Kinds of Saturation Effects

Saturation as described above can be observed either when all the molecules in the initial state have the same absorption probability, or when some of the state population is preferentially absorbed, depending on the spatial orientation of the molecules or on their transition frequency, selected from a certain frequency interval. The first case is called homogeneous line saturation, the second is due to inhomogeneous saturation or hole-burning. This different cases will be explained in more detail by discussing some relevant experiments. 

If the number of absorbed laser photons per second becomes larger than this relaxation rate, the initial state will be depopulated, thus decreasing the absorption rate, and therefore the fluorescence intensity will rise less than linearly with the laser intensity (Fig. 12). 

The absorption probability is proportional to the scalar product p = IM I 1 I cos ip, where p is the transition dipole moment, C the electric vector of the exciting light wave and p the angle between and p. 

This means that laser photons are preferentially absorbed by molecules with p // , which results in a partial polarization of the excited state population and with it the fluorescence. 

As soon saturation occurs, those ground-state molecules with p // die out first. This can be detected by observing the corresponding decrease in the fluorescence polarization. Fig. 12 shows the experimental results. This proves that the optical pumping by laser light is even faster than the relaxation rate between molecules of different spatial orientation. 

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Section IV explains a new approach to high resolution spectroscopy based on various kinds of saturation effects. Some of the experiments are performed inside the laser resonator, which implies the presence of coupling phenomena between the absorbing molecules under investigation and the laser oscillation itself. These feedback effects can be used for high-precision frequency stabilization and to measure frequency shifts and line profiles with an accuracy never... 

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