Optical Pumping Cell Research Articles

Atomic resonance behavior in laser-induced Rb atom desorption—the effect of ambient gas atoms and molecules

Abstract We report an observation that in a typical optical pumping cell containing Rb metal and 150 Torr N 2 gas, a cw laser beam of a few tens of mW and beam size 5.5 mm 2 can desorb Rb films on cell surfaces when the laser is tuned to the Rb D 1 line. The frequency dependence of the Rb desorption rate displays atomic resonance behavior. The desorption is suggested to be mediated by the ambient gas atoms (Rb) and molecules (N 2 ). The phenomenon was used to provide evidence for the existence of thin Rb films on seemingly clear cell surfaces.

Energy transport in high-density spin-exchange optical pumping cells.

We present in situ measurements of temperatures inside multi-atmosphere spin-exchange optical pumping cells using Raman scattering from the N2 quenching gas. Under conditions usually prevailing in spin-exchange optical pumping experiments, we find that gas temperatures can be elevated hundreds of degrees above ambient, and that convection plays a very important role in the heat transport of the system.

Coatings for optical pumping cells and short-term storage of hyperpolarized xenon

For a number of years now, siloxanes have been the materials of choice for coating vessels used in the production and short-term storage of hyperpolarized xenon. The methods used to apply this material, however, often vary from one research group to another and it is commonly reported that it is difficult to obtain cells with consistently long spin-lattice relaxation times (T1) and high-polarization levels. In a series of controlled experiments individual production variables were altered and optimized, leading to improved protocols for the reliable production of high-quality siloxane-coated cells. During these studies we discovered that the surface-induced relaxation rates in bare and coated Pyrex cells differ profoundly. This information on Xe relaxation helps to define the limits on the way pumping cells can be improved and suggests the need for further fundamental work on relaxation mechanisms.

Optimisation of optical pumping and filter cell development at the [formula omitted] filter project at HMI

The use of 3 He neutron spin filters opens new experimental fields, especially in the case of beams which are divergent or which have a broad bandwidth, in particular in the range of thermal or epithermal neutrons. Hence, investigations on such filter cells and the construction of a filling station to supply them is a focal point in the instrumental development programme at the HMI Berlin. An overview on the status of the project will be given. It shows results of our investigations on an advantageous design of filter cells and on the optimisation of optical pumping to polarise the 3 He gas with respect to its nuclear spins, as well.

Measurement of rubidium and xenon absolute polarization at high temperatures as a means of improved production of hyperpolarized (129)Xe.

We report on a rubidium-xenon (Rb-Xe) polarization unit for the continuous production of large quantities of hyperpolarized (129)Xe. The unit includes two diagnostic systems which enable the absolute measurement of both the (85)Rb and (129)Xe polarization in situ and at high temperatures. The Rb diagnostic system allows the measurement of one- or two-dimensional images of the absolute Rb polarization and thus enables the experimental study of light penetration into the optical pumping cell. The equilibrium Xe polarization measured in the optical pumping cell and in the freezing unit is typically approximately 20%, under optimal flowing conditions, and this is much lower than the volume-averaged rubidium polarization.

Absolute calibration of a double-cell polarized 3He gas target

We introduce a new method for determining the polarization of the atoms in a polarized 3He target. The new method makes use of special properties of the reaction 3He(d,p) 4He. We demonstrate that for protons produced by this reaction at θ=0°, the polarization observables T 20 and C z, z are related according to 2 T 20+3C z,z=−2 . Using this result, we have made an absolute measurement of the target polarization in the Wisconsin double-cell polarized 3He target. The calibration was carried out by observing the 3He(d,p) 4He reaction with vector- and tensor-polarized deuterons of known polarization. The analyzing power T 20(0°) at energy E d=2.1 MeV was first measured with the result T 20(0°)=−0.530±0.012. The corresponding spin correlation coefficient is C z, z (0°)=−0.417±0.006. The target polarization, P t, was then determined by measuring the 0° proton asymmetry with vector-polarized deuterons. During the measurement, the relative target polarization was monitored by measuring the circular polarization P ⊙ of the 668 nm emission light from the optical pumping cell. From the measured target polarization we obtain a calibration constant K= P t/ P ⊙=11.57±0.43. This result is in good agreement with the calibrations based on other techniques.

Coatings for optical pumping cells and extending the lifetime of hyperpolarized xenon

We propose evaporative induction heating as a method for the reliable production of coatings for glass cells suitable for the optical pumping and storage of hyperpolarized xenon. The long spin-lattice relaxation times of hyperpolarized xenon-129 contained in cells coated with polyethylene or dotriacontane showed that high quality coatings could be prepared this way. Measurements on cells coated with deuteriated versions of these compounds showed that the expected improvement in performance with isotopic substitution did not occur. This leads to some questions about the level of understanding of wall-induced relaxation in polarization cells.

Characterization and stabilization of fiber-coupled laser diode arrays

We have characterized the spectra and performance of an ensemble of 11 fiber-coupled laser diode arrays (LDAs) manufactured by Opto Power Inc. These high-power LDAs operate near 795 nm and are of a type commonly used for spin-exchange optical pumping of noble gases. We find the Opto Power LDAs to vary significantly in output power, spectral width, and other important characteristics, in a manner not correlated with age, operating lifetime, or information supplied by the manufacturer. In addition we have developed a two-loop feedback technique for use with LDAs that stabilizes the Rb magnetization in an optical pumping cell to better than one part in a thousand.

Three-dimensional imaging of spin polarization of alkali-metal vapor in optical pumping cells

We present detailed images of the Rb polarization in a high-pressure optical pumping cell. Images taken at high magnetic field strengths (≈37 G) yield precise information on the relative occupation probability of the hyperfine Zeeman sublevels. Low-field (⩽4 G) images yield very high resolution (less than 1 mm) maps of the spatial distribution of the Rb polarization. For optical pumping in the presence of high-pressure buffer gases, the measured populations of the hyperfine magnetic sublevels are shown to be well described by a spin temperature, even though the Rb–Rb spin exchange rate is much smaller than the rates of optical pumping or electron-randomizing Rb–Xe collisions in the buffer gas.

Spin exchange optical pumping of hydrogen and deuterium nuclei

The prospects for the direct production of a nuclear polarized atomic hydrogen or deuterium target by the use of spin exchange optical pumping is analyzed in both a low and a high magnetic field. In a low magnetic field it is found that the optical pumping cell wall coatings must have relaxation times that correspond to thousands of wall collisions if one is to produce a highly polarized target. In a high magnetic field it is found that wall coatings must have relaxation times that correspond to only a few hyndred wall collisions in order to produce a highly polarized atomic hydrogen target but must have relaxation times that correspond to thousands of wall collisions to produce a highly polarized atomic deuterium target.

Quenching, depolarization, and transfer of spin polarization in Rb-N2collisions

We have studied the transfer of electronic and nuclear spin polarization from the $5^{2}P_{\frac{1}{2}}$ state of Rb to the $5^{2}S_{\frac{1}{2}}$ state caused by quenching collisions of Rb atoms with ${\mathrm{N}}_{2}$ molecules. We evaluate the dependence of such polarization transfer upon the strength of the hyperfine interaction in the excited state. The nuclear spin independent relative probabilities connecting $J$, ${m}_{J}$ sublevels of the $5^{2}P_{\frac{1}{2}}$ state with the $S$, ${m}_{S}$ sublevels of the $^{2}S_{\frac{1}{2}}$ state are ${K}_{1}(^{2}P_{\frac{1}{2}},{m}_{J}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}\ensuremath{\rightarrow}^{2}S_{\frac{1}{2}},{m}_{S}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2})=0.33\ifmmode\pm\else\textpm\fi{}0.15,$ ${K}_{2}(^{2}P_{\frac{1}{2}},{m}_{J}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}\ensuremath{\rightarrow}^{2}S_{\frac{1}{2}},{m}_{S}=\ensuremath{\mp}\frac{1}{2})=0.67\ifmmode\pm\else\textpm\fi{}0.15.$ The results suggest that the quenching interaction is a sudden perturbation in which the orientation of the electronic spin is preserved. The experiment provides a determination of the cross section for the destruction of orientation within the $^{2}P_{\frac{1}{2}}$ state, ${\ensuremath{\Lambda}}_{\frac{1}{2}}^{1}$:${\ensuremath{\Lambda}}_{\frac{1}{2}}^{1}(\mathrm{Rb}^{2}P_{\frac{1}{2}},{\mathrm{N}}_{2})=(105\ifmmode\pm\else\textpm\fi{}25)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$.The value of ${\ensuremath{\Lambda}}_{\frac{1}{2}}^{1}$ for Rb in ${\mathrm{N}}_{2}$ is considerably larger than corresponding values for Rb in the noble gases. The experimental and theoretical results provide insight into the influence of quenching collisions on the efficiency of optical-pumping processes. They show why a small amount of ${\mathrm{N}}_{2}$ added to an optical-pumping cell improves pumping efficiency, whereas a large amount of ${\mathrm{N}}_{2}$ reduces it.

Efficient energy extraction by superradiant emission in NH 3

With a 320-cm long optical pumping cell, superradiant IR emission in NH 3 has been efficiently obtained. Oscillation occured dominantly in the forward direction. An output energy of 310 mJ per pulse has been extracted in the forward direction by superradiant emission. Including both the forward and backward output energies, photon conversion efficiency of up to 43% was achieved.

Spin-Exchange Cross Section forRb85-Rb87Collisions

The spin-exchange cross section for ${\mathrm{Rb}}^{85}$-${\mathrm{Rb}}^{87}$ collisions has been measured to be (1.70\ifmmode\pm\else\textpm\fi{}0.21)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$ using the techniques of optical pumping and an interferometric method for measuring the Rb vapor density. The ${\mathrm{Rb}}^{87}$ constituent in a vapor of natural Rb was polarized through absorption of circularly polarized resonance radiation, and the ${\mathrm{Rb}}^{87}$ polarization was partially transferred to the ${\mathrm{Rb}}^{85}$ constituent through spin-exchange collisions. A phenomenological theory of the optical pumping plus the exchange polarization process which neglects nuclear spin was used to relate the cross section to the ratio of the signal strengths for the ${\mathrm{Rb}}^{87}$ and ${\mathrm{Rb}}^{85}$ Zeeman resonances, the relaxation time, and the ${\mathrm{Rb}}^{87}$ vapor density. The density was measured by comparing spectral profiles of the resonance radiation transmitted through the optical pumping cell at room temperature with the resonance radiation transmitted at the temperature at which the signal strengths were measured. These spectral profiles were obtained through use of a scanning Fabry-Perot interferometer.