Fluorescent population of vibronic levels

Let us now consider the following experimental scheme (see Fig. 3.14). Let the level J be populated by fluorescence in the cycle J — J — J , that is, in a three-state cycle of so-called A-configurations, where state J has greater energy than J and J. To probe the polarization of the angular momenta created on level J laser-induced fluorescence in the second cycle J — J[ — J f may be used (see the broken lines in Fig. 3.14). 

When molecules arrive at the state with rotation quantum number J from the state J in the process of spontaneous radiation at rate (see Fig. 3.14), a photon possessing unit spin is emitted in an arbitrary direction. Let us assume that the angular momentum carried away by the emitted photon is small, as compared with both 3 [ and Jr. This means that the angular momentum vector of each separate molecule does not change its value and does not turn in space as a result of the spontaneous transition. Consequently, the angular momenta distribution pji(0,ip) is 

Here the first term in the righthand side determines the rate of creation of polarization moments in the spontaneous transition process, whilst the second term describes their relaxation. In writing Eq. (3.27) it is assumed that collisions do not lead to population of the state J , since it lies sufficiently high and is surrounded by non-populated rovibronic levels within the range of thermal energy kT. 

Let us find the magnitude of orientation and alignment of level J at its fluorescent population J —+ J, when J is excited by the action of weak light causing a transition J — J (see Fig. 3.14). The magnitude of moments j Pq created on level J may be found from (2.21), hence we know the rate of the generation of moments on level J . The 

If we compare these values with the corresponding ones, obtained in the optical polarization of angular momenta of the lower level by depopulation (see Figs. 3.3, 3.4 and 3.5), then we will see that similar values can be obtained only in the case of a large non-linearity parameter, x — 10 and more. 

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