Strongest magnetically induced transitions in alkali metal atoms

Atomic transitions in alkali metals that have zero probability in the absence of a magnetic field but have large probabilities in the presence of a magnetic field are called magnetically induced (MI). They are of interest because of their large probabilities, which exceed the probabilities of usual transitions in a wide magnetic field range. Magnetically induced transitions are classified as type-1 (MI1) and type-2 (MI2) and their total number is about $100$. In this work, MI2 transitions are examined between ground $F_g$ and excited levels $F_e$ of the hyperfine structure satisfying the condition $F_e-F_g = \Delta F = \pm 2$, which are forbidden in zero magnetic field but have large probabilities in the presence of a magnetic field. The probabilities of the MI2 transitions with $\Delta F = + 2$ and the MI transitions with $\Delta F = - 2$ are maximal in the case of optical radiation with the $\sigma^+$ and $\sigma^-$ polarizations, respectively. This difference is called type-1 magnetically induced circular dichroism (MICD1). It has been shown for the first time that the probability of the strongest MI2 transition in the $^{85}$Rb atom corresponding to the $D_2$ line in magnetic fields $>100$ G in the case of ${\sigma }^{ + }$ radiation is larger than the probability of the strongest MI2 transition in the case of $\sigma^-$ radiation by a factor of $2.5$. This difference is called type-2 magnetically induced circular dichroism (MICD2). It has been shown how to determine the strongest MI transition for any alkali metal atom, which is important for its application in magneto-optical processes. Theoretical curves reproduce well experimental results.