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Measurement of background gas in paraffin-coated alkali vapor cells

Naota Sekiguchi, A. Hatakeyama

Published Oct 20, 2015• 0 citations
OpenAlex

Abstract

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 $1 \times 10^{6}$ s$^{-1}$, 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.

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arXiv:0812.4959•Physical Review A•Mar 3, 2009•10 citations
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.
Observation of nonthermal, light-induced desorption of Cs and Rb atoms from paraffin coating is reported. Buffer-gas-free paraffin-coated cells containing Cs and Rb were exposed to laser light of various intensities and frequencies, and the change of the vapor densities as a function of time was investigated. Because atomic polarization relaxes very slowly in paraffin-coated cells (spin-relaxation times can be $\ensuremath{\gtrsim}1\mathrm{s}),$ they are applied in a variety of precision measurements (e.g., magnetometry, discrete symmetry tests, etc.). Light-induced desorption may be useful in such measurements as a method to control atomic density without changing the temperature of the cell.
Introduction and history Paraffin films and other surface coatings have played a decisive role in the emergence and development of optical magnetometry. When alkali atoms in the vapor phase collide with the bare surface of a glass container, they disappear inside the glass and are replaced in the vapor phase by another atom with random spin orientation. With a mean free path of the dimensions of the cell (typically on the order of 1 to several cm), the collision frequency is much too high, 10 4 s −1 , to maintain the substantial spin polarization required for practical applications. In order to prevent this detrimental effect, vapor cells include either an inert buffer gas [1–3] or an antirelaxation surface coating [4]. In the presence of a noble gas at a pressure from 10 −2 to a few atmospheres, the alkali atoms diffuse very slowly from the center of the cell to the glass walls, and their orientation is only very slightly affected by gas collisions. However, there are several advantages to the use of a surface coating instead of buffer gas. If the static magnetic field is not homogeneous, then resonance lines suffer from inhomogeneous broadening in the presence of the gas [5–7]. In addition, the optical pumping process is perturbed by the buffer gas [8, 9]: (i) it is more efficient at the center of the cell than near the uncoated walls, so that the atomic orientation is inhomogeneous inside the cell; (ii) the pump beam absorption line is broadened, and its profile varies with the distance from the entrance window. These effects are unfavorable for the production of alignment in the ground state.
The relaxation of atomic polarization in buffer-gas-free, paraffin-coated cesium vapor cells is studied using a variation on Franzen's technique of ``relaxation in the dark'' [Franzen, Phys. Rev. 115 850 (1959)]. In the present experiment, narrow-band, circularly polarized pump light, resonant with the Cs D2 transition, orients atoms along a longitudinal magnetic field, and time-dependent optical rotation of linearly polarized probe light is measured to determine the relaxation rates of the atomic orientation of a particular hyperfine level. The change in relaxation rates during light-induced atomic desorption (LIAD) is studied. No significant change in the spin relaxation rate during LIAD is found beyond that expected from the faster rate of spin-exchange collisions due to the increase in Cs density.
Abstract Anti-relaxation surface coatings can reduce collisions between alkali-metal atoms and cell walls, thereby prolonging the spin-polarization lifetime of alkali-metal atoms. This phenomenon can considerably improve the performance of quantum sensors. Researchers have focused on the surface topography and composition of anti-relaxation coatings in alkali-metal vapor cells and on methods that provide an increase in the relaxation time of alkali metals. We proposed a novel method to characterise the coating performance of alkali-metal cells by directly measuring the energy dissipation during collisions between the anti-relaxation coating surface and atoms in the cell. We found that the interactions between alkali-metal atoms and the anti-relaxation coating resemble elastic collisions, which can be indirectly characterised by the energy dissipation during interactions between an atomic force microscopy tip and atoms. In addition, we confirmed that the energy dissipation between collisions is positively correlated with the relaxation characteristics of alkali metals at various temperatures. This discovery is of great significance as it can be utilised to characterise the anti-relaxation coating performance of alkali-metal cells by directly analysing the energy dissipation of anti-relaxation coatings instead of measuring the relaxation time.
Alkali-metal vapor cells with antirelaxation coating (especially paraffin-coated cells) have been a central tool in optical pumping and atomic spectroscopy experiments for 50 years. We have discovered a dramatic change of the alkali-metal vapor density in a paraffin-coated cell upon application of an electric field to the cell. A systematic experimental characterization of the phenomenon is carried out for electric fields ranging in strength from 0--8 kV/cm for paraffin-coated cells containing rubidium and cells containing cesium. The typical response of the vapor density to a rapid (duration $\ensuremath{\lesssim}100\text{ }\text{ms}$) change in electric field of sufficient magnitude includes (a) a rapid (duration of $\ensuremath{\lesssim}100\text{ }\text{ms}$) and significant increase in alkali-metal vapor density followed by (b) a less rapid (duration of $\ensuremath{\sim}1\text{ }\text{s}$) and significant decrease in vapor density (below the equilibrium vapor density), and then (c) a slow (duration of $\ensuremath{\sim}100\text{ }\text{s}$) recovery of the vapor density to its equilibrium value. Measurements conducted after the alkali-metal vapor density has returned to its equilibrium value indicate minimal change (at the level of $\ensuremath{\lesssim}10%$) in the relaxation rate of atomic polarization. Experiments suggest that the phenomenon is related to an electric-field-induced modification of the paraffin coating.