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

Simple and efficient lambda-method and lambda/2-method (lambda 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 Rb-85 and Rb-87 D-1 line in an external magnetic field in the range of B = 0.5-0.7 T. It is experimentally demonstrated from 20 (12) Zeeman transitions allowed at low B-field in Rb-85 (Rb-87) spectra in the case of sigma(+) polarized laser radiation, only 6 (4) remain at B > 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 Rb-85 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 Rb-85 and from 12 to 4 for Rb-87 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. (C) 2014 Optical Society of America