  In this paper laser cooling of atoms with a narrow-line optical transition,
i.e. in regimes of quantum nature of laser-light interactions resulting in a
significant recoil effect, is studied. It is demonstrated that a minimum laser
cooling temperature for two-level atom in standing wave reached for red
detuning close to 3 recoil frequency greatly different from the theory used for
a semiclassical description of Doppler cooling. A set of dimensionless
parameters uniquely characterizing the time evolution and the steady state of
different atoms with narrow-line optical transitions in the laser field is
introduced. The results can be used for analysis of optimal conditions for
laser cooling of atoms with narrow lines such as $Ca$, $Sr$, and $Mg$, which
are of great interest for atomic clocks.
