Ultrafast Photochemistry of Ammonia Clusters: Formation and Decay of Hypervalent Molecular Clusters Containing the NH4 Radical

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Abstract
The photochemical reaction of ammonia clusters in the first excited state was studied by a femtosecond pump-probe technique and time-of-flight mass spectroscopy. Small ammonia clusters containing the NH4 radical were probed by a resonance enhanced two-photon ionization method. The NH4 radical is formed within 0.5 ps, and decomposes with a lifetime of 13 ps. The ammonia dimer ion was found to be formed mainly through the one-photon ionization of a new photolysis product such as an excited-state NH4*–NH2. This intermediate may correspond to a cage product in the predissociation process of larger ammonia clusters; its formation and decay times are ca. 1 ps and > 1 ns, respectively. We also examined the lifetimes of NH4(NH3)n using the pump-probe technique with nanosecond lasers. The lifetime of NH4 was found to be elongated by more than 106 times in ammonia clusters. The mechanism of the formation and decay processes for these species as well as for the resonance-enhanced two-photon ionization of ammonia clusters were discussed on the basis of the present results in conjunction with theoretical results from literature.