Kinetic Study of the Metastable Ammonia Cluster Ions Using Reflection Time-of-Flight Mass Spectroscope

The multiphoton ionization of the ammonia clusters at 355nm was studied using both conventionaland reflection time-of-flight mass spectrometers. Aseries of protonated ammonia clusters(NH3)nH+were obtained,and the hypervalent ammonia cluster ions (NH3)nH2+(n>2 ) were observed. The daughter ions which were produced from dissociation of the metastable am-monia cluster ions during their free flight were also observed. The dissociation rate constants were derived from the experimental result which depends upon the number of the ammnia molecules in a cluster ion. The life times of the metastable cluster ions were estimated, which range from 21ms to 122μs for different cluster size fromn=3 to 20. The larger the cluster size, the faster the dis-sociation rate and the shorter of the life time of the cluster ions. The kinetics shows that withn=5 or smaller the cluster ionsare more stable comparing to those withn≥6, which is consistant with the observation reported in literature.