A STUDY OF HYDROCARBON DISSOCIATION IN A MICROWAVE-INDUCED PLASMA SUSTAINED IN AR GAS

The dissociation processes occurring for a simple hydrocarbon introduc ed into a microwave induced plasma are characterized in terms of an eq uilibrium model. The plasma is sustained in Ar using a TM010 resonant cavity operating at 2.45 GHz. Propane was used as the test hydrocarbon . It has a relatively simple molecular structure and can easily be int roduced into the plasma in the gas phase, which requires that the plas ma only uses energy to dissociate and excite emission from the sample constituents. The dissociation of propane was studied by measuring emi ssion from C and C2 at different spatial positions in the plasma and a s a function of the amount of analyte introduced. The emission intensi ties are converted to number densities of each species using a combina tion of instrumental calibration factors obtained from photon counting and monochromator grating efficiency. An equilibrium constant express ion for the dissociation of C2 to C is calculated for both the side-on and end-on viewing of the plasma. The effect of contaminant water vap or and N2 on the formation of C and C2 is discussed. Analysis of equil ibrium constant data shows that the chemical reactions involving carbo n occurring in the plasma are not in equilibrium. Rather these process es are better characterized as occurring under steady state conditions in which the emission measurements are made prior to C and C2 reachin g their equilibrium concentrations. Equilibrium is precluded by gas fl ow turbulence and energy gradients in the plasma.