Effect of collisions on one-color polarization spectroscopy of OH A 2Σ+-X 2Π

The effect of collisions on the magnitude of polarization spectroscopy (PS) signals from the OH radical on the A (2)Sigma(+)-X (2)Pi (0,0) band has been studied. OH was produced by the 266-nm photolysis of H2O2 and detected by one-color degenerate PS, using approximate to5-ns pulses from a Nd:YAG pumped dye laser. Spectra with both circular and linear pump polarizations are reported, together with signal dependence on OH number density and pump pulse fluence. The relative line intensities in the spectra and measured square dependence on OH number density are consistent with a description of PS as a variant of four-wave mixing spectroscopy. The pump pulse fluence dependence is fitted well by literature saturation curves. The collisional dependence of the PS signal was investigated by adding increasing pressures of He, Ar, or N-2 collider gases for fixed overlapping pump and probe pulses. The principal finding is the very rapid loss of the PS signal with increasing collider pressure. The resulting phenomenological rate constants are in the range 5-9x10(-9) cm(3) s(-1). We discuss these rate constants with reference to the literature rotational energy transfer and dephasing rate constants. We propose that the very large observed values may be explained by the effect of elastic velocity changing collisions. (C) 2003 American Institute of Physics.