Resonant collisions of sodium atoms in a laser beam

Excited atoms inside a laser beam can undergo resonant collisions with atoms which are in the ground-state. The total cross-section of resonant collisions exceeds the one for non-resonant collisions by almost three orders of magnitude. An experiment is described which allows to measure the increase of the collision rate in sodium vapor due to the illumination with resonant D-light. In this experiment another phenomenon was observed: Under special conditions the momentum of part of the atoms can be increased considerably by the laser-light.