Electronic spectra of solvated NH4 radicals NH4(NH3)(n) for n=1-6

Electronic absorption spectra of NH4(NH3)(n) (n = 1 - 6) were measured in t he energy region of 4500-14000 cm(-1). The spectra were assigned to the tra nsition derived from a 3p(2)F(2) - 3s(2)A(1) Rydberg-Rydberg transition of NH4. A drastic decrease of excitation energy from the transition of NH4 (15 062 cm(-1)) down to 5800 cm(-1) for NH4(NH3)(4) cluster was observed, whil e there was no clear difference for n = 4, 5; and 6. Successive binding ene rgies of NH4(NH3)(n) in the 3p-type state determined from the spectra were found to decrease monotonically with increasing n, in a similar manner to t hose of a positive ion, NH4+. The drastic spectral change was ascribed to s pontaneous ionization of NH4 in ammonia clusters.