Excitation and relaxation of the asymmetric stretch mode of CO2 in a pulsed glow discharge

The excitation and relaxation of the vibrations of CO2 as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed: (1) time-resolved in situ Fourier transform infrared spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3-6.7 mbar and a current range of 10-50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T-3) to the rotational temperature (T-rot) is found to be only dependent on T-rot, in the conditions under study. The decay rate rho(T3-Trot) follows the relation rho(T3-Trot) = 388 s(-1) exp (T-rot - 273 K/154 K). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T-3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05-0.18 for 10-50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T-3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge.