Intensity and pressure dependence of resonance fluorescence of OH induced by a tunable uv laser

We have investigated the intensity and pressure dependence of the fluorescence spectrum of OH in the presence of ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$O molecules. molecules. Saturation of the absorption transition was observed at low pressures, and the corresponding fluorescence signal was found to vary as the square root of the exciting intensity. This observed dependence agreed with the predicted dependence which took into account the presence of laser modes in the spectrum of the exciting radiation. With full laser power incident, a saturation parameter as high as 3 × ${10}^5$ was observed. The fluorescence spectrum was found to peak at 3145 Å and at 3090 Å, with the relative peak intensities dependent upon gas pressures and upon the particular rotational electronic transition used for excitation. It is concluded that vibrational relaxation of the electronically excited OH due to water vapor in the system plays a dominant role in determining the observed flurescence spectrum.