Temperature measurements of the hydroxyl radical and molecular nitrogen in premixed, laminar flames by laser techniques.

Temperature profiles in an atmospheric pressure, premixed, laminar, methane-air flame were measured andsystematically compared by two different nonperturbing, laser light scattering methods. The rotationaltemperature of the lowest vibrational state of the electronic ground state of OH was measured by laser-induced fluorescence. The vibrational temperature of the electronic ground state of N(2) was also measured by laser Raman scattering. The first quantitative comparison of these two temperatures with a spatial resolution of less than 100 microm throughout both the reaction zone and the postflame zone is presented. The analysis of the data indicates that the N(2) vibrational temperature is in equilibrium with the OH rotational temperature at all points.

Hydroxyl and its concentration profile in methane-air flames.

Hydroxyl concentration profiles are measured in several atmospheric-pressure premixed laminar flames. The first quantitative comparison of hydroxyl concentration profiles to recent flame models is presented. These models include the influence of hydroxyl on fuel breakdown, on carbon monoxide oxidation, and on other species concentrations. The hydroxyl concentration profiles are measured by laser-induced fluorescence, and absolute mole fraction concentrations are determined by normalizing to laser absorption measurements in similar flames. The 100-microm spatial resolution of laser-induced fluorescence provides the first optical measurements of the flame's reaction zone at atmospheric pressure. The data are consistent with the electronic-excited state of hydroxyl having a negligible influence on the flame's structure.

Spontaneous Raman scattering by ground-state oxygen atoms.

We report the first known observation of Raman scattering by oxygen atoms. The (3)P(2)?(3)P(1) and (3)P(2)?(3)P(0) transitions in the electronic ground state that produced Raman shifts of 158 and 227 cm(-1) were detected. These transitions were observed in a fuel-lean atmospheric H(2) + O(2) flame. By comparing the O electronic and O(2) pure-rotational Raman-scattering intensities, we measured the polarized cross sections for the two lines to be 6 +/- 1 x 10(-31) and 4 +/- 1 x 10(-31) cm(2)/sr, respectively, with an excitation source at 532.1 nm. These cross sections are two to three times stronger than those predicted by a single-configuration single-excitation Coulomb approximation.

Coherent anti-Stokes Raman spectra of oxygen atoms in flames.

Coherent anti-Stokes Raman spectroscopy (CARS) was used to detect oxygen atoms (electronic Raman scattering) and oxygen molecules (rotational Raman scattering) in both hydrogen-oxygen and methane-oxygen flames. The high spectral resolution of CARS is useful for distinguishing the oxygen-atom signals from larger nearby rotational Raman signals. Saturation of the molecular CARS signal that is due to stimulated Raman scattering was observed. This effect limits the sensitivity of the CARS method.

Picosecond laser-spectroscopy measurement of hydroxyl fluorescence lifetime in flames.

Data are reported for the first known application of picosecond laser spectroscopy to the measurement of collisional quenching rates by time-resolved laser-induced fluorescence in flames. Collisional quenching rates are important for the determination of species concentrations by laser-induced fluorescence. The collisional quenching lifetime following excitation of the R(2)(4) A(2)Sigma(+) (nu' = 0) ? X(2)II (nu'' = 0) transition was measured to be 1.8 nsec in the burned-gas region of an atmospheric-pressure, premixed methane-air flame.