TG-spectroscopy

In the TG experiment (our setup) the sample is excited by two laser beams of spatial and longitudinal coherence. These beams of intensities Ia and Ib and wavelength lpump are crossed inside the sample with an angle Q. If the excitation wavelength is in resonance with an absorption band of the sample the complex refraction index of the sample is periodically modulated with a period Lambda: (n = refraction index of the sample). The period of the induced grating-like structure can thus be varied by simply changing the crossing angle of the two pump beams. The grating is detected by diffraction of a probe beam that in case of a "thick" grating has to meet the Bragg condition If the grating is "thin" the probe beam can hit the grating in any angle, in this case also self-diffraction of the pump beams into higher orders occurs. During the lifetime of the excited states the modulation of the complex refraction index comes from a variation of the population density. The excited states may decay via a photochemical reaction so that a chemical concentration grating rises up. They may also relax in a nonradiative way giving rise to a thermal grating. The different kinds of gratings can be identified by their characteristic lifetimes. Excited State Absorption of weakly absorbing samples can be detected by the use of a thermal grating.