The effect of significant decrease of water absorptivity for the intense picosecond laser radiation at lgr;=2.79 and 2.94 mgr;m being near the center of the OH stretching mode absorption band was discovered. In case of pure water a thermal mechanism dominated: A very fast temperature rise led to weakening of Hhyphen;bonds and consequently to the absorption band shift towards higher frequencies. As a result, a considerable (up to 10 times) decrease in the optical density at the laser frequency was obtained. In the second case of HDO diluted in D2O the temperature effects were eliminated and a pure spectroscopic saturation of thev=0 tov=1 vibrational transition was displayed. The saturation intensity as high asIs=2.5.1011W/cm2in this case gives the value of energy relaxation time of the OH excited state to be in the range 0.3 ps T10.6 ps. The width of the homogeneously broadened component of the fundamental OH band in HDO spectrum is evaluated to be ge; 40 cmminus;1.
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