Velocity distribution of mass-selected nano-size cluster ions

The paper reports on an experimental investigation of the ion velocity distribution function (IVDF) for mass-selected copper cluster ions with a diameter in the range 5–10 nm. The cluster ion beam is produced by a dc magnetron-based gas aggregation source. The measurements are carried out by means of a quadrupole mass filter combined with a basic retarding field analyser. The most probable cluster velocities are in the range 80–180 m s−1; decreasing with cluster size and increasing with buffer gas (Ar) flow. Experimental results are compared with the theoretical model of Smirnov et al. Noted discrepancies are explained by the interaction of clusters with buffer gas atoms outside the aggregation chamber during the passage from the orifice towards the detector. This interaction is not included in the theoretical model. Both positively and negatively charged clusters are investigated. The density ratio of positively to negatively charged clusters in the beam is evaluated from the measured IVDFs and compared with the theoretical model. (paper)