Electronic Spectra and Structures of Solvated NH4Radicals, NH4(NH3)n(n= 1−8)

Electronic spectra of ammoniated ammonium radicals, NH 4 (NH 3 ) n, produced through the photolysis of ammonia clusters, are investigated with a laser photodepletion spectroscopy. Vibrationaly resolved bands beginning at energy of 9305 ± 15 cm -1 are successfully assigned to NH 4 NH 3 . These bands are ascribed to the 2 2 A −1 2 A transition derived from the 3p 2 F 2 − 3s 2 A 1 excitation of NH 4 . A second group of bands beginning at energy 10073 ± 15 cm -1 is assigned to the 1 2 E−1 2 A transition. Electronic spectra of a series of NH 4 (NH 3 ) n ( n = 1−8) are also recorded with low resolution in the energy region of 4500−16000 cm -1 . A drastic decrease of the excitation energy from 15062 cm -1 for NH 4 to 5800 cm -1 for NH 4 (NH 3 ) 4 is observed, while no appreciable spectral change is found for n ≥ 5. The large spectral change is ascribed to the spontaneous ionization of NH 4 in ammonia clusters. Successive binding energies of NH 4 (NH 3 ) n in the excited state are determined from the spectral band positions.