FORMATION AND METASTABLE DECOMPOSITION OF UNPROTONATED AMMONIA CLUSTER IONS UPON FEMTOSECOND IONIZATION

The formation and metastable dissociation mechanism of unprotonated ammonia cluster ions, (NH3)+n, produced by multiphoton ionization (MPI) at 624 nm and a nominal pulse width of 350 fs, are investigated through a reflectron time‐of‐flight (TOF) mass spectrometric technique. Detection of the unprotonated ions after femtosecond and nanosecond multiphoton ionization under various intensity conditions is explained. The role of the energy of the ionizing photons, and the observation of these ions after femtosecond MPI is examined. The formation of the unprotonated series is found to be a function of intensity in the case of ionization on the nanosecond time scale, but not so for the femtosecond time domain. The results can be explained in terms of ionization mechanisms and ionizing pulse durations. The findings of the present study suggest that the unprotonated ions are trapped behind the barrier to intracluster proton transfer and/or concomitant NH2 loss. The studies of metastable decomposition also reveal t...