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Z. Xu, et al. [Phys. Rev. A 103, 023114 (2021)], have reported a measurement for the enhancement factor $\kappa$, which characterizes the overlap of the electron wavefunction with the noble-gas nucleus in Cs-$^{129}$Xe spin-exchange optical pumping (SEOP), that is more than an order of magnitude higher than previous experimental results and theoretical estimates. There is no plausible proposed physical basis for this discrepancy. As the authors repeatedly mischaracterize the nature and properties of the enhancement factor and do not completely describe their experimental technique, there is little reason to accept or consider this result in its current form in the context of future relevant work on Cs-Xe SEOP and its applications., Comment: 2 pages, 0 figures. Comment on: Z. Xu, et al., Phys. Rev. A 103, 023114 (2021)

We report a measurement of the ratio of dimensionless enhancement factors $\kappa_0$ for Cs-$^{129}$Xe and Rb-$^{129}$Xe in the temperature range $115-140\, ^{\circ}$C; both pairs are used in spin-exchange optical pumping (SEOP) to produce hyperpolarized $^{129}$Xe. $\kappa_0$ characterizes the amplification of the $^{129}$Xe magnetization contribution to the alkali-metal electronic effective field, compared to the case of a uniform continuous medium in classical magnetostatics. The measurement was carried out in "hybrid" vapor cells containing both Rb and Cs metal in a prescribed ratio, producing approximately the same vapor density for both. Alternating measurements of the optically detected EPR frequency shifts caused by the SEOP polarization and subsequent sudden destruction of the same quantity of $^{129}$Xe magnetization were made for $^{133}$Cs and either $^{87}$Rb or $^{85}$Rb. An important source of systematic error caused by power fluctuations in the pump laser that produced variable light shifts in the EPR frequency was characterized and then mitigated by allowing sufficient warm-up time for the pump laser. We measured $(\kappa_0)_{\rm CsXe}/(\kappa_0)_{\rm RbXe} = 1.215 \pm 0.007$ with no apparent temperature dependence. Based on our previous measurement $(\kappa_0)_{\rm RbXe}=518 \pm 8$, we determine $(\kappa_0)_{\rm CsXe} = 629 \pm 10$, more precise than but consistent with a previous measurement made by another research group., Comment: 10 pages, 7 figures, 47 references

We report a measurement of the dimensionless enhancement factor $\kappa_0$ for the Rb-$^{129}$Xe pair commonly used in spin exchange optical pumping (SEOP) to produce hyperpolarized $^{129}$Xe. $\kappa_0$ characterizes the amplification of the $^{129}$Xe magnetization contribution to the Rb electronic effective field, compared to the case of a uniform continuous medium in classical magnetostatics. The measurement is carried out in Rb vapor cells containing both $^3$He and $^{129}$Xe and relies on the previously measured value of $\kappa_0$ for the Rb-$^3$He pair. The measurement is based on (1) the optically detected (Faraday rotation) frequency shift of the $^{87}$Rb EPR hyperfine spectrum caused by the SEOP nuclear polarization and subsequent sudden destruction of nuclear polarization of both species and (2) a comparison of NMR signals for the two species acquired just prior to the EPR frequency shift measurements. We find $(\kappa_0)_{\rm RbXe} = 518\pm 8$, in good agreement with previous measurements and theoretical estimates but with improved precision., Comment: 21 pages, 7 figures

This article reviews the physics and technology of producing large quantities of highly spin-polarized, or hyperpolarized, $^3$He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping, and surveys applications of polarized $^3$He. Several recent developments are emphasized for each method. For SEOP, the use of spectrally narrowed lasers and Rb/K mixtures has substantially increased the achievable polarization and polarizing rate. MEOP in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and has led to the observation of a light-induced relaxation mechanism. In both methods the increased capabilities have led to more extensive study and modeling of the basic underlying physics. New unexplained dependences of relaxation on temperature and magnetic field have been discovered in SEOP cells. Applications of both methods are also reviewed, including targets for charged particle and photon beams, neutron spin filters, magnetic resonance imaging, and precision measurements., Comment: 68 pages, 25 figures, to appear in Reviews of Modern Physics. The introduction has be rewritten, plus changes in response to referee comments

We find that if solid xenon is formed from liquid xenon, denoted "ice", there is a 10% increase of $^{129}$Xe longitudinal relaxation $T_1$ time (taken at 77 K and 2 Tesla) over a trickle-freeze formation, denoted "snow". Forming xenon ice also gives unprecedented reproducibility of $^{129}$Xe $T_1$ measurements across a range of 77-150 K. This temperature dependence roughly follows the theory of spin-rotation mediated by Raman scattering of harmonic phonons (SRRS), though it results in a smaller-than-predicted spin-rotation coupling strength $c_{K0}/h$. Enriched ice $^{129}$Xe $T_1$ experiments show no isotopic dependence in bulk relaxation mechanisms at 77 K and at kilogauss fields., Comment: 7 pages, 4 figures

The sensitivity of the Rabi oscillations of a resonantly driven spin-1/2 system to a weak and slow modulation of the static longitudinal magnetic field, B_0, is studied theoretically. We establish the mapping of a weakly driven two-level system with modulation onto a strongly driven system without modulation. The mapping suggests that different regimes of spin dynamics, known for a strongly driven system, can be realized under common experimental conditions of weak driving (driving field B_ 1 << B_ 0) upon proper choice of the domains of modulation frequency, \omega_m, and amplitude, B_2. Fast modulation \omega_m >> \Omega_R, where \Omega_R is the Rabi frequency, emulates the regime of driving frequency much bigger than the resonant frequency. Strong modulation, B_2 >> B_1, emulates the regime B_1 >> B_0. Resonant modulation, \omega_m \approx \Omega_R, gives rise to an envelope of the Rabi oscillations. The shape of this envelope is highly sensitive to the detuning of the driving frequency from the resonance. Theoretical predictions for different domains of B_2 and \omega_m were tested experimentally using NMR of protons in water, where, without modulation, the pattern of Rabi oscillations could be observed over many periods. We present experimental results which reproduce the three predicted modulation regimes, and agree with theory quantitatively., Comment: 14 pages, 11 figures

The effect of dipolar and exchange interactions within pairs of paramagnetic electronic states on Pauli-blockade-controlled spin-dependent transport and recombination rates during magnetic resonant spin excitation is studied numerically using the superoperator Liouville-space formalism. The simulations reveal that spin-Rabi nutation induced by magnetic resonance can control transition rates which can be observed experimentally by pulsed electrically (pEDMR) and pulsed optically (pODMR) detected magnetic resonance spectroscopies. When the dipolar coupling exceeds the difference of the pair partners' Zeeman energies, several nutation frequency components can be observed, the most pronounced at sqrt{2} gamma B_1 (gamma is the gyromagnetic ratio, B_1 is the excitation field). Exchange coupling does not significantly affect this nutation component; however, it does strongly influence a low-frequency component < gamma B_1. Thus, pEDMR/pODMR allow the simultaneous identification of exchange and dipolar interaction strengths., Comment: 12 pages, 4 figures, to be submitted to Physical Review B, updated references

We study theoretically the spectrum, F(s), of spin-dependent transition rates within dipolar D and exchange J coupled pairs of two spins with S=1/2 undergoing Rabi oscillations due to a coherent magnetic resonant excitation. We show that the Rabi oscillation controlled rates exhibit a spectrum with three frequency components. When exchange is stronger than the Rabi frequency (J>>\Omega_R), the frequency components of the Rabi oscillation do not depend on J, rather they are determined by the relation between \Omega_R and D. We derive analytical expressions for the frequencies and the intensities of all three Rabi oscillation components as functions of \Omega_R/D and \delta/D, where \delta is detuning of the driving ac field from the Larmor frequency. When \Omega_R>>D, the two lower frequencies approach s=\Omega_R, while the upper line approaches s=2\Omega_R. Disorder of the local Larmor frequencies leads to a Gaussian broadening of the spectral lines. We calculate corresponding widths for different \Omega_R/D and \delta/D. Unexpectedly, we find that one of the frequency components exhibits an unusual evolution with \Omega_R: its frequency decreases with \Omega_R at \Omega_R

The Fermi-contact interaction that characterizes collisional spin exchange of a noble gas with an alkali-metal vapor also gives rise to NMR and EPR frequency shifts of the noble-gas nucleus and the alkali-metal atom, respectively. We have measured the enhancement factor $\kappa_{0}$ that characterizes these shifts for Rb-$^{129}$Xe in the high-pressure limit to be 493$\pm$31, making use of the previously measured value of $\kappa_{0}$ for Rb-$^{3}$He. This result allows accurate $^{129}$Xe polarimetry with no need to reference a thermal-equilibrium NMR signal.