High resolution degenerate four wave mixing spectroscopy of the OH radical: The doppler effect on the spectral lineshape and line broadening in the phase conjugate geometry

An experiment on high-resolution degenerate four wave mixing (DFWM) spectroscopy is performed at an unsaturated laser power and in a backward geometry. The lineshape and line broadening of the DFWM signal of the OH radical [(1,0) Q ₂(11) rotational line in X ²II↔A ²∑] were measured in a propane/air premixed flame. The lineshape dependence on the relative magnitude of the collisional linewidth and the Doppler width is discussed and the influence of the large Doppler broadening in the flame causes the lineshape to be close to a Lorentzian. By fitting the data to the DFWM moving absorber model, we found that the collisional linewidth (0.1136 cm ^-1) for the transition was larger than other reported values for a different vibrational band of this molecule; the reason for that is discussed.