FORMATION OF PROTONATED AMMONIA CLUSTER IONS - 2-COLOR 2-PHOTON IONIZATION STUDY

The mechanism of the resonance-enhanced two-photon ionization (RE2PI) and the dissociation of ammonia clusters has been investigated by using the two-color ionization technique and the reflectron time-of-flight mass spectrometer. The clusters are firstly excited to the intermediate state corresponding to the A1A2 state of an ammonia molecule. The lifetime of the intermediate species is found to be < 10 ns for the production of NH4+ and (NH3)2+, and longer than 1 mus for (NH3)nNH4+ (1 less-than-or-equal-to n less-than-or-equal-to 6). This result implies that NH4+ is produced by the ionization of the dimer followed by the dissociation of NH2 , whereas the higher protonated cluster ions are produced by the second photon ionization of the (NH3)nNH4 radicals produced by the dissociation of the parent clusters in the intermediate state. The appearance potentials of (NH3)2+ and (NH3)nNH + (0 less-than-or-equal-to n less-than-or-equal-to 4) are also determined by two-color RE2PI technique. Those values are discussed in connection with the two possible mechanisms for the observed ion production, which confirms the scheme obtained from the lifetime measurement.