Circular dichroism of atomic transitions of the $\mathrm{Rb}$ $D_{1}$ line in magnetic fields

The circular dichroism effect has been investigated for atomic transitions of the $\mathrm{Rb}$ $D_{1}$ line in magnetic fields of up to $3$ kG using circularly polarized $\sigma^+$ and $\sigma^-$ radiation. The process of selective reflection from a $350$-nm-thick nanocell has been used, which makes it possible to form narrow atomic lines and observe separately the behavior of individual transitions. Two groups consisting of six (${}^{85}\mathrm{Rb}$ atoms) and four (${}^{87}\mathrm{Rb}$ atoms) transitions are formed in magnetic fields $B>0.5$ kG upon $\sigma^+$ and $\sigma^-$ laser excitation. All transitions have been identified. It is shown that the strongest transitions for ${}^{87}\mathrm{Rb}$ and ${}^{85}\mathrm{Rb}$ atoms in magnetic fields of up to several kG are formed under $\sigma^-$ irradiation. A further increase in the magnetic field makes it possible to attain the Paschen–Back regime on a hyperfine structure, for which the probabilities of transitions upon $\sigma^+$ and $\sigma^-$ excitation become identical. The theoretical model and experiment are in good agreement.