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^-(NH₃)_n (n ≤ 16), Na^-(NH₃)_n (n ≤ 12), and Na^-(H₂O)_n (n ≤ 7), as well as the ionization potentials (IPs) of Li(NH₃)_n (n ≤ 28) and Li(H₂O)_n (n ≤ 46), are examined. PESs of Li^-(NH₃)_n (n ≤ 10) exhibit three bands derived from the Li(3²S)-Li^-(¹S), Li(2²P)-Li^-(¹S), and Li(2²S)-Li^-(¹S) transitions. The vertical detachment energies of the 3²S- and 2²P-type states decrease dramatically with increasing n. For n ≥ 11, the transitions to the 2²P- and 3²S-type states almost become degenerate with the transition of the neutral ground (2²S) state. In addition to these observations, we also find the red shift of the 2²S-type transition with a much slower rate. The similar spectral trends are also observed for the Na(3²P)-Na^-(¹S) and Na(3²S)-Na^-(¹S) transitions of Na^-(NH₃)_n. On the other hand, the transitions of Na^-(H₂O)_n exhibit the opposite shifts, and the ²P-²S energy separation does not change. As for Li(H₂O)_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(H₂O)_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.