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 flame. OH radicals were produced in the post flame gases of a methane/air flame. Transient signals from absorption in theA2Σ-X2Π (0-0) electronic transition were studied by pump-probe experiments using different P- and Q-branch 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 relaxation times for these flame conditions are of the order of 240–590 ps depending on the rotational state. We found slightly larger values for the relaxation times of molecular orientation than for molecular alignment. These results are relevant to the interpretation and modeling of four-wave mixing spectra.