Microscopic Solvation Process of Alkali Atoms in Finite Clusters: Photoelectron and Photoionization Studies of M(NH3)n and M(H2O)n (M = Li, Li-, Na-)

Photoelectron spectra (PESs) of Li - (NH 3 ) n ( n ≤ 16), Na - (NH 3 ) n ( n ≤ 12), and Na - (H 2 O) n ( n ≤ 7), as well as the ionization potentials (IPs) of Li(NH 3 ) n ( n ≤ 28) and Li(H 2 O) n ( n ≤ 46), are examined. PESs of Li - (NH 3 ) n ( n ≤ 10) exhibit three bands derived from the Li(3 2 S)−Li - ( 1 S), Li(2 2 P)−Li - ( 1 S), and Li(2 2 S)−Li - ( 1 S) transitions. The vertical detachment energies of the 3 2 S- and 2 2 P-type states decrease dramatically with increasing n . For n ≥ 11, the transitions to the 2 2 P- and 3 2 S-type states almost become degenerate with the transition of the neutral ground (2 2 S) state. In addition to these observations, we also find the red shift of the 2 2 S-type transition with a much slower rate. The similar spectral trends are also observed for the Na(3 2 P)−Na - ( 1 S) and Na(3 2 S)−Na - ( 1 S) transitions of Na - (NH 3 ) n . On the other hand, the transitions of Na - (H 2 O) n exhibit the opposite shifts, and the 2 P− 2 S energy separation does not change. As for Li(H 2 O) n, we find a monotonous decrease in IPs with n ≤ 4 and a constant IP behavior for n ≥ 5. The limiting value for n → ∞ (3.12 eV) is comparable to the estimated photoelectric threshold of ice as in the case of Cs(H 2 O) n reported previously. On the basis of these results as well as those of the ab initio calculations, we discuss the early stage of solvated-electron formation in finite clusters.