EVAPORATIVE DISSOCIATION OF AMMONIA CLUSTER IONS - QUANTIFICATION OF DECAY FRACTIONS AND ISOTOPE EFFECTS

A modified version of the evaporative ensemble model is employed to deduce relative binding energies for protonated cluster ions using previously measured metastable dissociation fractions of ammonia cluster ions, (NH3)nH+, n = 4-22, obtained by using a laser-based time-of-flight mass spectrometer equipped with a reflection. The newly derived values are found to be in good agreement with those reported earlier based on high-pressure mass spectrometry (HPMS) and several other methods employing data on dissociation dynamics. The various methods are found to be complementary. New measurements are reported for the metastable decay of deuterated cluster ions to investigate the influence of mass effects on the dissociation rate. The decay fractions of the cluster ions (ND3)nD+ are found to be consistently higher than those of the corresponding (NH3)nH+ by 15%. The isotope effect can be well accounted for by considering the differences of their bulk heat capacities, and such studies are found to be a promising method for measuring the heat capacities of small, unsupported cluster systems.