Relaxation of Optically Pumped Rb Atoms on Paraffin-Coated Walls

Relaxation of optically pumped Rb atoms on paraffin-coated walls has been studied using Franzen's sequence (relaxation in the dark) for observables 〈Sz〉 (mean electronic longitudinal polarization), and <S·{}I> (population difference between the two hfs levels). The physical meaning of optical signals under different conditions is analyzed in some detail, and it is pointed out that different observables relax differently. In the interpretation of our results, extensive use has been made of the theoretical analysis of the relaxation of a spin S (coupled to a spin I in the Rb atom) caused by a weak interaction of the type _γsH(t)·S, $H(t)$ being a random magnetic field acting on S during the dwell time of a Rb atom on the wall. Experimental results are in excellent agreement with this analysis, if one assumes the existence of two uncorrelated interactions of the above type. The first is the dipole-dipole interaction between S and the nuclear spins K of the protons (or deuterons) of the coating. This interaction has a long correlation time τc1≈4×10^-10 sec and dominates the relaxation of 〈Sz〉 in low fields. The second interaction which is independent of spins K, has a short correlation time τc2≈10^-12 sec, and dominates the relaxation of <S·{}I> in low fields, and of 〈Sz〉 in large fields. The values of these correlation times and of the strengths of the two interactions have been measured, first in low fields by combining results on 〈Sz〉 and <S·{}I> for the two Rb isotopes and for two types of coatings [(CH₂)ₙ and (CD₂)ₙ], and second, by studying the variations of the "pseudo" relaxation time T₁ of 〈Sz〉 for magnetic-field values up to 5000 G. We arrive at a detailed picture of physical adsorption of Rb on paraffin-coated walls, and find 0.1 eV for the adsorption energy. These results are compatible with those obtained for the relaxation of oriented Rb atoms in collisions with buffer-gas molecules.