Detection of ground-state atomic hydrogen in a dc plasma using third-harmonic generation

Third-harmonic generation (THG) was used to monitor ground-state atomic hydrogen H(1s ²S_1/2) in a dc plasma system. A 364.6 nm laser beam focused through H₂ or CH₄/H₂ plasmas induced THG at 121.5 nm, near the atomic hydrogen 2p ²P^o_J→1s ²S_1/2 Lyman-α transition. Both the intensity and frequency shift of the excitation spectra exhibited dependence on the plasma power. Absolute H atom concentration was estimated by comparing the frequency shift to that obtained in a calibrated microwave discharge flow system. The sensitivity was ∼4×10¹³ cm^-3 (100 ppm). The measured atomic hydrogen densities were substantially less than in other diamond chemical vapor deposition methods and may explain the lower diamond deposition rates obtained with dc plasma systems of this type.