Two-color, two-photon ionization of ammonia has been used to investigate the ionization and dissociation mechanisms of ammonia clusters. By observing the ion signals in a reflectron time-of-flight mass spectrometer as a function of the delay time between the two laser pulses, it is found that the lifetime of the intermediate to formation of (NH3)2H+ and higher mass cluster ions is at least 500 times longer than that to formation of NH4+ and (NH3)2+. Similar results are found for deuterated ammonia. The intermediate is formed by one-photon excitation in the energy range accessing the Ã 1A2 state of ammonia. This observation confirms that the mechanism of ionization of the dimer in this energy region is two-photon ionization followed by the rapid intracluster proton-transfer reaction (NH3)2+ → NH4+ + NH2. The mechanism for ionization of larger clusters is discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry