Abstract Selective reflection of a laser radiation from an interface formed by a dielectric window and a potassium atomic vapour confined in a nano-cell with 350 nm gap thickness is implemented for the first time to study the atomic transitions of K D 2 line in external magnetic fields. In moderate B -fields, there are 44 individual Zeeman transitions which reduce to two groups (one formed by σ + , the other one by σ − circularly-polarised light), each containing eight atomic transitions, as the magnetic field increases. Each of these groups contains one so-called ‘guiding’ transition whose particularities are to have a probability (intensity) as well as a frequency shift slope (in MHz G −1 ) that are constant in the whole range of 0–10 kG magnetic fields. In the case of π -polarised laser radiation, among eight transitions, two are forbidden at B = 0, yet their probabilities undergo a giant modification under the influence of a magnetic field. We demonstrate that for B -fields > 165 G a complete hyperfine Paschen–Back regime is observed. Other peculiarities of K D 2 line behaviour in magnetic field are also presented. We show a very good agreement between theoretical calculations and experiments. The recording of the hyperfine Paschen–Back regime of K D 2 line with high spectral resolution is demonstrated for the first time.
We precisely investigate the Zeeman and Paschen-Back effects of the hyperfine structure during the rubidium $5{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}5{\phantom{\rule{0.16em}{0ex}}}^{2}{D}_{5/2}$ two-photon transition. Owing to the high sensitivity of the
D state of atoms to a magnetic field, the splittings of the spectra of the
D state are sufficiently large in a weak magnetic field of
<10 G. We experimentally observe how in the hyperfine coupling in the
52D5/2 state is disrupted even in weak magnetic fields around 10 G and the energy levels are arranged according to their total angular momentum
J and nuclear spin
I. The transient of the spectra known as the Paschen-Back effect is observed even in a weak magnetic field with an order of magnitude of several tens of gauss. The results of theoretical calculations for energy shifts and transition intensities under a magnetic field are in quantitative agreement with the experimental results.