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 --> infinity) 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 NH4+-NH3 comparing with that of NH4+-H2O.