Diffusion coefficients of Rb-inert gas mixtures using coherent scattering from optically pumped population gratings*

We present determinations of binary diffusion coefficients D at 24 °C for trace amounts of naturally abundant Rb atoms in naturally abundant He, Ne, N₂, Ar, Kr and Xe buffer gases

(A. Pouliot et al., Phys.Rev. A, Submitted). D has been measured by establishing spatially periodic population gratings in Rb vapor using two laser beams that intersect at a small angle θ of a few milliradians. The population gratings decay exponentially in time due to diffusive motion through the buffer gas, as well as due to other processes which alter the angular momentum of the Rb atoms. We distinguish the contribution of diffusion from these other processes by varying θ and observing the characteristic θ² dependance of the exponential decay rate. We have measured the decay rate as a function of the buffer gas pressure over a range of 7 000 to 90 000 Pa. We obtain the following values of the diffusion coefficient at 24 °C and scaled to atmospheric pressure of 101 325 Pa: 0.33(5) cm²/s, 0.214(14) cm²/s, 0.132(7) cm²/s, 0.123(9) cm²/s, 0.093(3) cm²/s and 0.073(4) cm²/s, for Rb in He, Ne, N₂, Ar, Kr and Xe respectively. We have also determined the diffusion coefficient using quantum, classical and semi-classical methods based on the most accurate inter-atomic potentials from the literature. We find that near room temperature, simulations of D using classical and quantum methods agree within their intrinsic, sub 1% standard uncertainties, and that these results agree with our experimental determinations. Our measurements and modeling are relevant to the optimization of magnetometers, biomedical imaging using spin-polarized noble gases, tests of collision models based on interatomic potentials, and the development of pressure sensors.