Photofragmentation - Laser Induced Fluorescence Detection of NO2

This paper presents the first results from a newly designed system for the purpose of detecting NO2 using the method of photofragmentation - laser induced fluorescence (PF-LIF). This method was proposed by Davis, et al. (1979) and by Rodgers, et al. (1979,80) for the detection of several fluorescing and non-fluorescing compounds. In this technique, the species to be detected is made to decompose into one or more vibrationally excited fluorescent species. One of these vibrationally excited photofragments is then optically pumped into an electronically excited state. A fraction of this resulting fluorescence, occurring at shorter wavelengths than the laser pump, is then sampled using appropriate long wavelength blocking filters. This allows for major discrimination against the white fluorescence background from aerosols, gas phase impurities, and the scatter chamber walls. In conventional laser induced fluorescence, this white fluorescence background can never be entirely eliminated as some of it must always fall within the bandpass of the detection system. In the PF-LIF approach, the white fluorescence noise can be reduced, at least in principle, to the level of the PMT dark count by the extensive application of long wavelength blocking filters.