Spin Relaxation in Optical Pumping

The spin relaxation of optically aligned rubidium vapor has been studied as a function of helium buffer gas pressure. Relaxation times as long as 0.68 sec were observed in helium at ½-atm pressure. The diffusion constant D0 for rubidium in helium at 50°C was evaluated as 0.54 cm2 sec—1. The disorientation cross section σ for rubidium-helium collisions was found to be 6.2×10—25 cm2. A mechanism for spin relaxation in optically aligned alkali vapors is proposed and discussed. It is found that the relaxation arises from the coupling of the alkali electron spin to orbital and rotational motion accompanying the collision. The variation of σ for rubidium in the presence of other inert gases is also discussed.