MEASUREMENT OF ORIENTATIONAL RELAXATION TIMES OF OH (A(2)SIGMA-X-2-PI) TRANSITIONS IN ATMOSPHERIC-PRESSURE FLAMES USING PICOSECOND TIME-RESOLVED NONLINEAR SPECTROSCOPY

Time-resolved picosecond pump-probe polarization and degenerate four w ave mixing (DFWM) spectroscopy have been applied for the observation o f coherently excited states in the (0,0) band of the electronic A(2) S igma-X-2 Pi transition of OH in atmospheric pressure CH4/air and H-2/O -2 flames. From the exponential decays of the signal after excitation with a 20 ps pump pulse the decay of orientation and alignment through dephasing collisions and rotational energy transfer takes place withi n 150-370 ps depending on the rotational state quantum number. For sim ultaneously excited transitions within the bandwidth of the picosecond laser pulse, oscillations in the transient DFWM signal superimposed o n the exponential decay have been observed for cross-polarized pump an d probe laser beams. (C) 1997 Elsevier Science B.V.