The results of the recent infrared pump-probe experiment are analyzed, in which the time evolution of the spectrum of the OH-stretching vibration excited by an ultrashort laser pulse in the dilute solution HDO/D_2O was measured. To interpret the results of the experiment, a formalism developed earlier for a different application, optical transitions of the chromophoric molecules in polar solvent, is used. A mechanism of the IR shift dynamics in water is suggested, involving α-relaxation (structural relaxation), which may also be related to the dielectric behavior of water. A suggestion is made for computer simulations as well as for an experimental test of the proposed mechanism.
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