We present a comprehensive theory of nuclear spin polarization of 3 He and 129 Xe gases by spin-exchange collisions with optically pumped alkali-metal vapors. The most important physical processes considered are ͑1͒ spin-conserving spin-exchange collisions between like or unlike alkali-metal atoms; ͑2͒ spin-destroying collisions of the alkali-metal atoms with each other and with buffer-gas atoms; ͑3͒ electron-nuclear spin-exchange collisions between alkali-metal atoms and 3 He or 129 Xe atoms; ͑4͒ spin interactions in van der Waals molecules consisting of a Xe atom bound to an alkali-metal atom; ͑5͒ optical pumping by laser photons; ͑6͒ spatial diffusion. The static magnetic field is assumed to be small enough that the nuclear spin of the alkali-metal atom is well coupled to the electron spin and the total spin is very nearly a good quantum number. Conditions appropriate for the production of large quantities of spin-polarized 3 He or 129 Xe gas are assumed, namely, atmospheres of gas pressure and nearly complete quenching of the optically excited alkali-metal atoms by collisions with N 2 or H 2 gas. Some of the more important results of this work are as follows: ͑1͒ Most of the pumping and relaxation processes are sudden with respect to the nuclear polarization. Consequently, the steady-state population distribution of alkali-metal atoms is well described by a spin temperature, whether the rate of spin-exchange collisions between alkali-metal atoms is large or small compared to the optical pumping rate or the collisional spin-relaxation rates. ͑2͒ The population distributions that characterize the response to sudden changes in the intensity of the pumping light are not described by a spin temperature, except in the limit of very rapid spin exchange. ͑3͒ Expressions given for the radio-frequency ͑rf͒ resonance linewidths and areas can be used to make reliable estimates of the local spin polarization of the alkali-metal atoms. ͑4͒ Diffusion effects for these high-pressure conditions are mainly limited to thin layers at the cell surface and at internal resonant surfaces generated by radio-frequency magnetic fields when the static magnetic field has substantial spatial inhomogeneities. The highly localized effects of diffusion at these surfaces are described with closedform analytic functions instead of the spatial eigenmode expansions that are appropriate for lower-pressure cells. ͓S1050-2947͑98͒07408-3͔

ly. The oxygen fugacity (/02) of the experiments was not buffered externally. However, /02 calculations based on biotite-sanidine-magnetite-H20-02 equilibrium [D. R. Wones, Kozan Chishitsu 31, 191 (1981)] yield /02 values 1.5 to 2 log units above the nickel-nickel oxide buffer. This is consistent with the estimated f02 conditions of natural epidote-bearing magmas {6, 16). Quenched experimental charges were sectioned longitudinally, polished, and examined with reflected-light microscopy and backscattered electron imaging. Rim widths were measured with an optical microscope equipped with a graduated ocular lens; the rim widths reported are the average of 20 to 30 measurements. Rim width data are as follows: experiment Ep-10, t = 51.17 hours, rim width = 2.74 ± 0.9 fjim; experiment Ep-12, t = 141.20 hours, rim width = 4.95 ± 0.8 fjtm; and experiment Ep-11, t = 378.78 hours, rim width = 8.18 £ 1.3 fjtm. 9. D. C. Rubie and A.

The precision of nuclear magnetic resonance spectroscopy 1 (NMR) is limited by the signal-to-noise ratio, the measurement time T m and the linewidth ν = 1/(πT 2 ). Overcoming the T 2 limit is possible if the nuclear spins of a molecule emit continuous radio waves. Lasers 2,3 and masers 4-13 are self-organized systems which emit coherent radiation in the optical and microwave regime. Both are based on creating a population inversion of specific energy states. Here we show continuous oscillations of proton spins of organic molecules in the radiofrequency regime (raser 5 ). We achieve this by coupling a population inversion created through signal amplification by reversible exchange (SABRE) 14-16 to a high-quality-factor resonator. For the case of 15 N labelled molecules, we observe multi-mode raser activity, which reports di erent spin quantum states. The corresponding 1 H-15 N J-coupled NMR spectra exhibit unprecedented sub-millihertz resolution and can be explained assuming two-spin ordered quantum states. Our findings demonstrate a substantial improvement in the frequency resolution of NMR.Radio-wave masers (rasers) arise from the radiofrequency Zeeman splittings of nuclear spins such as 1 H, 3 He, 29 Al or 129 Xe. Rasers using 3 He and 129 Xe gas as the rasing medium employ spin exchange optical pumping 17 (SEOP) at T > 400 K to create sufficient population inversion 6-8 , whereas 29 Al solid 9 or 1 H Zeeman liquidstate masers 10 rely on dynamic nuclear polarization (DNP) techniques 18 or photochemical excitation 11 to invert populations. Solidstate maser action has been observed in pulsed mode at room temperature with pentacene 12 , and a continuous-mode solid-state maser based on nitrogen-vacancy centres in diamond has been proposed 13 .Here we report the observation of a liquid-state para-hydrogen pumped molecular raser that operates at 300 K with protons of organic molecules in solution and thereby avoids costly high magnetic fields, high vacuum, optical pumping or DNP techniques. We continuously supply para-hydrogen (p-H 2 ) gas into a solution containing the raser active molecules and an iridium-based SABRE catalyst [14][15][16] . This spin-order transfer catalyst creates population inversion, equivalent to a negative spin temperature, on target molecules without altering their molecular structure. Coupling of these hyperpolarized molecules to a high-Q resonator 19 produces a sustained raser signal comprised of frequencies that originate from the scalar couplings of nuclei within the molecule.The operating principles for the 3 He Zeeman maser 6-8 are a starting point to assess challenges associated with the design of a SABRE-pumped room-temperature proton raser working at low frequencies. Masing starts once the radiation-damping rate 1/τ rd , which quantifies the coupling between the resonator and the nuclear spins, satisfies the conditionHere the apparent transverse relaxation rate 1/T * 2 = 1/T 2 + 1/τ p is the sum of the transverse relaxation rate 1/T 2 and the pumping rate 1/τ p . The radiation-dampi...