The straightforward application of energy and linear momentum conservation to the
absorption/emission of photons by atoms allows us to establish the essential features of laser
cooling of two level atoms at low laser intensities. The lowest attainable average kinetic energy of
the atoms depends on the ratio ##IMG##
[http://ej.iop.org/images/0143-0807/36/6/065008/ejp518689ieqn1.gif] {${rm{Gamma
}}/{E}_{{rm{R}}}$} between the natural linewidth and the recoil energy and tends to E R as ##IMG##
[http://ej.iop.org/images/0143-0807/36/6/065008/ejp518689ieqn2.gif] {${rm{Gamma
}}/{E}_{{rm{R}}}$} tends to zero (in one dimension). This treatment, like the quantum mechanical
ones, is valid for any value of the ratio ##IMG##
[http://ej.iop.org/images/0143-0807/36/6/065008/ejp518689ieqn3.gif] {${rm{Gamma
}}/{E}_{{rm{R}}}$} and contains the semiclassical theory of laser cooling as the limiting case in
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