ELECTRON-BOMBARDMENT INDUCED FRAGMENTATION OF SIZE-SELECTED NEUTRAL (D(2)O)(N) CLUSTERS

The fragmentation behaviour of size selected neutral (D2O)n clusters with n less-than-or-equal-to 4 after ionization with 70 eV electrons is subject of this work. Size selection by scattering the cluster beam from a He target beam in combination with a quadrupole mass filter and time resolved measurements at specific laboratory angles enables us to determine the neutral precursor masses of the detected ions. The measured fragment pattern is dominated by deuterated ions of the form (D2O)n-xD+ with x greater-than-or-equal-to 1. The dimer fragmentation which leads with a probability of 62.5% to the D3O+ ion and with 37.5% to D2O+ can be explained by fast intracluster ion-molecule reactions of charged monomer fragments reacting with the partner molecule. For larger clusters the fragmentation process can be rationalised by the creation of an initially highly excited D3O+ (D2O)x complex which is stabilized by evaporating additional monomer units with the main fragment channel (D2O)D+ for n = 3 and (D2O)2D+ for n = 4. With increasing cluster size an increasing tendency of evaporation of more than one water monomer unit has been observed.