High-velocity dark states in velocity-selective coherent population trapping

We have observed long-lived dark states in velocity-selective coherent population trapping (VSCPT) in a one-dimensional lin-angle-lin configuration with metastable He atoms using the {ital J}=1{r_arrow}1 transition at {lambda}=1.083 {mu}m. These states are characterized by two peaks in the atomic momentum distribution at {plus_minus}{ital Q}{h_bar}{ital k} ({ital Q}=positive integer), where {ital k} is the magnitude of the optical wave vector. VSCPT states previously observed are characterized by {ital Q}=1, but we have observed long-lived states with {ital Q}=2. Optical Bloch equation calculations show that these high-velocity dark states appear at higher {ital Q} for greater light intensity. {copyright} {ital 1996 The American Physical Society.}