SIGMA+-SIGMA- OPTICAL MOLASSES IN A LONGITUDINAL MAGNETIC-FIELD

We investigate the effect of a longitudinal magnetic field on the orientational cooling mechanism that operates both in sigma+-sigma- optical molasses and in a magneto-optical trap. For an atom with different Lande g factors in its ground and excited states, single-photon scattering and stimulated Raman scattering are resonant at different field-dependent velocities. The sharp polarization-gradient feature in the force-versus-velocity curve may therefore be translated to a velocity at which the Doppler force is dominant. In addition to this, we find that the momentum diffusion coefficient can grow as the field is increased. By numerically solving a semiclassical Fokker-Planck equation, we show that these effects lead to a field-induced inhibition of efficient sub-Doppler cooling for atoms with unequal g factors. Our treatment also enables us to discuss other phenomena that are accessible only if both Doppler and polarization-gradient effects are included in the calculation.