Light absorption saturation studies of H 20 and HDO near 3400 cm −1 with intense laser radiation

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 centre of the OH stretching mode absorption band was discovered. In the 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 D 2O the temperature effects were eliminated and a pure spectroscopic saturation of the v = 0 to v =1 vibrational transition was displayed. The value of the saturation intensity as high as I s=2.5 × 10 11W cm −2 in this case gives the value of energy relaxation time of the OH excited state to be T 1=0.6 ps. The width of the homogeneously broadened component of the fundamental OH band in HDO is evaluated to be greater than or equal to 50 cm −1.