A level-crossing model for infrared pump-probe spectroscopy of water

The level-crossing model is applied to pump-probe spectroscopy of the intra molecular vibration of hydrogen-bonded HDO molecules dissolved in H2O. The parameters of the model are extracted from experimental data. It is shown t hat within the linear approximation for the dependence of the vibrational t ransition frequency on the hydrogen-bridge length, the pumping light satura tes not only the upsilon = 0 --> upsilon = 1, but also the upsilon = 1 --> upsilon = 2 transition, resulting in the suppression of superabsorption in transmission spectra. The non-linearity of the transition frequency may be introduced through either anharmonic corrections to the vibrational energy levels, or different curvatures of parabolic levels. As the result, the 1 - -> 2 transition shifts out of resonance with the pumping field, no populati on of the upsilon = 2 level appears, and the transmission spectra fit bette r to experimental observations.