  We present experimental results and a theoretical model that illustrate how
competing eigenbases can determine the dynamics of a fluorescing atom. In the
absence of a magnetic field, the atom can get trapped in a dark state, which
inhibits fluorescence. In general, this will happen when the magnetic
degeneracy of the ground state is greater than the one of the excited state. A
canonical way to avoid optical pumping to dark states is to apply a magnetic
field at an angle with respect to the polarization of the exciting light. This
generates a competition of eigenbases which manifests as a crossover between
two regimes dominated either by the laser or the magnetic field. We illustrate
this crossover with fluorescence measurements on a single laser-cooled calcium
ion in a Paul trap and find that it occurs at a critical laser intensity that
is proportional to the external magnetic field. We contrast our results with
numerical simulations of the atomic levels involved and also present a simple
theoretical model that provides excellent agreement with experimental results
and facilitates the understanding of the dynamics.
