Alkali metal atoms in strong magnetic fields: “Guiding” atomic transitions foretell the characteristics of all transitions of the D$_1$ line

It has been shown that the D$_1$ line of atomic vapors of alkali metals excited by $\pi$-polarized radiation in a strong transverse magnetic field includes specific “guiding” (indicating) atomic transitions between the magnetic sublevels of the hyperfine structure. The dependence of the frequency shift of the guiding transitions on the magnetic field, as well as on their dipole moments, is asymptotic for all other transitions. An experiment with a nanocell with Rb vapor with a thickness of half the wavelength ($\lambda/2$ method) for ensuring a sub-Doppler spectral resolution has completely confirmed the presence of guiding transitions. Two groups of six transitions for $^{85}$Rb and two groups of four transitions for $^{87}$Rb have been detected in the absorption spectra in magnetic fields above 4 kG. A guiding transition has been identified in each of four groups. Four transitions forbidden at $B=0$ have been also detected; with an increase in the magnetic field, their probabilities also approach the probabilities of the guiding transitions.