Saturation studies of H2O and HDO near 3400 cm−1 using intense picosecond laser pulses

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Abstract
The effect of significant decrease of water absorptivity for the intense picosecond laser radiation at λ=2.79 and 2.94 μm 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 H‐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 the v=0 to v=1 vibrational transition was displayed. The saturation intensity as high as Is=2.5.1011 W/cm2 in this case gives the value of energy relaxation time of the OH excited state to be in the range 0.3 ps <T1 −1.