  In this paper, we develop an analytical approach to Doppler cooling of atoms
by one- or two-photon transitions when the natural width of the excited level
is so small that the process leads to a Doppler temperature comparable to the
recoil temperature. A ``quenching'' of the sharp line is introduced in order to
allow control of the time scale of the problem. In such limit, the usual
Fokker-Planck equation does not correctly describe the cooling process. We
propose a generalization of the Fokker-Planck equation and derive a new model
which is able to reproduce correctly the numerical results, up to the recoil
limit. Two cases of practical interest, one-photon Doppler cooling of strontium
and two-photon Doppler cooling of hydrogen are considered.
