Rotational level dependence of ground state recovery rates for OH X 2Π(v″=0) in atmospheric pressure flames using the picosecond saturating-pump degenerate four-wave mixing probe technique

Abstract

Femtosecond degenerate four-wave mixing of carbon disulfide: High-accuracy rotational constants Polarization and probe delay effect on degenerate four wave mixing of pyrazine We report the first direct measurement of the rotational level dependency of the rate of recovery of initially depleted levels in the electronic ground state X 2 ⌸(vЉϭ0) of OH produced in different flame environments at atmospheric pressure. The initial depopulation of a specific rotational level is accomplished by an intense picosecond pump pulse at 308 nm to partially saturate the electronic A 2 ⌺ -X 2 ⌸(0,0) transition. The recovery of the depleted ground state population then is monitored by probing the same level via the ͑1,0͒ band at 283 nm using picosecond degenerate four-wave mixing ͑DFWM͒. Both laser wavelengths were derived from the pulse-amplified and frequency doubled output of two independently tunable distributed feedback dye lasers operated with Rh101 and Rh6G in ethanol, respectively, and pumped with the second harmonic of a frequency doubled ps-Nd:YAG laser. It is shown that the rate of repopulation of the depleted ground state levels decreases by 54% and 50% with increasing rotational quantum number, NЉ, ranging from 2-16 and 2-13 for stoichiometric CH 4 /air and H 2 /O 2 /He flames, respectively. Within experimental error their absolute values in both flames are equal and are not noticeably sensitive to an unequal depletion of the Zeeman sublevels, as created for different polarization configurations of the saturating pump beam and the DFWM probe beams. The rate of (1.8Ϯ0.4)ϫ10 9 s Ϫ1 averaged over all rotational transitions investigated is smaller by a factor of 3 than the corresponding average rate of the temporal DFWM signal intensity decay determined by us previously. The rate also is smaller than total depopulation rates obtained in the excited A 2 ⌺ ϩ state of OH for similar flame conditions.

Simultaneous OH-PLIF and PIV measurements in a gas turbine model combustor

“…The spectral laser intensities in the imaged area was approximately 3.1 MW/cm 2 cm −1 for OH. The applied laser intensities were above the saturation intensities of approximately 1 MW/cm 2 cm −1 [32] for the chosen transitions of OH and thereby a considerable degree of saturation is to be expected in measurements. By means of a beam splitter, a small portion of the incoming laser beam is directed into a CH 4 /air reference flame in a McKenna burner regulated by means of digital mass flow controllers, operating under laminar conditions.…”

Phase-Locked Temperature Measurements by Two-Line OH PLIF Thermometry of a Self-Excited Combustion Instability in a Gas Turbine Model Combustor

“…Saturation has to be minimised, in order to reduce its influence on the temperature determination. The influence on the temperature is due to different rotational energy transfer rates (RET) in the ground states of the selected transitions [19,20].…”

Laser induced fluorescence in atmospheric pressure discharges

“…RET rate coefficients in the ground state are not well known, but are thought to be slightly larger than those of the A state [63]. In [64] and references therein, the recovery rate of an initially depleted level of the X state was measured in various combustion mixtures at high temperature. For low N level rates of the order of (3 − 6) × 10 9 s −1 where found, almost independent of the gas mixture.…”