Measurement of force-assisted population accumulation in dark states.

Atoms can be accumulated by velocity-selective coherent population trapping (VSCPT) in dark states of very highly monovelocity, resulting in very narrow distributions. The optical pumping process that permits the population accumulation proceeds by random walk in momentum space and is of limited efficiency. Several authors have predicted that damping forces can enhance VSCPT in carefully chosen laser fields. We present corroboration of this idea with measurements showing increased efficiency for VSCPT.