Measurement of atomic hydrogen in a hot filament reactor by two-photon laser-induced fluorescence

It is now well known that hydrogen atom supersaturation is a prerequisite for diamond film growth. However, the exact role played by atomic hydrogen remains unclear. Thus determination of the H concentration under various experimental conditions is of great interest in understanding the role of hydrogen in diamond deposition processes. It has been demonstrated previously that two-photon laser induced fluorescence (LIF) can be used to determine quantitative H-atom concentrations. We measured the spatial profile of the concentration and translational temperature of atomic hydrogen in a hot filament reactor during diamond deposition process by two-photon LIF. Hydrogen atoms were excited to the n = 3 state by two photons at 205 nm wavelength and the emission of the H_α transition at 656 nm (n = 3 → n = 2) was observed. UV light at 205 nm was produced by an excimer pumped dye laser using rhodamine B at 615 nm. This light was frequency doubled to produce 307.5 nm which was then mixed with 615 nm to give 205 nm. The energy obtained was typically 50 μJ for a pulse duration of about 30 ns and a laser bandwidth of 2 pm. This work is complementary to measurements of atomic hydrogen performed by intracavity laser absorption in CH₄-H₂ mixtures.