusters with 2<=n<=13 have been measured. The cluster ionization potentials are shifted to lower energies compared with the bare Al atom and they decrease almost monotonically with increasing cluster size. This is consistent with a model in which the metal valence electron is delocalized in a Rydberg-like surface state. For small clusters the difference between the binding energy of the ion and the neutral complexes is compared with energetics calculated by a density functional method.