Ion plating discharges: evidence of cluster formation during metal evaporation

Previous work has demonstrated the possible presence of metal clusters in ion plating discharges that utilise evaporative sources. In the present study, cathode sheath thickness measurements under ion plating conditions and mass spectroscopy studies (up to 2500 a.m.u.) of gas evaporation have provided further evidence of metal clusters in the vapour phase. The view that these clusters nucleate and grow by vapour cooling, through metal atoms losing energy by collisions with gas atoms, is supported by the following observations: (i) for evaporation at a fixed gas pressure, the abundance of clusters seems to be enhanced if argon is used instead of neon, indicating the greater cooling effect from a gas with larger atomic mass; (ii) cluster detection rate appears to be increased with increasing gas pressure, and this can be attributed to higher collision frequencies leading to enhanced cooling.