Absolute concentration and loss kinetics of hydrogen atom in methane and hydrogen plasmas

A measurement technique of the absolute concentration of hydrogen (H) atoms in methane (CH4) and/or hydrogen molecule (H-2) plasmas has been established. The H-atom concentration was evaluated by vacuum ultraviolet absorption spectroscopy (VUVAS) using a high-pressure H-2 microdischarge hollow cathode lamp (H-2-MHCL) as the Lyman alpha (L-alpha 121.6 nm) light emission source. A measurement technique of the background absorption caused by species other than H atoms at the L-alpha line was developed by using the VUVAS technique with the MHCL employing nitrogen molecules (N-2-MHCL). The lines around L-alpha used for the background absorption measurements are 2p(2)3s P-4(5/2)-2p(3) S-4(3/2)0 at 119.955 nm, 2p(2)3s P-4(3/2)-2p(3) S-4(3/2)0 at 120.022 nm, and 2p(2)3s P-4(1/2)-2p(3) S-4(3/2)0 at 120.071 nm of the N atom. By using the VUVAS technique with the MHCLs and subtracting the background absorption from the absorption of H atoms at L-alpha, we have achieved the measurement of the H-atom concentration in an inductively coupled plasma operated in CH4 and/or H-2. The H-atom concentration increased from 2x10(11) to 3x10(12) cm(-3) when increasing the CH4 flow rate ratios from 0% to 50% in the CH4-H-2 mixture and was almost constant in its range between 50% and 100% at a pressure of 1.33 Pa, a radio frequency power of 200 W, and a total flow rate of 100 sccm. The behavior of the H-atom concentration was compared with that of the Balmer alpha emission intensity. The decay of the H-atom concentration in the H-2 plasma afterglow was investigated to clarify the loss kinetics of H atoms. The dependence of the decay time constant on the pressure showed that H atoms were dominantly lost through diffusion to the wall surface. The diffusion constant of H atoms in H-2 plasmas was determined to be 3.0x10(5) cm(2) Pa s(-1) at 400 K. The surface loss probability of H atoms on the stainless-steel and the hydrocarbon walls were estimated to be 0.15 and 0.07, respectively. (C) 2001 American Institute of Physics.