Magnetically induced transitions in spectral vicinity of the $D_2$ line of cs atoms: giant growth of transition probabilities and different asymptotic behavior in increasing transverse magnetic field

Two types of magnetically induced (MI) transitions in cesium atoms were studied experimentally and theoretically. The MI transitions are forbidden in the absence of magnetic field. Probabilities of MI transitions rapidly grow with increase in magnetic field and can exceed those of transitions allowed in the absence of magnetic field. Asymptotic behavior of probabilities of MI transitions in strong magnetic fields is different. In the case of magnetically induced transitions of the first type (MI1), transition probabilities experience giant increase with increase in the applied magnetic field, and with a further increase of the magnetic field, the probabilities of these transitions tend to asymptotic large value. Probabilities of magnetically induced transitions of the second type (MI2) also experience giant increase with increase in the applied magnetic field. However, probabilities of such transitions tend to zero again with further increase in the magnetic field. It is demonstrated that measurement of the second derivative (SD) of absorption spectra of Cs vapor contained in a nanocell with thickness $L$ = 426 nm corresponding to half the wavelength of the $D_2$ line of cesium ($\lambda$ = 852 nm) enables conducting Doppler-free spectroscopy. Narrow width of atomic lines and linear relation between the SD signal amplitudes and transition probabilities allows studying individual atomic transitions in an external transverse magnetic field with induction varying from 0.5 to 5.3 kG. In particular, four MI transitions were studied: two MI1 and two MI2 ones. Theoretical calculations agree well with the experimental results.