Spin Relaxation of Optically Aligned Rubidium Vapor

A new optical method for studying the spin relaxation of optically aligned rubidium vapor is described. In this method, the pumping radiation, consisting of circularly polarized $D_1$ resonance radiation, is suddenly shut off and then turned on again after a known time interval. The relaxation which takes place during the interval of darkness causes the vapor to become more opaque to the pumping radiation at a rate which is determined by the relaxation time. A large degree of alignment, as well as a relaxation time of about 80 milliseconds, were observed in a closed-off evacuated cylindrical glass cell completely lined, except for $\frac{1}{200}$ of the wall area, with a thin film of tetracontane (${\mathrm{C}}_{40}$ ${\mathrm{H}}_{82}$). The variation of relaxation time with buffer gas pressure was studied in this cell and in an unlined glass cell. From the observations with the unlined cell, diffusion coefficients for rubidium in neon and argon of 0.31 ${\mathrm{cm}}^2$/sec and 0.24 ${\mathrm{cm}}^2$/sec, respectively, can be deduced. Observed cross sections for disorientation collisions between aligned ground-state rubidium atoms and neon, argon, krypton, and xenon atoms are 5.2×${10}^{-23\mathrm{}}$ ${\mathrm{cm}}^2$, 3.7×${10}^{-22\mathrm{}}$ ${\mathrm{cm}}^2$, 5.9×${10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$, and 1.3×${10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$, respectively. In the evacuated tetracontane-lined cell, the relaxation time decreased by 30% for a tenfold increase in rubidium vapor pressure. An explanation for this relatively weak dependence is suggested. The longest observed relaxation time was approximately 0.4 seconds in a tetracontanelined cell filled with neon to a pressure of 3 cm Hg.