  A one-dimensional nano-metric-thin cell (NC) filled with potassium metal has
been built and used to study optical atomic transitions in external magnetic
fields. These studies benefit from the remarkable features of the NC allowing
one to use $\lambda/2$- and $\lambda$-methods for effective investigations of
individual transitions of the K D_1 line. The methods are based on strong
narrowing of the absorption spectrum of the atomic column of thickness L equal
to $\lambda/2$ and to $\lambda$(with $\lambda = 770\un{nm}$ being the resonant
laser radiation wavelength). In particular, for a $\pi$-polarized radiation
excitation the $\lambda$-method allows us to resolve eight atomic transitions
(in two groups of four atomic transitions) and to reveal two remarkable
transitions that we call Guiding Transitions (GT). The probabilities of all
other transitions inside the group (as well as the frequency slope versus
magnetic field) tend to the probability and to the slope of GT. Note that for
circular polarization there is one group of four transitions and GT do not
exist. Among eight transitions there are also two transitions (forbidden for
$B$ = 0) with the probabilities undergoing strong modification under the
influence of magnetic fields. Practically the complete hyperfine Paschen-Back
regime is observed at relatively low ($\sim 1\un{kG}$) magnetic fields. Note
that for K $D_2$ line GT are absent. Theoretical models describe the experiment
very well.

Atomic Physics