  We study a two-level atom in a double--well potential coupled to a continuum
of electromagnetic modes (black body radiation in three dimensions at zero
absolute temperature). Internal and external degrees of the atom couple due to
recoil during emission of a photon. We provide a full analysis of the problem
in the long wavelengths limit up to the border of the Lamb-Dicke regime,
including a study of the internal dynamics of the atom (spontaneous emission),
the tunneling motion, and the electric field of the emitted photon. The
tunneling process itself may or may not decohere depending on the wavelength
corresponding to the internal transition compared to the distance between the
two wells of the external potential, as well as on the spontaneous emission
rate compared to the tunneling frequency. Interference fringes appear in the
emitted light from a tunneling atom, or an atom in a stationary coherent
superposition of its center--of--mass motion, if the wavelength is comparable
to the well separation, but only if the external state of the atom is
post-selected.
