Electronic structure and stability of NH₄(NH₃)_n and NH₄(NH₃)_m(H₂O)_n

The photoionization process of NH₄(NH₃)_n and NH₄(NH₃)_m(H₂O)_n radicals produced by an ArF excimer laser photolysis of ammonia clusters and ammonia-water mixed clusters are examined using time-of-flight mass spectroscopy. The ionization potentials (IPs) of NH₄(NH₃)_n (n=0-35) and NH₄(NH₃)_m(H₂O)_n (m-0-4, n=0-3) are determined by the photoionization threshold measurements. The binding energies of NH₄(NH₃)_n-1-NH₃ (n=1-6) are estimated from IPs. The results indicate that the bonding between NH₄ and NH₃ is semi-ionic. The IPs for the large ammoniated NH₄ clusters decrease with increasing n up to 35. The limiting value (n → ∞) is found to be 1.33 eV, which coincides with the photoemission threshold of liquid NH₃. This feature is similar to those found recently for alkali-atom-ammonia clusters. A clear trend is found for the IPs of NH₄(NH₃)_m(H₂O)_n; the clusters containing more water molecules have higher IP. This trend is ascribed to the large binding energy of NH⁺₄-NH₃ comparing with that of NH⁺₄-H₂O.