Saturation and lifetime effects on degenerate four-wave mixing spectroscopy

The effect of rotational predissociation on the degenerate four-wave mixing (D4WM) spectrum of the A((2)A") <-- X((2)A') band of HCO is investigated. We find that D4WM signal intensities depend on the excited state lifetime to the fourth power in the limit that the rotational predissociation rate is much faster than the collision depopulation rate. The effect can significantly reduce the apparent rotational line strength even when the predissociation is not readily observable through its influence on the line profile as a result of the concomitant homogeneous line broadening. The effects of rotational predissociation are generally more pronounced for the higher rotational states, and, consequently, failure to take account of its influence on the D4WM rotational line strength can lead to a significant distortion of the band contour. Since D4WM has been widely applied as a temperature probe, in, for example, combustion, the conclusion is that this effect must be taken into account when interpreting D4WM spectra from species which exhibit rotational predissociation. (C) 1999 American Institute of Physics. [S0021-9606(99)00809- 0].