Laser cooling can be achieved by exciting an atomic system, in order to "pump" atoms in the excited state. This gives a recoil momentum $\hbar k$ the the atom, which then emits spontaneously. As spontaneous emission is an isotropic process, this does not give (statistically speaking) e recoil momentum to the atom, which is in this way "slowed down" by the laser. What is was wondering is: in which way stimulated emission can be ignored in this process ? Maybe both spontaneous and stimulated emission happen, but as the latter does not give momentun change (considering both absorption and emission), is considered as irrelevant? Or maybe that's linked to the choice of alkali, which have some sort of particular property?
Answer
You've hit the nail on the head.
Maybe both spontaneous and stimulated emission happen, but as the latter does not give momentum change (considering both absorption and emission), is considered as irrelevant?
This is exactly what happens. If the absorbed photon is emitted via stimulated emission, there is no net change in the momentum and the atom is neither cooled nor heated.
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