Hyperfine Paschen-Back regime of Potassium D${}_2$ line observed by Doppler-free spectroscopy

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 ${\sigma }^{+}$ the other one by ${\sigma }^{-}$ 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) that are constant in the whole range of $0-10$kG magnetic fields. In the case of $\pi$-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.

13 pages, 7 figures