  We numerically study the slow (subradiant) decay of the fluorescence of
motionless atoms after a weak pulsed excitation. We show that, in the
linear-optics regime and for an excitation detuned by several natural
linewidths, the slow decay rate can be dominated by close pairs of atoms
(dimers) forming superradiant and subradiant states. However, for large-enough
resonant optical depth and at later time, the dynamics can also be dominated by
radiation trapping due to classical multiple scattering. Signatures of this
regime are found in the spatial distribution of excited atoms, which fulfills
the prediction of the diffusion equation, in the polarization of the emitted
light, which is depolarized at late time, and in the spectrum, which is mainly
at the atomic resonance.
