Anisotropy of angular momenta ideas and methods

It is rather likely that understanding of the anisotropic distribution of the angular momenta of atoms was enhanced by Hanle s [184] discovery in 1924. Hanle observed the effect of the disappearance of linear polarization in fluorescence in a weak magnetic field directed at right angles to the E- 

The most natural method of detecting the optical polarization produced in the excited state consists of using the polarization characteristics of molecular fluorescence from this state in some transition b — c which is convenient for observation see Fig. 1.2. The same radiation, in the cycle a — b c may also provide information on the initial state a if the process of absorption is non-linear. A more direct way of probing the anisotropy of angular momenta distribution in the ground state consists of monitoring the absorption from this level by means of a test beam. 

The description used in the present book does not, however, confine itself to the limit of an infinitely large angular momentum. As is well known, the problem is in its simplest form in the case of either small momenta (J 1), or in that of very large ones (J 1). We will also cover the possibility of approaching the problem in the most unfavourable case of an arbitrary angular momentum. 

Less popular (undeservedly, in our opinion) is the employment of optical polarization of angular momenta and of external field effects for the determination of structural parameters of molecules. In particular, unique information can be obtained from Lande factor measurements, the magnitude and sign of which form a singularly sensitive indicator of intramolecular interactions of various types [267, 294]. 

finally, one must keep in mind the fact that non-spherical distribution of the angular momenta of a molecular gas can affect the results of quantitative spectroscopy. And if one uses laser sources, so popular today, non-linear effects, including non-linear optical polarization of ground state angular momenta, are by no means rare phenomena, and the art of their evaluation may become necessary in the everyday practical work of the spectroscopist. 

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