Hyperfine Paschen-Back regime in alkali metal atoms: Consistency of two theoretical considerations and experiment

Simple and efficient λ-method and λ?2-method (λ is the resonant wavelength of laser radiation) based on a nanometric-thickness cell filled with rubidium (Rb) are implemented to study the splitting of hyperfine transitions of an ⁸⁵Rb and ⁸⁷Rb D₁ line in an external magnetic field in the range of B0.5-0.7 T. It is experimentally demonstrated from 20 (12) Zeeman transitions allowed at low B-field in ₈₅Rb (₈₇Rb) spectra in the case of δ+ polarized laser radiation, only 6 (4) remain atB > 0.5 T, caused by decoupling of the total electronic momentum J and the nuclear spin momentum I (hyperfine Paschen-Back regime). The expressions derived in the frame of completely uncoupled basis (J;m_J ; I;m_I ) describe the experimental results extremely well for 85Rb transitions at B > 0.6 T (that is a manifestation of hyperfine Paschen-Back regime). A remarkable result is that the calculations based on the eigenstates of the coupled (F;m_F ) basis, which adequately describe the system for a low magnetic field, also predict reduction of the number of transition components from 20 to 6 for ⁸⁵Rb and from 12 to 4 for ⁸⁷Rb spectrum at B > 0.5 T. Also, the Zeeman transition frequency shifts, frequency intervals between the components and their slope versus B, are in agreement with the experiment.