"Fluorescence" interference with Raman measurements in nonpremixed flames of methane

What appears to be fluorescence has been measured in the blue (visibly soot free) regions of laminar and turbulent nonpremixed flames of methane at atmospheric pressure. The measurements are part of an experiment to measure the spontaneous Raman scattering of eight species in the flame. The flames studied range from ones with low mixing rates to flames close to extinction. The "fluorescence" is nonresonantly excited by a pulsed dye laser and has been routinely monitored at λ = 516.5 nm which is a bandhead of diatomic carbon, C₂. It is Stokes "fluorescence" that interferes with the Raman signal detection. It is found that the "fluorescence" is broadband covering all the visible spectrum, and is observed only on the rich side of stoichiometry with noticeable intensities for fuel number densities in the range 0.1 to {reversed tilde equals} 1.5 × 10¹⁸ cm^-3. The "fluorescence" intensity is of the order of the Raman signals, increases with the fourth power of the laser flux, is unaffected by flame stretch, and decreases with dilution of the unburned methane. No "fluorescence" is detected at temperatures below {reversed tilde equals} 1000°C. Although the sources of this "fluorescence" are not fully known, it is likely that C₂ is only one of many contributing radicals and molecules. The possible contribution of incandescence from vaporizing soot particle is also discussed. Irrespective of the sources of the "fluorescence," its interference with the Raman lines of stable species in the flame can be corrected for using Stokes and anti-Stokes "fluorescence" monitors, and the Raman scattering technique can therefore be used for measurements in the blue regions of methane flames and may be extendable to higher hydrocarbon flames.