Time Decay Spectrum of NO2 Fluorescence Radiation

Investigation of the optical absorption and fluorescence of NO2 molecule has long been of interest because it is not only one of the key substances of air pollution, but also a stable molecule of nonzero spin and has many special properties such as that the vibronic levels of the first excited state are coupled strongly to the high vibration levels of the ground state, so that once NO2 molecules are excited, they must undergo complicated quenching process. The quenching mechanism influences the lifetime of the excited molecule severely. In the present paper, the fluorescence lifetime of NO excited electronic states are observed experimentally by the technique of LIF time decay spectroscopy and with an optical parameter generator and amplifier pumped by a Nd : YAG laser as excitation source. The results show that the fluorescence lifetime of excited NO molecules depends on the excitation wavelength and sample pressure. The time decay curves present a property of bi-exponential when the excitation wavelength is selected as 429.0, 452.0, 509.0 and 532.0 nm, respectively. This indicates that the fluorescence is composed of two components. One has a long lifetime, while the other has a short one. The short-lived component comes from the radiation of the molecules excited by A(2)B(2), (BB1)-B-2<--X(2)A(1) transition. And the long one is owing to the radiation of the molecules excited to the high rovibronic levels of the ground electronic state. These levels are correlated with A(2)B(2) state. The de-excitation mechanism of the excited molecules is investigated by measuring the variation in fluorescence lifetime versus the sample pressure. The conclusion is that the excited molecules corresponding to the short lifetime quench mainly through the process of radiation and fast inner conversion. As to the excited molecules with long lifetime, the de-excitation process is not only radiation, but also the non-radiation process of collision.