Electronic states of sodium dimer in ammonia clusters:: Theoretical study of photoelectron spectra for Na2-(NH3)n (n=0-6)

The geometries, energetics, and vertical detachment energies of Na-2(-)(NH3)(n) (n = 0-6) were examined by ab initio molecular orbital methods in connection with their photoelectron spectra. One of the Na atoms is selectively solvated in the most stable structures for each n. The solvated Na is spontaneously ionized and the formation of a solvated electron occurs with increasing n, giving rise to the Na--Na+(NH3)(n)(e(-))-type state. The ground and two lowest-lying excited states derived from the 1(1)Sigma(+)(g), 1(3)Sigma(+)(u), and 1(3)Pi(u) states of Na-2, respectively, are of ion-pair character though the 1(3)Sigma(+)(u)-type state has an intermediate nature slowly changing to the radical-pair state with increasing n. On the other hand, the higher states stemming from the 1(1)Sigma(+)(u), 1(3)Sigma(+)(g), and 1(1)Pi(u) states of Na-2 show a developing radical-pair nature as n increases. The size dependences of the photoelectron spectra such as the near parallel shifts of the first and second bands, as well as the rapid red shifts of the higher bands, are studied on the basis of the electronic change of the neutrals by solvation.