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Physical Review A

Optical pumping of Rb in the presence of high-pressure He3 buffer gas

B. Larson, O. Häusser, P. P. J. Delheij, D. M. Whittal, Douglas Michael Thiessen

Published Sep 1, 1991• 80 citations
Publisher

Abstract

A cryogenic technique has been used to produce polarized $^{3}\mathrm{He}$ targets of up to relative density p=12 atm (\ensuremath{\approxeq}3\ifmmode\times\else\texttimes\fi{}${10}^{20}$ $^{3}\mathrm{He}$ atoms/${\mathrm{cm}}^{3}$; p=1 atm corresponds to 760 Torr or 101.3 kPa pressure at 273 K). In these targets $^{3}\mathrm{He}$ nuclei are polarized by spin-exchange collisions with optically pumped rubidium atoms. From transmission measurements at wavelengths of 790--800 nm, we have determined pressure shifts, linewidths, and line-shape asymmetries for the Rb 5${\mathit{S}}_{1/2}$\ensuremath{\rightarrow}5${\mathit{P}}_{1/2}$ D1 transition. The Rb spin-destruction rate was found to exhibit a quadratic increase versus $^{3}\mathrm{He}$ pressure, which indicates the importance of Rb${\mathrm{\ensuremath{-}}}^{3}$He${\mathrm{\ensuremath{-}}}^{3}$He collision processes. The transmission results for circularly polarized light are well described by a model that predicts the dependence of the average Rb polarization on Rb density, $^{3}\mathrm{He}$ pressure, light intensity, and cell geometry. The Rb${\mathrm{\ensuremath{-}}}^{3}$He spin-exchange cross section, 〈${\mathrm{\ensuremath{\sigma}}}_{\mathrm{SE}}$v〉=6.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, was found to be independent of $^{3}\mathrm{He}$ pressure up to p=12.1 atm. Maximum $^{3}\mathrm{He}$ polarizations of 72--79% were observed with cells of 17 ${\mathrm{cm}}^{3}$ volume that contained $^{3}\mathrm{He}$ at p=6--9 atm.

DOI
10.1103/physreva.44.3108

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A cryogenic technique has been used to produce polarized 3He^{3}\mathrm{He}3He targets of up to relative density p=12 atm (\ensuremath{\approxeq}3\ifmmode\times\else\texttimes\fi{}1020{10}^{20}1020 3He^{3}\mathrm{He}3He atoms/cm3{\mathrm{cm}}^{3}cm3; p=1 atm corresponds to 760 Torr or 101.3 kPa pressure at 273 K). In these targets 3He^{3}\mathrm{He}3He nuclei are polarized by spin-exchange collisions with optically pumped rubidium atoms. From transmission measurements at wavelengths of 790--800 nm, we have determined pressure shifts, linewidths, and line-shape asymmetries for the Rb 5S1/2{\mathit{S}}_{1/2}S1/2​\ensuremath{\rightarrow}5P1/2{\mathit{P}}_{1/2}P1/2​ D1 transition. The Rb spin-destruction rate was found to exhibit a quadratic increase versus 3He^{3}\mathrm{He}3He pressure, which indicates the importance of Rb\ensuremath−3{\mathrm{\ensuremath{-}}}^{3}\ensuremath−3He\ensuremath−3{\mathrm{\ensuremath{-}}}^{3}\ensuremath−3He collision processes. The transmission results for circularly polarized light are well described by a model that predicts the dependence of the average Rb polarization on Rb density, 3He^{3}\mathrm{He}3He pressure, light intensity, and cell geometry. The Rb\ensuremath−3{\mathrm{\ensuremath{-}}}^{3}\ensuremath−3He spin-exchange cross section, 〈\ensuremathσSE{\mathrm{\ensuremath{\sigma}}}_{\mathrm{SE}}\ensuremathσSE​v〉=6.1\ifmmode\times\else\texttimes\fi{}10\ensuremath−20{10}^{\mathrm{\ensuremath{-}}20}10\ensuremath−20 cm3{\mathrm{cm}}^{3}cm3 s\ensuremath−1{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}s\ensuremath−1, was found to be independent of 3He^{3}\mathrm{He}3He pressure up to p=12.1 atm. Maximum 3He^{3}\mathrm{He}3He polarizations of 72--79% were observed with cells of 17 cm3{\mathrm{cm}}^{3}cm3 volume that contained 3He^{3}\mathrm{He}3He at p=6--9 atm.
We report investigations of spin exchange between 21Ne^{21}\mathrm{Ne}21Ne nuclei and optically pumped Rb vapor. Polarization of 30% (〈Kz〉=0.46〈{K}_{z}〉=0.46〈Kz​〉=0.46) of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ has been achieved with a vapor pressure of 3\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ of Rb. This high density of alkali metal can be efficiently optically pumped only when light trapping is suppressed by inclusion of at least 100 Torr of N2{\mathrm{N}}_{2}N2​. In addition, we have made the first accurate measurement of the binary-collision spin-exchange rate: $〈{\ensuremath{\sigma}}_{\mathrm{ex}}v〉=(4.66\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ cm3{\mathrm{cm}}^{3}cm3-${\mathrm{s}}^{\ensuremath{-}1}$. Extension of these techniques to 3He^{3}\mathrm{He}3He has yielded 5% polarization of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ in a preliminary experiment.
The use of high-power ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Alx{\mathrm{Al}}_{\mathit{x}}Alx​As diode laser arrays for optical pumping of Rb vapor has been examined both theoretically and experimentally. Our studies show that large atomic polarizations can be obtained using the broad spectral output of a diode laser array if the Rb resonance line is collisionally broadened by several atmospheres of a buffer gas. The objective was to study the effectiveness of diode laser arrays as light sources for optically pumped spin-exchange polarized 3He^{3}\mathrm{He}3He targets. The average Rb polarization was deduced at three different Rb vapor pressures using the polarization induced in the 3He^{3}\mathrm{He}3He buffer gas. The largest 3He^{3}\mathrm{He}3He polarization observed was 53% using a 20-W diode array with 1.9-nm full width at half maximum spectral width in 10.5 amagat of 3He^{3}\mathrm{He}3He. This result compares favorably with other light sources for this target. The measured average Rb polarizations are compared to theoretical calculations.
We have studied the optical pumping of mixtures of Rb vapor and N2{\mathrm{N}}_{2}N2​ buffer gas by laser light tuned to the D1{D}_{1}D1​ transition having a spectral width of  500~500 500 MHz. The Rb densities are of the order of 1013{10}^{13}1013 cm${}^{\ensuremath{-}3}$, while the buffer-gas pressures range from 0.1 to 10 torr. As the frequency of the right-hand circularly polarized laser is varied across the D1{D}_{1}D1​ absorption profile, the electron spin polarization of the Rb is found to take on negative values for small negative values of pump detuning from the absorption profile center. This occurs for N2{\mathrm{N}}_{2}N2​ pressures below  1~1 1 torr; at 10 torr the electron spins consistently point in the same direction as the angular momentum of the pump light. The spin-reversal effect can be understood in terms of populations of the F=2F=2F=2 (85Rb^{85}\mathrm{Rb}85Rb) and F=1F=1F=1 (87Rb^{87}\mathrm{Rb}87Rb) states caused by small unpolarized fractions in the pump beam and its elimination in terms of pressure broadening caused by the N2{\mathrm{N}}_{2}N2​ buffer gas. We speculate that this effect could be used for fast Rb spin modulation.
We have studied the efficiency of spin-exchange optical pumping (SEOP) in cells that contain either pure rubidium, mixtures of rubidium and potassium, or pure potassium. Optical pumping was performed with spectrally narrowed diode array lasers operating at either the wavelength of the Rb $5{s}_{1∕2}\ensuremath{-}5{p}_{1∕2}$ transition (795nm)(795\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(795nm) or the K $4{s}_{1∕2}\ensuremath{-}4{p}_{1∕2}$ transition (770nm)(770\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(770nm). Calculations of the expected efficiency are shown for practical conditions and compared to measurements. We have measured the vapor density ratio in Rb-K mixture cells using absorption of white light. Our focus is on the conditions that are relevant for neutron spin filters, but the results are relevant to all applications of SEOP. We find that both Rb-K and pure K cells yield improvements in the spin-exchange efficiency, and each approach has its own advantages. Overall, the best results for high polarizing rate with high polarization were obtained with mixture cells with K-Rb vapor density ratios between 2 and 6.
We have made a systematic study of rubidium optical pumping in a simple cylindrical cell geometry with a high-power 10W10\phantom{\rule{0.3em}{0ex}}\mathrm{W}10W diode laser array, low magnetic fields, and buffer-gas pressures of less than 50torr50\phantom{\rule{0.3em}{0ex}}\text{torr}50torr. We have determined rubidium polarizations experimentally for H2{\mathrm{H}}_{2}H2​, N2{\mathrm{N}}_{2}N2​, He, and Ar buffer gases, with Rb number densities from ${10}^{12}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Comparison to a relatively simple optical pumping model allows us to extract useful information about radiation trapping and quenching effects.
We have investigated high-density Rb optical pumping in polarized 3He^{3}\mathrm{He}3He targets and present measurements of collisional relaxation rates and studies of diffusion-driven wall relaxation of the Rb polarization. We show that a boundary layer resides in the vicinity of the cell walls within which a polarization gradient is established by diffusion. Absorption of the incident laser radiation within this layer leads to a striking decrease in optical-pumping efficiency for resonant light but has little effect for off-resonant light. Our theory introduces polarization-dependent slowing of optical pumping rates due to the Rb nuclear spin. We have observed a strong frequency dependence of the optical-pumping efficiency, consistent with our theoretical predictions. We report the following rate constants for collisional relaxation of Rb polarization: due to Rb (${\mathit{k}}_{\mathrm{Rb}\mathrm{\ensuremath{-}}\mathrm{Rb}}$=8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ cm3{\mathrm{cm}}^{3}cm3/s), due to N2{\mathrm{N}}_{2}N2​ (${\mathit{k}}_{\mathrm{N}2\mathrm{\ensuremath{-}}}$Rb=8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ cm3{\mathrm{cm}}^{3}cm3/s), and due to 3He^{3}\mathrm{He}3He, an upper limit of (k3{\mathit{k}}^{3}k3He-Rb\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ cm3{\mathrm{cm}}^{3}cm3/s). The data and model presented are used to predict laser power and intensity requirements for high-density polarized 3He^{3}\mathrm{He}3He targets.