  Second-stage laser cooling of thulium atoms at the 530.7 nm transition with a
natural linewidth of 350 kHz offers an interesting possibility to study
different regimes of a magneto-optical trap (MOT). The intermediate value of
the spectral linewidth of the cooling transition allows the observation of
three distinct regimes depending on intensity and detuning of the cooling
beams. Namely, the "bowl-shaped" regime when light pressure force competes with
gravity, the "double structure" regime with interplay between Doppler and
polarization-gradient (sub-Doppler) cooling, and the "symmetric" regime when
Doppler cooling dominates over sub-Doppler cooling and gravity. The
polarization-gradient cooling manifests itself by a two-temperature momentum
distribution of atoms resulting in a double-structure of the spatial MOT
profile consisting of a cold central fraction surrounded by a hot halo. We
studied the "double structure" regime at different saturation parameters and
compared observations with calculations based on semiclassical and quantum
approaches. The quantum treatment adequately reproduces experimental results if
the MOT magnetic field is properly taken into account.
