Investigation of weak transient absorptions by the method of induced gratings

Self diffraction of the pump beams at the thermal grating can be enhanced by deposing additional energy in the "transparent" parts of the grating within the lifetime of the excited states. A condition for this is that the wavelength of the applied light is in resonance with an absorption band of the excited state. By this one can detect weak transient absorptions by tuning the wavelength of the second light pulse. As an example for such an experiment the ESA of a 5µm-thick layer of benzophenone in benzene with an OD of about 10-5 around the wavelength of the pump lightis given.