Electron bombardment fragmentation of size selected NH3 clusters

Abstract

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 NH+4 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 NH+4 ion is still the largest fragment channel. The other protonated ions NH+4(NH3)x start to contribute to the fragmentation with increasing cluster size but never exceed the contribution at mass 18 amu.