The applicability of commonly used 129Xe nuclear polarization (PXe) buildup models to large optical cell polarizers, which operate at lower gas pressures when compared to small cell polarizers for which spin-exchange optical pumping (SEOP) models were originally developed, has yet to be systematically evaluated. In addition, spatial heterogeneity of Rb vapor density ([Rb]) and its impact on SEOP in low pressure, large optical cell polarizers remains poorly understood. In this theoretical study, it was shown that 1D analytical and numerical PXe buildup models yield diverging PXe and critical flow rate values at low flow rates and high laser absorption, contributing to the discrepancy between theoretical and experimentally measured production rates. Additionally, it was found that [Rb] heterogeneity leads to lower PXe buildup and an undesirable increase in optimal cell temperature, while the optimal laser absorption remains relatively unchanged. The updated simulation framework presented here can be utilized to model other SEOP systems.
The spectroscopic characteristics of the polarization of 129Xe nuclei in Xe gas by spin exchange with optically pumped Rb atoms is examined in high magnetic field. The high field Zeeman effect provides the spectral dispersion necessary to separate the effects of different light polarizations and incident wavelengths on the pumping cycle. Indirect detection of the D1 line in Rb using 129Xe nuclear magnetic resonance in combination with direct optical detection indicates that conventional container construction significantly decreases the efficiency of the 129Xe polarization. The decreased pumping efficiency is due to randomly polarized photons that can be eliminated by using containers with good optical quality windows. Study of these effects is facilitated by the large Zeeman shifts obtained with multi-tesla fields which resolve the D1 multiplet structure even in the presence of the significant pressure-broadening incells with typical gas pressures of 0⋅1-1 atm.
The polarization of 129Xe gas by spin exchange with optically pumped Rb vapor is investigated in high magnetic field. Operation in a high field provides added spectral dispersion via Zeeman shifts of the electronic transitions. This simplifies the physics of the pumping process in such a way as to make it more amenable to treatment with simple rate theory. A relationship between the steady state 129Xe polarization and the applied laser power is derived which agrees with experimental results. This theory serves as a guide to the manufacture of very high polarizations of 129Xe gas.
The ability to produce liter volumes of highly-spin-polarized
129Xe enables a wide range of investigations, most notably in the fields of materials science and biomedical magnetic resonance imaging. However, for nearly all polarizers built to date, both peak
129Xe polarization and the rate at which it is produced fall far below those predicted by the standard model of Rb metal vapor, spin-exchange optical pumping (SEOP). In this work we comprehensively characterized a high-volume flow-through
129Xe polarizer using three different SEOP cells with internal volumes of 100, 200, and
300cm3 and two types of optical sources: a broad-spectrum 111-W laser [full width at half maximum (FWHM) equal to 1.92 nm] and a line-narrowed 71-W laser (FWHM equal to 0.39 nm). By measuring
129Xe polarization as a function of gas flow rate, we extracted the peak polarization and polarization production rate across a wide range of laser absorption levels. Peak polarization for all cells consistently remained a factor of 2--3 times lower than predicted at all absorption levels. Moreover, although production rates increased with laser absorption, they did so much more slowly than predicted by the standard theoretical model and basic spin-exchange efficiency arguments. Underperformance was most notable in the smallest optical cells. We propose that all these systematic deviations from theory can be explained by invoking the presence of paramagnetic Rb clusters within the vapor. Cluster formation within saturated alkali-metal vapors is well established and their interaction with resonant laser light was recently shown to create plasmalike conditions. Such cluster systems cause both Rb and
129Xe depolarization, as well as excess photon scattering. These effects were incorporated into the SEOP model by assuming that clusters are activated in proportion to excited-state Rb number density and by further estimating physically reasonable values for the nanocluster-induced velocity-averaged spin-destruction cross section for Rb $(\ensuremath{\langle}{\ensuremath{\sigma}}_{\text{cluster-Rb}}v\ensuremath{\rangle}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, the
129Xe relaxation cross section $(\ensuremath{\langle}{\ensuremath{\sigma}}_{\text{cluster-Xe}}v\ensuremath{\rangle}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, and a non-wavelength-specific photon-scattering cross section $({\ensuremath{\sigma}}_{\mathrm{cluster}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2})$. The resulting modified SEOP model now closely matches experimental observations.
We have made a systematic study of rubidium optical pumping in a simple cylindrical cell geometry with a high-power
10W diode laser array, low magnetic fields, and buffer-gas pressures of less than
50torr. We have determined rubidium polarizations experimentally for
H2,
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.
Optical pumping of an optically thick atomic vapor typically requires a quenching buffer gas, such as N2, to prevent radiation trapping of unpolarized photons which would depolarize the atoms. We show that optical pumping of a trace contamination of Rb present in K metal results in a 4.5 times higher polarization of K than direct optical pumping of K in the absence of N2. Such spin-exchange polarization transfer from optically thin species is useful in a variety of areas, including spin-polarized nuclear scattering targets and electron beams, quantum-nondemolition spin measurements, and ultrasensitive magnetometry.