Time resolved tunable infrared diode laser absorption spectroscopy has been
used to detect the methyl radical and four related stable molecules, CH4,
C2H2, C2H4 and C2H6, in H-2 surface wave plasmas (f = 2.45 GHz, power densi
ty approximate to 10-50 W cm(-3)) containing 10% methane under static condi
tions at different pressures (p = 0.1-4 Torr). For the first time, the time
dependence of the conversion of methane to the methyl radical and three st
able C-2 hydrocarbons was studied in a fixed discharge volume nearly up to
a stationary state. The degree of dissociation of the methane precursor was
found to increase by up to 96% in the stationary state, and the methyl rad
ical concentration was measured to be in the range of 10(12)-10(13) molecul
es cm(-3). The concentrations of both C2H2 and C2H4 produced in the plasma
showed a maximum at a distinct time before decreasing. In contrast, the C2H
6 concentration was observed to increase with time to a nearly constant val
ue between 6 X 10(12) and 2 X 10(14) molecules cm(-3) varying with pressure
.
Based on time resolved concentrations, conversion rates to the measured C-2
hydrocarbons (R-C(C2Hy) = 10(11)-10(13) molecules J(-1)) could be estimate
d in dependence on pressure in a surface wave discharge. The influence of d
iffusion on the spatial distribution of the hydrocarbon concentration in th
e discharge tube was considered. A qualitative model has been developed in
order to describe the chemical processes and to identify the main plasma ch
emical reaction paths.