RESUMO
With 532 nm laser as excitation source, the excitation and relaxation process of NO2 molecule was investigated by the technique of photoacoustic and fluorescence emission spectra. The results show that NO2 molecules will be pumped to the first excited electronic state by laser photon. When the sample pressure is lower, some of the excited molecules relax to the ground state by radiation process directly; the other parts are redistributed to a few of the excited rovibronic energy levels by the process of fast internal energy transfer. With the increase in the sample pressure, continual collisions dominate the relaxation process gradually. This makes the excited molecules to be redistributed to many excited rovibronic energy levels. Emission from these excited levels forms a continuous spectrum. Just then, the efficiency of fluorescence emission from laser excited level decreases and the fluorescence intensity on the long wavelength side increases. The intensity of PA signals increases also. These phenomena indicate that besides the relaxation process of radiation, there is a strong relaxation process of continual collision under the condition of higher sample pressure. It converts vibration energy of the excited molecules into translation one. This induces the increase in gas temperature and a sound wave is produced.
