MEASUREMENT OF ORIENTATIONAL RELAXATION-TIMES OF OH IN A FLAME USING PICOSECOND TIME-RESOLVED POLARIZATION SPECTROSCOPY

Picosecond time-resolved polarization spectroscopy in the ultraviolet has been used to measure relaxation times of the laser-induced optical anisotropy of OH in an atmospheric pressure name. OH radicals were pr oduced in the post flame gases of a methane/air flame. Transient signa ls from absorption in the A(2) Sigma-X(2) Pi (0-0) electronic transiti on were studied by pump-probe experiments using different P-and Q-bran ch transitions. A theoretical approach has been developed to interpret experimentally observed transient signals in terms of the relaxations of molecular orientation and alignment. The observed effective relaxa tion times for these flame conditions are of the order of 240-590 ps d epending on the rotational state. We found slightly larger values for the relaxation times of molecular orientation than for molecular align ment. These results are relevant to the interpretation and modeling of four-wave mixing spectra.