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.
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