Rotational temperature measurements of excited and ground states of C2(d 3Πg−a 3Πu) transition in a H2/CH4 915 MHz microwave pulsed plasma

Abstract

The rotational temperature of a low energy (0.09 eV), C2 Swan band state (a 3Πu), obtained by white light absorption, is compared to the rotational temperatures of three electronic excited states [C2(d 3Πg),CH(A2Δ) and CN(B 2Σ+)] in a high power, H2/CH4 microwave plasma used for diamond deposition. All temperatures are measured at 50 mbar, and both continuous (as a function of microwave power) and pulsed (as a function of time after the pulse) modes of operation are investigated. The rotational temperature of C2’s excited state is found to be higher than that of the low energy state (assumed equal to the gas temperature), indicating that the excitation of C2 is to a large extent the result of chemical reactions (chemiluminescence) rather than electronic excitation. The rotational temperatures of CH(A 2Δ) and CN(B 2Σ+) excited states are also higher than that for C2’s low energy state temperature.