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

Abstract

Electronic absorption spectra of NH₄(NH₃) _n , (n = 1 – 6) were measured in the energy region of 4500–14000 cm⁻¹. The spectra were assigned to the transition derived from a 3p ²F₂ − 3s ²A₁ Rydberg-Ryclberg 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, while there was no clear difference for n = 4, 5, and 6. Successive binding energies 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 those of a positive ion, NH ⁺₄ . The drastic spectral change was ascribed to spontaneous ionization of NH₄ in ammonia clusters.