Ammoniated NH4 radicals produced by an ArF excimer laser photolysis of ammonia clusters are studied by one-photon ionization and time-of-flight mass spectroscopy. The ionization potentials (IPs) of NH4(NH3)n (n = 0-35) 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 larger clusters are found to decrease monotonically with increasing n to a limit of 1.33 eV, which coincides with the photoemission threshold of liquid NH3. This feature is similar to those found recently for alkali atoms-ammonia clusters.
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