Measurements of velocity-selective resonances from adiabatic rapid passage

Adiabatic Rapid Passage (ARP) allows the inversion of atomic states much faster than absorption-spontaneous-emission cycles with a concomitant large increase in momentum exchange rate and, hence, the applied optical forces. We have implemented ARP with appropriate modulators and polarizers on metastable He atoms on the 23S→23P transition at λ≈ 1083.3 nm. We have measured the velocity dependence of this force and have been surprised by the appearance of large peaks in the magnitude of this force at regularly spaced velocity intervals. Such unexpected behavior suggests that unexpected coherence effects come into play in optical forces, including those used for laser cooling.