Laser Raman scattering in fuel-rich flames: background levels at different excitation wavelengths

A comparative study has been performed in order to quantify the signal-to-background ratio of Raman measurements in fuel-rich flames using pulsed laser radiation at 532, 489, 355 and 266 nm. For the comparison, spectra were recorded in stable premixed, sooting and non-sooting C 2 H 4 /air flames with equivalence ratios of ϕ = 1.58, 1.92 and 2.0. The spectra show that the background, consisting mainly of laser-induced emissions from polycyclic aromatic hydrocarbons and from C 2 radicals, changes significantly with the laser wavelength. Although the Raman cross sections increase at shorter wavelength the signal-to-background ratio in the flames investigated is clearly the best for λ = 532 nm and the worst for λ = 266 nm. The presented results show the potential and limitations of concentration measurements by laser Raman scattering in fuel-rich flames and are a valuable aid for the decision of what laser system should be used for measurements in laboratory or practical flames.