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

We present experimental data and theoretical interpretation of NMR spectra of remotely magnetized samples, detected in an unshielded environment by means of a differential atomic magnetometer. The measurements are performed in an ultra-low-field at an intermediate regime, where the J-coupling and the Zeeman energies have comparable values and produce rather complex line sets, which are satisfactorily interpreted.

The response of a spin exchange relaxation free atomic magnetometer to a repetitive short-pulsed pump was investigated. Quantum sub-resonances at a repetition rate of 1/n of the Larmor frequency of the magnetic field inside the shield are experimentally observed and theoretically explained. This is a type of synchronization phenomenon. Investigations in single alkali atom cells and mixed alkali atoms of K and Rb are presented. In the latter, one species is pumped while the probe is on the other species polarized by spin exchange. The effect of spin destruction, spin exchange, and collisions is studied in order to account for the width of the resonances. Quantum calculations of a three-level Λ model for this phenomenon exhibit a dip at the resonance frequency in the absorption spectrum for both cases of pulsed and cw pump modes and an evidence for electromagnetically induced transparency.