Deceleration of an Atomic Beam

Abstract

The origin of optical forces on atoms has been discussed in Chapter 3, and here a specific application is introduced. The use of electromagnetic forces to influence the motion of neutral atoms has been a subject of interest for some years, and several review articles and books on the subject are listed in Appendix B. The force caused by radiation, particularly by light at or near the resonance frequencies of atomic transitions, originates from the momentum associated with light. In addition to energy E = ℏω, each photon carries momentum ℏk and angular momentum ℏ. When an atom absorbs light, it stores the energy by going into an excited state; it stores the momentum by recoiling from the light source with a momentum ℏk; and it stores the angular momentum in the form of internal motion of its electrons. The converse applies for emission, whether it is stimulated or spontaneous. It is the velocity change of the atoms, v_r = ℏk/M ≃ few cm/s, that is of special interest here, and although it is very small compared with thermal velocity, multiple absorptions can be used to produce a large total velocity change. Proper control of this velocity change constitutes a radiative force that can be used to decelerate and/or to cool free atoms.