Enhancement of optically pumped spin orientation via spin-exchange collisions at low vapor density

An effect is demonstrated where off-resonant optical pumping is enhanced by spin-exchange collisions, which enables a high degree of spin orientation to be obtained in alkali-metal atoms. The experiment with room-temperature cesium vapor employs radio-frequency spectroscopy under conditions of the nonlinear Zeeman effect that allow the populations of the individual Zeeman sublevels to be determined. The improved efficiency of atomic orientation leads to a significant reduction of population relaxation out of the state with maximum magnetic quantum number m (stretched state), which is observed as a narrowing of radio-frequency resonances. Spin-exchange collisions play a crucial role in this effect although the applied atomic density 10^11 / cm^3 corresponds to collision rates not greater than several tens of Hz. A simplified theoretical model has been developed to reproduce the measurement results numerically.