Magnetically induced anomalous dichroism of atomic transitions of the cesium D$_2$ line

Transitions $F_{\text{e}}-F_{\text{g}}=\Delta F=\pm2$ between the excited and ground levels of the hyperfine structure of the Cs D$_2$ atomic line in an external magnetic field of $300$–$3000$ G have been studied for the first time with the use of $\sigma^+$ and $\sigma^-$ circularly polarized radiation. Selection rules forbid these transitions in zero magnetic field. At the same time, the probabilities of these transitions in a magnetic field increase significantly; for this reason, we refer to these transitions as magnetically induced transitions. The following rule has been found for the intensities of $24$ magnetically induced $F_{\text{g}}=3\to F_{\text{e}}=5$ and $F_{\text{g}}=4\to F_{\text{e}}=2$ transitions: the intensities of magnetically induced transitions with $\Delta F=+2$ are maximal (the number of such magnetically induced transitions is also maximal) in the case of $\sigma^+$ polarized radiation, whereas the intensities of magnetically induced transitions with $\Delta F=-2$ are maximal (the number of such transitions is also maximal) in the case of $\sigma^-\sigma^+$ and $\sigma^-$ polarized radiation can reach several orders of magnitude; i.e., anomalous circular dichroism is observed. For an experimental test, absorption spectra of a Cs-filled nanocell with the thickness equal to half the wavelength of resonant laser radiation $= 852$ nm have been analyzed in order to separately detect magnetically induced transitions. The experiment is in good agreement with the theory. Possible applications have been discussed.