Loss kinetics of carbon atoms in low-pressure high density plasmas

Vacuum ultraviolet absorption spectroscopy (VUVAS) with a carbon hollow cathode lamp was applied to the measurement of decay rate of C atom density in the afterglow of CO and CO/H₂ inductively coupled plasmas. The transition line used for the measurement was 2p3s³ P₂-2p²³P₂ at 165.7 nm. The influence of background absorption by the species in plasma other than C atoms on the transition line of C atoms was found to be negligible. This was clarified by measuring the absorption intensities around the center wavelength of C atoms in plasmas with VUVAS employing a xenon microhollow cathode lamp. Moreover, the dependence of the decay rate of C atom density on pressure revealed that C atoms were dominantly lost at the surface rather than in the gas phase in both CO and CO/H₂ plasmas. However, in the case of CO/H₂ plasma at higher pressures over about 5.0 Pa, C atoms were lost in the gas phase as well as at the surface. The diffusion constants of C atoms in both CO and CO/H₂ plasmas were also determined to be 3.1×10⁴ and 3.7 × 10⁴ cm² Pas^-1, respectively.