Time evolution of vibrational temperatures in a CO2 glow discharge measured with infrared absorption spectroscopy

Vibrational temperatures of CO2 are studied in a pulsed glow discharge by means of time-resolved in situ Fourier transform infrared spectroscopy, with a 10 mu s temporal resolution. A method to analyze the infrared transmittance through vibrationally excited CO2 is presented and validated on a previously published CO2 spectrum, showing good agreement between fit and data. The discharge under study is pulsed with a typical duty cycle of 5-10 ms on-off, at 50 mA and 6.7 mbar. A rapid increase of the temperature of the asymmetric stretch vibration (T-3) is observed at the start of the pulse, reaching 1050 K, which is an elevation of 550 K above the rotational temperature (T-rot) of 500 K. After the plasma pulse, the characteristic relaxation time of T-3 to Trot strongly depends on the rotational temperature. By adjusting the duty cycle, the rotational temperature directly after the discharge is varied from 530 to 860 K, resulting in relaxation times between 0.4 and 0.1 ms. Equivalently, as the gas heats up during the plasma pulse, the elevation of T-3 above T-rot decreases strongly.