Laser ablation source for formation and deposition of size-selected metal clusters

This work describes construction of a source and optimisation of its parameters for production of cluster ion beams using material ablation by the second harmonic of a Nd:YAG laser (532 nm). The influence of different source parameters such as carrier gas pressure, laser power, delay time between gas, and laser pulses as well as nozzle configuration on the cluster formation are studied. For the current experiments the laser ablation cluster source was optimized for production of Co+ cluster ions. Clusters with n up to 150 atoms are registered by a time-of-flight mass spectrometer. Deposition of size-selected Co50 clusters with kinetic energies in the interval of
250–4850 eV/cluster on highly ordered pyrolytic graphite is studied. At the highest impact energies the clusters are implanted. Craters and well-like structures can be seen by scanning tunneling microscopy at impact spots. A decrease in cluster kinetic energy leads to formation of bumplike structures which probably represent damaged graphite areas with incorporated Co atoms. Further decrease in the cluster impact energy to the level of 450–250 eV/cluster creates condition for so-called cluster pinning when the cluster constituents are intact but the energy transferred to the graphite is still enough to produce radiation defects to which the cluster is bound.