Solvation process of Na-m in small ammonia clusters: photoelectron spectroscopy of Na-m(-)(NH3)(n) (m <= 3)

Photoelectron spectra (PESs) of Na-m(-)(NH3)(n) (m less than or equal to 3) are investigated to explore the solvation of sodium atom and its aggregate s in small ammonia clusters. For Na-(NH3)(n), we examine the PESs with an i mproved resolution and confirm the spontaneous ionization of the Na atom in small clusters. As for Na-2(-) (NH3)(n) (n less than or equal to 8), verti cal detachment energy (VDE) of the transition from the anion to the neutral ground state is found to shift to the red with respect to that of Na-2(X(1 )Sigma(g)(+)), while that of the first excited state derived from Na-2(a(3) Sigma(u)(+)) increases gradually for n greater than or equal to 4. In addit ion, the transitions to the higher-excited stares derived from those correl ated to the 3(2)S+3(2)P asymptote are found to be shifted rapidly to the re d and become almost degenerate with the a (3)Sigma(u)(+)-type transition. T he extensive spectral change is ascribed to the asymmetrical ammoniation an d spontaneous ionization of Na-2 in small clusters. We also find a drastic change in the PESs of Na-3(-)(NH3)(n); the neutral ground ((2)Sigma u(u)(+) -type) and first excited ((2)Sigma(g)(+)-type) states of these clusters bec ome degenerate with each other as the number of NH3 molecules increases. Wi th the aid of the theoretical calculations, this spectral change is ascribe d to the dissociation of the Na-3(-) core in clusters. This may be the firs t observation of the dissolution of metal aggregates in small clusters. (C) 2000 Elsevier Science B.V. All rights reserved.