  We observe bimodal fluorescence patterns from atoms in a fast atomic beam
when the laser excitation occurs in the presence of a magnetic field and the
atoms sample only a portion of the laser profile. The behavior is well
explained by competition between the local intensity of the laser, which tends
to generate a coherent-population-trapping (CPT) dark state in the $J=1$ to
$J'=0$ system, and the strength of an applied magnetic field that can frustrate
the CPT process. This work is relevant for understanding and optimizing the
detection process for clocks or other coherent systems utilizing these
transitions and could be applicable to in situ calibration of the laser-atom
interaction, such as the strength of the magnetic field or laser intensity at a
specific location.
