Temperature determination of transient species by degenerate four wave mixing: Application of the independently determined power law of the transition dipole moment and geometric factors

We investigated the dependence of the degenerate four wave mixing (DFWM)signal intensities on the electronic transition dipole moment of isolated lines in the A (1) Sigma(+)<--X (1) Sigma(+) transition band of NaH. We applied a new method to determine this dependence in transient species without previous knowledge of the sample temperature. By using different appropriate pairs of DFWM lines sharing common lower level, the relative population difference is eliminated. We found that this ratio is well described by a power law (mu(1)/mu(2))(x) where mu(i) is the electronic one-photon transition dipole moment. As a result of saturation the exponent x depends on the total laser energy deposited in the medium. The observations are in good agreement with a modified two-level model, which includes the effects of polarization of the laser beams used in the experimental setup by introducing geometric factors. The exponent of the integrated signal intensities varies between 3 and 8 for high and low laser intensities with a rapid decrease to high intensities. This makes it possible to compare signal intensities at different DFWM laser intensities in order to obtain relative level populations, i.e., temperature, with high sensitivity. (C) 1996 American Institute of Physics.