ELECTRONIC STRUCTURE AND STABILITY OFNH4(NH3)nANDNH4(NH3)m(H2O)n

The photoionization process ofNH4(NH3)nandNH4(NH3)m(H2O)nradicals 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) ofNH4(NH3)n(n=0−35)andNH4(NH3)m(H2O)n(m=0−4, n=0−3)are determined by the photoionization threshold measurements. The binding energies ofNH4(NH3)n−1−NH3(n=1−6)are estimated from IPs. The results indicate that the bonding betweenNH4andNH3is semi-ionic. The IPs for the large ammoniatedNH4clusters decrease with increasingnup to 35. The limiting value(n→∞)is found to be 1.33 eV, which coincides with the photoemission threshold of liquidNH3. This feature is similar to those found recently for alkali-atom–ammonia clusters. A clear trend is found for the IPs ofNH4(NH3)m(H2O)n; the clusters containing more water molecules have higher IP. This trend is ascribed to the large binding energy of${\rm NH}_4^+ -{\rm NH}_3$comparing with that of${\rm NH}_4^+ -{\rm H}_2 {\rm O}$.