Solvation of sodium dimer in ammonia clusters: Photoelectron spectroscopy and ab initio study of Na-2(-)(NH3)(n)

Photoelectron spectra (PESs) of Na-2(-)(NH3)(n) (n = 0-5) are investigated to explore the solvation of sodium dimer in small ammonia clusters. We also calculate the structures and energetics of Na-2(-)(NH3)(n) (n = 0-3) as we ll as the vertical detachment energies (VDEs) associated with the transitio ns to various neutral states for n up to 2 by using an ab initio MO method. The PES of Na-2(-)(NH3) exhibits five bands at the VDE of 0.41, 1.36, 1.86 , 2.11, and 2.40 eV. These bands are reproduced theoretically with a reason able accuracy and assigned to the transitions from the anion ground state, 1(2)Sigma(u)(+)(X-), to the neutral ground, 1(1)Sigma(g)(+)(X), and low-lyi ng four excited states derived from the 1(3)Sigma(u)(+)(a), 1(3)Pi(u)(b), 1 (1)Sigma(u)(+)(A), and 1(3)Sigma(g)(+) states of Na-2, respectively. The VD Es to the neutral ground state for all clusters examined are found to be sl ightly smaller than or equal to that of Na-2(1(1)Sigma(g)(+)), while that t o the first excited state derived from Na-2(1(3)Sigma(u)(+)) increases grad ually for n = 4 and 5. In addition, the higher-energy transitions derived f rom the 1(3)Pi(u), 1(1)Sigma(u)(+), and 1(3)Sigma(g)(+) states correlated t o the Na(3(2)S) + Na(3(2)P) asymptote are found to be shifted rapidly to th e red and almost become degenerate with the 1(3)Sigma(u)(+)-type transition . On the basis of these experimental and theoretical results, we discuss th e structure of the anion clusters and the solvation state of Na-2 in (NH3)( n).