Beats in laser-interrogated dichroism

Magnetic quantum beats in the transient process after pulsed depopulation of the ground state may be observed not only in fluorescence, but also in a more direct way, namely in absorption. In connection with what was discussed in Section 3.5, one must expect maximum sensitivity if the experiment is conducted according to the laser interrogated dichroism method see Fig. 3.17. To this end it is convenient to direct the external magnetic field B along the 2-axis as shown in Fig. 4.21 where the probe beam E-vector can be either in the xy plane (Em) or in the yz plane (Epr2). 

Let us assume that the pumping beam is of the 6-pulse type, i.e. is switched on at time t = 0 for a short time as compared with the relaxation time, whilst the probe beam acts continuously. Substituting (3.31) into Eq. (3.32) in which only the first and the last terms are left and taking 

If the magnetic field is switched off, then, according to (4.49), one may observe relaxation of the anisotropy of the angular momenta, and the transient process will be determined by only one rate of relaxation of alignment 72, which is also an advantage against the situation that one faces in the case of registering the transient process in fluorescence (Section 3.6). 

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