Time-resolved spectroscopic study of photofragment fluorescence in methane/air mixtures and its diagnostic implications

In this work 80-picosecond laser pulses of 266-nm wavelength with intensities up to (2.0 +/- A 0.5) x 10(11) W/cm(2) were used for fragmentation of methane/air gas mixtures at ambient pressure and temperature. Emission spectra are, for the first time, studied with ultrahigh temporal resolution using a streak camera. Fluorescence spectra from CH(A(2)Delta-(XI)-I-2 , (BI)-I-2 (-)-(XI)-I-2 pound , (CI)-I-2 (+)-(XI)-I-2 pound ), CN((BI)-I-2 (+)-(XI)-I-2 pound (+) pound, Delta v = 0 and Delta v = +/- 1), NH(A(3)I (-)-(XI)-I-3 (-)) pound, OH(A(2)I (+)-(XI)-I-2 pound ) and N-2 (+)((BI)-I-2 pound (u) (+) -(XI)-I-2 pound (g) (+) ) were recorded and analyzed. By fitting simulated spectra to high-resolution experimental spectra, rotational and vibrational temperatures are estimated, showing that CH(C), CN(B), NH(A), and OH(A) are formed in highly excited vibrational and rotational states. The fluorescence signal dependencies on laser intensity and CH4/air equivalence ratio were investigated as well as the fluorescence lifetimes. All fragments observed are formed within 200 ps after the arrival of the laser pulse and their fluorescence lifetimes are shorter than 1 ns, except for CN(B-X) Delta v = 0 whose lifetime is 2.0 ns. The CN(B-X) Delta v = 0 fluorescence was studied temporally under high spectral resolution, and it was found that the vibrational levels are not populated simultaneously, but with a rate that decreases with increasing vibrational quantum number. This observation indicates that the rate of the chemical reaction that forms the CN(B) fragments is decreasing with increasing vibrational state of the product. The results provide vital information for the application of laser diagnostic techniques based on strong UV excitation, as they show that such methods might not be entirely non-intrusive and suffering from spectral interferences, unless the laser intensity is kept sufficiently low. Finally, equivalence ratios were determined from "unknown" spectra using multivariate analysis, showing a good agreement with theoretical compositions with an error of 4 %. The method is expected to be a useful diagnostic tool for measurements of local equivalence ratios in for example combustion environments