Electron impact fragmentation of size-selected krypton clusters

Clusters of krypton are generated in a supersonic expansion and size selected by deflection from a helium target beam. By measuring angular distributions for different fragment masses and time-of-flight distributions for fixed deflection angles and fragment masses, the complete fragmentation patterns for electron impact ionization at 70 eV are obtained from the dimer to the heptamer. For each of the neutral Kr-n clusters studied, the main fragment is the monomer Kr+ ion with a probability f(n1) > 90%. The probability of observing dimer Kr-2(+) ions is much smaller than expected for each initial cluster size. The trimer ion Kr-3(+) appears first from the neutral Kr-5, and its fraction increases with increasing neutral cluster size n, but is always much smaller than that of the monomer or dimer. For neutral Kr-7, all possible ion fragments are observed, but the monomer still represents 90% of the overall probability and fragments with n > 3 contribute less than 1% of the total. Aspects of the Krn cluster ionization process and the experimental measurements are discussed to provide possible reasons for the surprisingly high probability of observing fragmentation to the Kr+ monomer ion.