Resonance enhanced multiphoton ionisation probing of H atoms in a hot filament chemical vapour deposition reactor

We demonstrate some of the merits and limitations of using multiphoton ionisation (MPI) spectroscopy, resonance enhanced at the two photon energy by the 2s(1);S-2(1/2) state, to detect H atoms within a hot filament reactor used for diamond chemical vapour deposition (CVD). Subsequent analysis of the Doppler broadened lineshapes obtained in this way allows determination of spatially resolved, relative H atom number densities and gas temperatures. The effects of H-2 pressure and flow rate, filament temperature and radial distance from the filament on the relative H atom number densities and the gas temperature profiles have each been investigated for the case of pure H-2 and, in some cases, in the presence of added CH4. The present findings complement and extend previous measurements obtained using alternative in situ detection methods, and are generally consistent with current models of the gas phase chemistry prevailing in low power diamond CVD reactors. They also serve to refine the role of the hot filament in the H atom production process, and to illustrate continuing ambiguities regarding the way in which the local relative H atom number densities vary with filament temperature, with the presence of trace quantities of added hydrocarbon, and the interpretation of these dependencies.