In Doppler cooling, laser light is tuned slightly below an atomic resonance frequency. When an atom moves towards the laser, it perceives the light as being closer to the resonance frequency due to the Doppler shift, increasing the likelihood of absorbing a photon. Absorbing this photon exerts a force opposite to the atom's motion, effectively slowing it down. Subsequent spontaneous emission of photons occurs in random directions, but on average, this process reduces the atom's velocity.
