Nonlinear spectroscopy of photocoloured polytungstic acid nanocomposites

Picosecond absorption spectroscopy was used in an investigation of nonlinear optical processes in nanostructured composites based on polytungstic acid, which were photocoloured by preliminary UV irradiation. The absorption bands responsible for the photocoloration and associated with intervalent charge transfer were bleached by the action of powerful laser pulses. The characteristic monoexponential relaxation time of the process was ~300 ps. The results obtained were analysed taking account of the special features of the splitting of the energy levels of tungsten ions with different degrees of oxidation and of delocalisation of the valence states within separate elements of the polytungstic acid structure.