Kinetic energy release measurements of ammonia cluster ions during metastable decomposition and determination of cluster ion binding energies

The dissociation dynamics of protonated ammonia clusters is investigated following their production from neutrals using multiphoton ionization. A very useful method of measuring the kinetic energy release arising from evaporative unimolecular dissociation is established; it employs a time-of-flight/reflectron to separate daughters and parents and enables the energy release to be determined from the peak shapes. The method is found to lead to values of high precision. Using a modified statistical theory analysis suggested by Engelking, it is shown that binding energies for large cluster ions can be readily determined. The results compare very well with those derived by other methods, ones which generally have much more severe size limitations. In the case of (NH3)n H+, the observed drop in binding energy from n=5 to 6 is consistent with the particularly stable protonated pentamer structure. Heretofore, unavailable results are available from n=7 to 17, with hints of slightly more stable cluster ions at n=12 and 14, than for neighboring size clusters. The unimolecular dissociation is found to proceed by a statistical evaporative mechanism, with average kinetic energy releases showing a general decrease with size beyond n=5; small local maxima are found in regions of particularly stable structures.