Relaxation mechanism and the detection of collision quenching rate of excited NO2 molecules

Laser induced fluorescence dispersive (LIFD) spectrum of NO2 is obtained at 2.66 Pa sample pressure and with 532 nm laser as radiation source. The spectrum is composed of four vibration series which comes from the transition of excited molecules distributed in different energy levels. It will develop into a continuous envelope centered at the position of 630.7 nm when the pressure exceeds 66.5 Pa. The center wavelength presents red shift with the increase of pressure. These results show that apart from radiation, the dominant depopulation way of excited molecules is collision. The excited energy is converted into translation one by collision and generates sound wave further. The photoacoustic (PA) spectrum of NO2 in 520-600 nm wavelength region is collected. It consists of banded structure which is assigned to the first excited electronic state. The collision quenching rate is acquired by measuring the variation of PA signal versus sample pressure. The optimum excitation and detection wavelengths are decided for the detection of NO2.