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arXiv:1807.02329 · Physical Review A

Scattering of an alkali-metal atomic beam on anti-spin-relaxation coatings

Naota Sekiguchi, A. Hatakeyama, Kazane Okuma, Hiroaki Usui

Published Oct 29, 2018• 17 citations
arXivHTMLPDF

Abstract

We performed scattering experiments using a rubidium (Rb) atomic beam on paraffin films and measured the angular and velocity distributions of scattered atoms. The paraffin films were prepared in various ways and characterized by atomic force microscopy and x-ray diffraction. The films exhibited various roughnesses and crystal structures. The paraffin films preserved the spin polarization of the scattered atoms. The measured angular distributions of all prepared films were consistent with Knudsen's cosine law. The velocity distributions were well fitted by Maxwell's distribution, characterized by a temperature much closer to the film temperature than to the atomic-beam temperature. We therefore concluded that the translational motion of the scattered atoms was thermalized with the paraffin films via single-scattering events.

DOI
10.1103/physreva.98.042709

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•Oct 20, 2015•0 citations
We performed scattering experiments using a rubidium (Rb) atomic beam on paraffin films and measured the angular and velocity distributions of scattered atoms. The paraffin films were prepared in various ways and characterized by atomic force microscopy and x-ray diffraction. The films exhibited various roughnesses and crystal structures. The paraffin films preserved the spin polarization of the scattered atoms. The measured angular distributions of all prepared films were consistent with Knudsen's cosine law. The velocity distributions were well fitted by Maxwell's distribution, characterized by a temperature much closer to the film temperature than to the atomic-beam temperature. We therefore concluded that the translational motion of the scattered atoms was thermalized with the paraffin films via single-scattering events.
Abstract With an objective to explore the surface interaction between alkali-metal atoms and an inert organic coating, we used an atomic beam to measure the velocity distribution of gaseous rubidium atoms scattered from a low-temperature tetracontane film. When the film temperature decreased from 300 K to 200 K, the number of scattered atoms did not change. At below 200 K, the number began to decrease until, finally, no scattered atoms were observable at 140 K. X-ray photoelectron spectroscopy at 160 K or below revealed rubidium atoms on the film surface. The temperature of the scattered atoms was the same as that of the film. Thermalization of scattered atoms at room temperature was reported previously, but this study is the first to report thermalization at low temperatures.
Many technologies based on cells containing alkali-metal atomic vapor benefit from the use of antirelaxation surface coatings in order to preserve atomic spin polarization. In particular, paraffin has been used for this purpose for several decades and has been demonstrated to allow an atom to experience up to 10 000 collisions with the walls of its container without depolarizing, but the details of its operation remain poorly understood. We apply modern surface and bulk techniques to the study of paraffin coatings in order to characterize the properties that enable the effective preservation of alkali spin polarization. These methods include Fourier transform infrared spectroscopy, differential scanning calorimetry, atomic force microscopy, near-edge x-ray absorption fine structure spectroscopy, and x-ray photoelectron spectroscopy. We also compare the light-induced atomic desorption yields of several different paraffin materials. Experimental results include the determination that crystallinity of the coating material is unnecessary, and the detection of C[Double Bond]C double bonds present within a particular class of effective paraffin coatings. Further study should lead to the development of more robust paraffin antirelaxation coatings, as well as the design and synthesis of new classes of coating materials.
The interaction of spin polarized cesium atoms on coated surfaces is examined. Coated surfaces find extensive applications in atomic devices-hydrogen masers and advanced rubidium frequency standards utilize wall coatings to obtain very narrow hyperfine resonances. A Cs atomic beam is hyperfine or spin polarized by optical pumping techniques using a single mode AlGaAs diode laser. The ground state distribution is probed before and after scattering from coated surfaces using laser induced fluorescence spectroscopy. The coatings under investigations are paraffin and organosilanes and these are known to be very weakly relaxing surfaces for polarized alkali atoms. The spin relaxation properties of paraffin and organosilane coatings have been studied in glass cell-vapor environment. In our experiments using polarized atomic beams, our goal is to examine in detail the hyperfine and spin relaxation properties of these coatings in a carefully controlled environment which is not possible in the gas cell environment. We measure the spin relaxation probability per wall collision as a function of temperature of the surface. Results of our studies will be presented.
The scattering of Rb atoms on an antirelaxation coating was studied. No significant change in the spin relaxation probability of Rb atoms by single scattering from a tetracontane surface was observed by cooling the film from 305 to 123 K. The mean surface dwell time was estimated using a time-resolved method. Delay-time spectra, from which mean surface dwell times can be estimated, were measured at 305, 153, and 123 K, with a time window of $9.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ s. The increase in mean surface dwell time with cooling from 305 to 123 K was smaller than $4.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ s, which is significantly smaller than the value expected from the mean dwell time at room temperature measured using the Larmor frequency shift. These results can be explained by assuming a small number of scattering components, with a mean surface dwell time at least three orders of magnitude longer than the majority component.
Paraffin and octadecyltrichlorosilane (OTS) coatings can alleviate collisions between alkali-metal atoms and cell walls and then prolong the atomic spin-polarization lifetime. The surface structure and collision effects of these antirelaxation coatings, as well as the methods to avoid antirelaxation invalidity, have been the focus of researchers. This study investigated the thermolability of coating surface structure and the collision interactions between alkali metal atoms and coatings, considering the influence of various coating preparation factors, where this collision interaction is indirectly analyzed by measuring the collision energy dissipation between an atomic force microscopy (AFM) probe and the atoms on the coating surface. We found that appropriate evaporation time, carbochain length, and postannealing process can enhance the thermostability of the paraffin coating and eliminate its morphological defects. Furthermore, the OTS/water concentration, the soaking time, and the type of solvent have different levels of influence on the cluster formation and the thermostability of the OTS coatings. Moreover, the antirelaxation performance of coatings has been shown to be characterized by counting the energy dissipated when the AFM probe collides with the antirelaxation coating, replacing the conventional light-atom interaction- based method for measuring the relaxation characteristics, but requiring specific coating preparation factors to be maintained.
We measured the rate of velocity-changing collisions (VCCs) between alkali atoms and background gas in buffer-gas-free anti-spin-relaxation-coated cells. The average VCC rate in paraffin-coated rubidium vapor cells prepared in this work was 1×1061 \times 10^{6}1×106 s−1^{-1}−1, which corresponds to $\sim$1 mm in the mean free path of rubidium atoms. This short mean free path indicates that alkali atoms do not travel freely between the cell walls. In addition, we found that a heating process known as increases the VCC rate, and also confirmed that ripening improves the anti-relaxation performance of the coatings.