Electron impact detachment of small negative clusters

Results for electron impact detachment of internally cold C _n^- and Al_n^- clusters are presented. The experiments were performed using an electrostatic ion beam trap fitted with an internal electron target. The measured electron impact detachment cross sections for the C_n^- (n=1-9) clusters exhibit an even-odd oscillation, which is in agreement with the familiar even-odd variation in the binding energy, namely higher cross section for weaker binding. However, the cross sections increase on average with the cluster size, in spite of the average increase in the binding energy. We suggest, based on the data and on preliminary calculations, that the size dependent polarizability is the cause of this effect.