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

The fragmentation behaviour of size selected neutral (D2O)n clusters w ith 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 f rom 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 meas ured fragment pattern is dominated by deuterated ions of the form (D2O )n-xD+ with x greater-than-or-equal-to 1. The dimer fragmentation whic h 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 c harged monomer fragments reacting with the partner molecule. For large r clusters the fragmentation process can be rationalised by the creati on of an initially highly excited D3O+ (D2O)x complex which is stabili zed by evaporating additional monomer units with the main fragment cha nnel (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 mon omer unit has been observed.