The creation of lasers stimulated the rapid development of many new directions in atomic physics. In a number of them, the role of laser radiation is restricted to “preparation”of the polarized excited target. Consider the main features of the density matrix and the statistical tensors of the excited atomic state after optical pumping by a laser within the two-level approximation. Each of the two levels, the ground state α 0 J 0 and the excited state α 1 J 1, includes, generally, a set of sublevels with different projections of the total angular momentum: M 0= -J 0,…,J 0 and M 1 =-J 1,…,J 1. Assume that the laser field is weak and does not split the sublevels. In practice, the laser radiation in experiments has a high degree of linear or circular polarization and we consider here laser pumping by linearly and circularly polarized light. For pumping by linearly polarized light, the stimulated transitions α 0 J 0 →αJ 1 and α 1 J 1→α 0 J 0 proceed only between magnetic sublevels with M 0 = M 1 (the quantization axis along the polarization vector of the laser field), while for pumping by circularly polarized light, the stimulated transitions proceed between the sublevels with M 1 = M 0 ± 1 (the quantization axis along the laser beam; the plus and minus signs are for right and left circular polarization, respectively). The quantization z-axis in this subsection will always be chosen as stated above. So the stimulated transitions take place only within definite pairs of magnetic sublevels, while transitions between sublevels from different pairs occur only as a result of spontaneous photoemission.
KeywordsAngular Momentum Orbital Angular Momentum Total Angular Momentum Quantum Beat Reduce Matrix Element
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