Degenerate four-wave mixing of S-2 and OH in fuel-rich propane/air/SO2 flames

Absolute concentration profiles of S-2 and OH in a premixed propane/air/SO2 dame at atmospheric pressure are determined by DFWM and absorption spectroscopy in the wavelength range from 308.5 to 310.5 nm. The OH radical is monitored via the well known (0,0) band of the electronic A((2) Sigma(+))-X((2) Pi) system. Upon addition of SO2 to the fuel stream numerous transitions of sulfur containing species are observed in addition. Recent molecular parameters for the B((3) Sigma(u)(-))-X((3) Sigma(g)(-)) transition of S-2 are used to compute a synthetic spectrum of the molecule within the accuracy of the laser system (approximate to 0.15 cm(-1)) that overlaps favorably with major features in the experimental spectra. In spite of the dense spectrum, isolated transitions are observed that are suitable for concentration measurements. Relative concentrations of OH and S-2 are mapped by taking advantage of the high sensitivity of the DFWM technique. Absorption spectroscopy, on the other hand, is used to obtain an absolute number density of OH at positions in the flame where a significant level of the radical occurs. These measurements are linked to the relative OH profile and yield the absolute concentration of the hydroxyl as a function of height above the burner surface. Furthermore, the relative S-2 profile obtained by DFWM is put on an absolute scale by quantitative comparison with the OH profile. The required Einstein B coefficients for the B-X transition of S-2 are obtained from a calculation of Honl-London factors and taking into account the Franck-Condon factors and the electronic transition moment from the literature. The measured profiles of S-2 and OH are in good qualitative agreement with a recent theoretical model of the sulfur chemistry in flames.