Alkali-metal-atom polarization imaging in high-pressure optical-pumping cells

We present a detailed experimental analysis of Rb-polarization imaging in high-pressure gas cells. The Rb vapor in these cells is optically pumped by high-power diode-laser arrays. We present images for high ~35 G! and low ~ 4G !magnetic fields and for different He and Xe buffer-gas mixtures. We demonstrate that high-field imaging provides an absolute measurement of the Rb-polarization distribution in the cell, based on the fact that a spin-temperature distribution of the hyperfine magnetic sublevels is established in high-pressure buffer gases. A survey of various mechanisms that broaden the Rb magnetic-resonance lines is presented. These broadening mechanisms determine the limits of the spatial resolution achievable for images of the Rb-polarization distribution. @S1050-2947~98!04109-2# PACS number~s!: 32.80.Bx, 32.80.Cy, 32.70.Jz The production of hyperpolarized noble gases through spin exchange with optically pumped alkali-metal atoms has been important for a large variety of fields, among them magnetic resonance imaging @1‐6#, materials and surface sciences @7‐10#, atomic physics @11‐13#, and in nuclear physics measurements such as the determination of the spin structure of the neutron @14#. The present work is motivated by a desire to increase the quantity and the polarization of the hyperpolarized noble gas produced. The total noble-gas nuclear polarization produced by spin-exchange optical pumping depends on the three-dimensional distribution of the Rb spin polarization. A reliable method of measuring the spatial distribution of the alkali-metal polarization should provide helpful information in the development of effective optical-pumping techniques. An imaging technique in very low magnetic field using laser optical pumping in a rotating magnetic field gradient has recently been published @15# .I n this paper we present an imaging technique for high-pressure optical-pumping cells, in high and low magnetic fields @16#. We focus on Rb vapor pumped by broadband high-power diode-laser arrays @2‐4 nm full width at half maximum ~FWHM!#. The high-pressure buffer gas broadens the Rb optical transition linewidth, leading to a more efficient optical coupling to the diode-laser array emission. At the buffergas pressures explored in this work ( ’10 atm!, the spatial diffusion of the Rb atoms is negligible @17#. The Rb polarization is effectively ‘‘frozen’’ into the vapor. An additional, interesting finding is that optical pumping under highpressure conditions generally leads to a spin-temperature distribution of Rb atoms between the hyperfine magnetic sublevels.