High resolution degenerate four wave mixing spectroscopy of OH radical: the Doppler effect an the lineshape and linebroadening in the backward geometry

The degenerate four wave mixing spectroscopy makes the spatial separation of signal from background noise possible and it has recently emerged as one of the nonlinear spectroscopic techniques to measure the temperature and concentration of trace molecules in a hostile environment due to its high signal-to-noise ratio. The temperature of the transient molecule like the OH radical in combustion can be usually measured by a Boltzman plot or a spectral simulation of many rotational lines. The basic assumption of these methods is that the DFWM signal is produced by a population grating between the probe beam and one of the pump beams. It is, however, reported that the thermal grating is dominant at high pressure and low temperature, and the DFWM signal generated in this condition can give poor information for the temperature measurement. How the environments, like the polarization of input beams, the geometry of DFWM experiment, pressure and temperature of the gas etc., will influence the DFWM signal, needs to be answered to get the best condition for accurate measurement. The high temperature of the flame gives a relatively large Doppler linebroadening compared to the collisional linebroadening. Thus, the lineshape of the OH radical in a flame is expected to be closer to Lorentzian than to a Lorentzian cubed profile, which is for the case of small Doppler linebroadening.