Laser Technique Precisely Maps How Rubidium Atoms Disperse in Gases

Researchers have meticulously measured diffusion coefficients for rubidium in mixtures with helium, neon, nitrogen, argon, krypton and xenon, providing crucial data for a range of scientific applications. Alexander Pouliot, Eduardo Chomen Ramos, and Gehrig Carlse, all from York University, led the study, working in collaboration with Jacek Kłos from University of Maryland and Eite Tiesinga from both National Institute of Standards and Technology and University of Maryland. This research establishes precise values for these coefficients using a novel coherent scattering technique, monitoring the decay of optically pumped population gratings to isolate diffusive motion. The resulting data, offering coefficients from cm²/s to cm²/s, is significant as it validates theoretical models of atomic collisions and has direct relevance to optimising the performance of sensitive instruments like magnetometers and enhancing techniques in spin-polarized noble gas imaging.

This work addresses a critical need in the development of quantum sensors and advanced imaging techniques, both of which rely on a detailed understanding of atomic behaviour within gas mixtures.

This grating, a pattern of differing atomic densities, decays over time as rubidium atoms collide with the surrounding buffer gas. By meticulously monitoring this decay and its dependence on the angle of the laser beams, the team isolated the contribution of diffusion from other collisional processes. Measurements were conducted at a standard atmospheric pressure of 101 325 Pa and a temperature of 24.0°C, spanning a pressure range of 7 000 Pa to 90 000 Pa.

The resulting diffusion coefficients, 0.33 cm²/s for rubidium in helium, 0.214 cm²/s in neon, 0.132 cm²/s in nitrogen, 0.123 cm²/s in argon, 0.093 cm²/s in krypton, and 0.073 cm²/s in xenon, were then compared with predictions from quantum, classical, and semi-classical theoretical models. The team’s calculations, based on the most accurate interatomic interaction potentials available, demonstrated strong agreement with the experimental data after accounting for systematic effects. This research not only refines our understanding of atomic collisions but also provides crucial data for optimising magnetometers, enhancing biomedical imaging with spin-polarized noble gases, and advancing the development of highly sensitive pressure sensors.

Laser-induced population gratings measure rubidium diffusion in noble gases and nitrogen

A coherent transient technique underpinned this work, enabling precise measurements of binary diffusion coefficients for rubidium in helium, neon, argon, krypton, xenon, and molecular nitrogen near room temperature. The experiment established a spatially periodic population grating within a rubidium sample using two intersecting laser beams, each with perpendicular linear polarizations, operating at a wavelength of 795nm.

These beams intersected at a small angle, adjustable between 1.5 milliradians and 4.0 milliradians, creating a grating period ranging from approximately six to fifteen periods across a 3mm spatial extent. This geometry maximised the spatial frequency of the grating and facilitated accurate diffusion measurements. The laser light, locked 60MHz below resonance on the 85Rb D2 line, induced optical pumping, forming population gratings in the magnetic sublevels of the rubidium ground state.

These atomic population gratings mirrored the spatial characteristics of the polarization grating, providing a sensitive probe of atomic motion. A subsequent read-out pulse, aligned along one of the initial beam directions, detected coherent scattering from this atomic lattice, generating a measurable signal along the other beam direction. The exponential decay of this scattered signal directly reflects the diffusive decay of the population grating, driven by momentum-changing collisions between rubidium atoms and the buffer gas.

To isolate the contribution of diffusion, measurements were performed across a buffer gas pressure range of 50 Torr to 700 Torr, equivalent to approximately 6.7 to 93.3 Pascals. By varying the intersection angle of the laser beams, the researchers exploited the characteristic dependence of the decay rate on this angle, specifically a θ−2 relationship for diffusion. This angular dependence allowed for the differentiation of diffusive decay from other mechanisms, such as spin-exchange or spin-destruction collisions, and ensured the accuracy of the diffusion coefficient determinations.

Rubidium diffusion coefficients quantified across a range of buffer gases using population grating decay

Diffusion coefficients for rubidium atoms in various buffer gases have been comprehensively determined, revealing values of 0.89 cm²/s in helium, 0.62 cm²/s in neon, 0.56 cm²/s in nitrogen, 0.48 cm²/s in argon, 0.41 cm²/s in krypton, and 0.36 cm²/s in xenon at a standard atmospheric pressure of 101325 Pa and a temperature of 24.0°C. These measurements, obtained using a single technique, represent precise determinations of atomic motion within different gaseous environments.

The study establishes a spatially periodic population grating in rubidium, monitoring its decay to extract diffusion coefficients, and achieves a precision of a few percent in measuring decay rates. The research distinguishes diffusive motion from other collisional processes by exploiting the characteristic angular dependence of the grating decay, finding that the decay time constant scales proportionally to the inverse square of the angle between the intersecting laser beams.

Data were collected across a pressure range, ensuring the reliability of the extracted diffusion coefficients at standard atmospheric pressure. These experimentally determined values are directly comparable with theoretical calculations based on quantum, semi-classical, and classical models utilising established interatomic interaction potentials. Agreement between the experimental data and theoretical predictions is observed when systematic effects are accounted for, validating the accuracy of both the measurements and the underlying theoretical framework. The consistency of results across six different buffer gas systems, obtained with minimal experimental changes, strengthens the reliability of the findings and provides a robust benchmark for future studies.

The Bigger Picture

Scientists have long recognised that precisely knowing how atoms move through gases is crucial, yet surprisingly difficult to determine accurately. This isn’t merely an academic exercise; it underpins the performance of technologies ranging from atomic clocks to sensitive magnetometers and even advanced imaging techniques relying on spin-polarised noble gases.

The challenge lies in isolating the true diffusion coefficient from a host of other collisional processes that simultaneously affect atomic motion. Previous methods often relied on theoretical approximations or indirect measurements prone to systematic errors. This new work offers a significant step forward by employing a clever technique, monitoring the decay of a laser-created atomic population grating.

By meticulously measuring the rate at which this grating disperses, the researchers have directly determined diffusion coefficients for rubidium atoms in a range of inert gases with unprecedented precision. The agreement between these experimental values and sophisticated theoretical modelling is particularly noteworthy, validating both the measurements and the underlying physics.

However, the study doesn’t resolve all uncertainties. While the technique excels at isolating diffusion, extending it to more complex gas mixtures or at significantly different temperatures and pressures remains a challenge. Furthermore, understanding the subtle interplay between diffusion and other collisional phenomena, such as spin-exchange, requires continued investigation.

Future work might focus on adapting this grating technique for in-situ measurements within operational devices, providing real-time feedback for optimisation and control, or exploring the behaviour of different atomic species. Ultimately, a deeper understanding of atomic diffusion will unlock further improvements in a diverse array of precision technologies.