ELECTRON-BOMBARDMENT FRAGMENTATION OF SIZE SELECTED NH3 CLUSTERS

Ammonia clusters that are generated in a supersonic expansion with He are size selected by a scattering process with He atoms. By measurements of angular dependent mass spectra and time-of-flight distributions at all fragment masses, the complete fragmentation pattern for electron impact ionization of clusters from the dimer to the pentamer are obtained for electron energies of 70 eV. The dimer fragmentation which is dominated by the NH4+ ion (0.58) can be explained completely by fast ion-molecule reactions of the monomer fragments with the partner molecule. For the larger clusters up to n = 11 the NH4+ ion is still the largest fragment channel. The other protonated ions NH4+ (NH3)x start to contribute to the fragmentation with increasing cluster size but never exceed the contribution at mass 18 amu. © 1990 American Institute of Physics.