Electronic spectra and structures of solvated NH4 radicals, NH4(NH3)n (n=1-8)

Electronic spectra of ammoniated ammonium radicals, NH4(NH3)(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 NH4NH3. These bands are ascribed to the 2 (2)A -1 (2)A transition derived from the 3p F-2(2)-3s (2)A(1) excitation of NH4. 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 NH4(NH3), (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 NH4 to 5800 cm(-1) for NH4(NH3)(4) is observed, while no appreciable spectral change is found for n greater than or equal to 5. The large spectral change is ascribed to the spontaneous ionization of NH4 in ammonia clusters. Successive binding energies of NH4(NH3)(n) in the excited state are determined from the spectral band positions.