REDUCTION OF THE 2ND-ORDER DOPPLER BROADENING IN 2-PHOTON ATOMIC-BEAM SPECTROSCOPY - A PROPOSAL

The paper describes a proposal for a new method to overcome the limitations introduced by the second-order Doppler effect in two-photon spectroscopy experiments on atomic beams. Suppression of the second-order Doppler shift and significant reduction (at least by a factor 10) of the Doppler broadening can be achieved when two counterpropagating laser beams of slightly different frequencies v+ and v- in the laboratory frame are used. These frequencies have to be adjusted in such a way that, in the frame where the atoms of the monokinetic atomic beam are at rest, frequencies of both laser beams are exactly equal to the half of the frequency v0 of the atomic line, satisfying the relativistic resonance condition v0 = 2 square-root v+v-. The feasibility and the performance of the proposed method are discussed, as well as the influence of the velocity distribution in the atomic beam.