"Unmoved" Atomic Transitions of Alkali Metals in External Magnetic Fields

The "mixing" of magnetic sublevels of the ground and excited levels of an atomic transition with magnetic sublevels of a nearby transition (in the hyperfine structure of atoms of alkali metals) is responsible for a strong modification of its probability. A dramatic manifestation of this mixing effect is a huge increase in the probability of the atomic transition in magnetic fields that is forbidden by selection rules in zero magnetic field B = 0; the latter is called a magnetically induced (MI) transition. Two MI transitions with different dependences on B have been studied. Another dramatic manifestation of the mixing effect is that there are atomic transitions whose frequency is practically fixed in a wide range of magnetic fields, i.e., the slope S [MHz/G] of the frequency shifts as a function of B is practically zero, with the transition probability being significant. We call these transitions "unmoved" transitions (UTs). We have studied the UTs of ⁸⁷Rb atoms, the D₂ line, 1, +1〉 → 1', +1'〉 and Cs atoms, the D₂ line, 3, –3〉 → 5', –4'〉, where the quantum numbers F and m_F of the excited and ground levels are marked with and without primes, respectively. We show that in the range B = 180–705 G for ⁸⁷Rb, the D₂ line, S ≤ 0.03 MHz/G; for comparison, S ≈ 3.6 MHz/G for ⁸⁷Rb in the same B range for the MI transition 1, +1〉 → 3', +2'〉 is greater by a factor of 120. A second laser was used to control the UT amplitude. Our theoretical model describes well the experiment.