We investigate the temperature dependence of diffusion coefficients of Rb atoms DRb(T,p) in the presence of N2 buffer gas. This work follows recent determinations of DRb(T,p) in 6 buffer gas mixtures over the pressure range of p = 50 to 760 Torr and at a constant temperature of T = 24°C. These measurements agree with quantum mechanical and classical calculations of collisions based on ab-initio interatomic potentials, to an accuracy ranging from 5.5% to 15% [Pouliot et. al., Phys. Rev. A 111, 033108 (2025)], suggesting that the determinations of diffusion coefficients D(T,p) can become the basis for a quantum pressure sensor. We report progress on these measurements and discuss suppression of systematic uncertainties arising from the heterodyne detection system.
We present comprehensive determinations of the diffusion coefficients
D at $T=24\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$ for trace amounts of naturally abundant Rb atoms in inert, naturally abundant He, Ne,
N2, Ar, Kr, and Xe buffer gases using a single measurement technique. They have been measured by establishing a spatially periodic population grating in the Rb sample using two laser beams that intersect at a small angle $\ensuremath{\theta}$ of a few milliradians. The atomic population grating decays exponentially in time due to diffusive motion induced by momentum-changing elastic collisions between Rb and buffer gas atoms or molecules. This decay is monitored by observing the scattered field from a readout beam aligned along the direction of one of the excitation beams. We distinguish the contribution of diffusion from other collisional processes by measuring the characteristic ${\ensuremath{\theta}}^{2}$ dependence of the decay rate. We also measure the systematic dependence of the decay rate on the buffer gas pressure over a range of 7000 to
90000 Pa. In this manner, we obtain diffusion coefficients at standard atmospheric pressure of
101325 Pa and at a temperature of $24.0(5)\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$. We use two models to correct for systematic effects due to the transit time, one assuming a rectangular profile of the population distribution and a rectangular readout beam profile, and a second using Gaussian profiles. We obtain weighted averages of 0.33(5), 0.214(14), 0.132(7), 0.123(9), 0.093(9), and
0.073(4)cm2/s for Rb in He, Ne,
N2, Ar, Kr, and Xe, respectively. The number in parentheses represents one standard deviation of the combined statistical and systematic uncertainty. We also compare these data with diffusion coefficients obtained using quantum, classical, and semiclassical theoretical methods based on the most accurate interatomic interaction potentials from the literature. Near room temperature, simulations of
D using classical and quantum methods agree within their intrinsic, sub-1% standard uncertainties. We find that the semiclassical model only gives the correct orders of magnitude for
D. Our computed diffusion coefficients based on the quantum theory agree with the experimental determinations when systematic effects are taken into account. 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.
We demonstrate a technique for the accurate measurement of diffusion coefficients for alkali vapor in an inert buffer gas. The measurement was performed by establishing a spatially periodic density grating in isotopically pure 87Rb vapor and observing the decaying coherent emission from the grating due to the diffusive motion of the vapor through N2 buffer gas. We obtain a diffusion coefficient of 0.245 ± 0.002 cm2 /s at 50°C and 564 Torr. Scaling to atmospheric pressure, we obtain D0 = 0.1819 ± 0.0024 cm2 /s. To the best of our knowledge, this represents the most accurate determination of the Rb-N2 diffusion coefficient. Our measurements can be extended to different buffer gases and alkali vapors used for magnetometry and can be used to constrain theoretical diffusion models for these systems
We apply an innovative method called Optically-Detected Spin-Echo to measure the intrinsic coherence relaxation time in the buffer-gas vapor cell of a Rb atomic clock [1]. This method suppresses the influence of static-magnetic-field-gradients across the cell which is a source of relaxation processes. Such studies are of interest for high-performance Rb atomic clocks, where both intrinsic population and coherence relaxation times (T1and T2, respectively) of the “clock transition” (52S1/2|Fg= 1, mF= 0〉 ↔ |Fg= 2, mF= 0〉) are relevant.
Relaxation experiments have been performed between 0–200 G on optically polarized Rb atoms in argon, krypton or xenon, at pressures ranging from 0.1 to 20 torr. They clearly show the existence of Rb–Ar, Rb–Kr and Rb–Xe molecules of low binding energy (smaller than kT). Molecular parameters such as lifetime, spin-orbit coupling constant, formation rate, constant of mass action, etc., are measured, or evaluated when they cannot be directly measured. Experimental values of the diffusion coefficient of Rb in the gas (except xenon) and of the disorientation cross sections by usual binary collisions are also obtained.
We demonstrate a technique for the accurate measurement of diffusion coefficients for alkali vapor in an inert buffer gas. The measurement was performed by establishing a spatially periodic density grating in isotopically pure
87Rb vapor and observing the decaying coherent emission from the grating due to the diffusive motion of the vapor through
N2 buffer gas. We obtain a diffusion coefficient of $0.245\ifmmode\pm\else\textpm\fi{}0.002\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{2}/\text{s}$ at $50{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and 564 Torr. Scaling to atmospheric pressure, we obtain ${D}_{0}=0.1819\ifmmode\pm\else\textpm\fi{}0.0024\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{2}/\text{s}$. To the best of our knowledge, this represents the most accurate determination of the $\mathrm{Rb}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ diffusion coefficient to date. Our measurements can be extended to different buffer gases and alkali vapors used for magnetometry and can be used to constrain theoretical diffusion models for these systems.