POPULATION ACCUMULATION IN DARK STATES AND SUBRECOIL LASER COOLING

The Lambda system resulting from the interaction of counterpropagating orthogonally polarized beams of light with a J(g) = 1 --> J(e) = 1 tr ansition is known to exhibit laser cooling without the presence of a d amping force, through velocity selective coherent population trapping. This occurs independent of the sign of the laser's detuning from the atomic resonance frequency. We present measurements and calculations t hat show a detuning-dependent transient peak or dip in the ground-stat e momentum distribution centered at p(g) = 0, however, which is remini scent of laser cooling by a damping force. We explain this in terms of the character of the quantum-mechanical eigenstates of the total grou ndstate Hamiltonian of the system, using both analytic expressions and numerical calculations.