Electronic absorption spectra of NH 4 (NH 3 )n (n = 1 − 6) were measured in the energy region of 4500-14 000 cm −1 . The spectra were assigned to the transition derived from a 3p 2 F 2 − 3s 2 A 1 RydbergRydberg transition of NH 4 . A drastic decrease of excitation energy from the transition of NH 4 (15 062 cm −1 ) down to 5800 cm −1 for NH 4 (NH 3 ) 4 cluster was observed, while there was no clear difference for n = 4, 5, and 6. Successive binding energies of NH 4 (NH 3 )n in the 3p-type state determined from the spectra were found to decrease monotonically with increasing n, in a similar manner to those of a positive ion, NH + 4 . The drastic spectral change was ascribed to spontaneous ionization of NH 4 in ammonia clusters.
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