LIF Thermometry in a Sooty Flame: An Application of Excitation-Emission Spectrum

A new method for the temperature measurement in a sooty flame was developed, based on an excitation scan. A tunable, narrowband ArF excimer laser was used to excite various absorption lines in D²Σ⁺ ← X²Π(0,1) band of NO. Spectrum of the laser-induced emissions from the flame was obtained by using a spectrograph. In the case of a propane diffusion flame, this emission spectrum consisted of NO and O₂ fluorescence, and off-resonance emissions related to the soot particles. These off-resonance emissions were usually stronger than the fluorescence. However the off-resonance emissions in the shorter wavelength region than 250 nm were weaker than the fluorescence. In other words, the ε(0,3) band of NO fluorescence which appeared in the ultra-violet region (around 208nm), was stronger than the off-resonance emissions, and was free from the O₂ fluorescence. Therefore, the NO rotational temperature in a sooty flame could be deduced from the excitation spectrum based on the ε(0,3) band. In order to obtain the excitation spectrum based on the ε(0,3) band, an excitation-emission spectrum (EES) was obtained. NO rotational temperature in a propane diffusion flame was derived from this EES.