Rotational level dependence of ground state recovery rates for OH X2Π(v"'=0) in atmospheric pressure flames using the picosecond saturating-pump degenerate four-wave mixing probe technique

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)Pi(v"=0) 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)Sigma-X (2)Pi(0,0) transition. The recovery of the depleted ground state population then is monitored by probing the same level via the (1,0) band at 283 nm using picosecond degenerate four-wave mixing (DFWM). 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, N-', ranging from 2-16 and 2-13 for stoichiometric CH4/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 (1.8+/-0.4)x10(9) s(-1) 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)Sigma(+) state of OH for similar flame conditions. (C) 2002 American Institute of Physics.