Spontaneous recoil effect in a light beam of finite size

A theoretical analysis shows that the finite time of atom transit through a light beam leads to a nonequilibrium correction to the distribution function due to the spontaneous recoil effect, which depends on the transverse velocity and the coordinate of the atom. As a result, atoms recoil from the beam both in the transverse and in the longitudinal direction. Here, a dense gradient arises across the beam and a slow drift arises along the beam. Results are presented on a resonant line of Na in the center of a light beam.