Clusters

An alternative to the Laser Vapourisation Source (LVS) is the Pulsed Arc Cluster Ion Source (PACIS), where the laser is replaced by a pulsed high current arc between two electrodes. As in the LVS, this discharge is timed to coincide with a pulse of carrier gas. This carries the plasma through the channel and into the expansion cone where supersonic expansion results in clustering.

A big advantage of the PACIS is that it is a cheap alternative to LVS. No laser is required for the production of clusters and, since mostly cluster ions are formed, subsequent ionisation by a laser is not required either. The only expense is for a power supply and a pulser that can deliver a high voltage, high current arc through the target rod.

The cluster beam which is formed by the PACIS has three main components. There is a fast beam of positively charged atoms and two slower moving components of neutral and positively charged clusters. Of these components the positively charged clusters are the most intense and have a smaller spatial distribution than the neutral clusters.

By varying source parameters such as arc energy, the delay between arc discharge and opening of the pulsed valve, volume of the thermalisation zone and the nozzle diameter it is possible to control the characteristics of the produced beam. In particular, it is possible to control the mass distribution, kinetic energies and temperatures of the produced clusters. For example, increasing the delay between the arc discharge and the pulse of carrier gas leads to the formation of larger clusters.

In general, by varying the source parameters it is possible to produce cluster beams that have different clustering and expansion stories and therefore have different mass distributions, temperatures and velocities.