Rotational level dependence of ground state recovry rates for OH X~2II(v''=O) in atmospheric pressure flames using the picosecond saturating-pump degenerate four-wave mixing probe technique

摘要

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~2II(V''=0) of OH produced in different flame environments at atmospheric pressure.The initial depopulation of a specfic rotational levelis accomplished by an intense picosecond pump pulse at 308 nm to partially saturate the electronic A ~2sum-~2II(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 decrease by 54% and 50% with increasing rotational quantum number, N'', 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 (1.8+-0.4)X10~9 s ~(-1) average rate of the temporal DFWM signal intensity decay determined by us previously. The rate also is smaller that total depopulation rates obtained in the excited A ~2smu~+ state of Oh for similar flame conditions