Theory of spin - exchange optical pumping of 3 He and 129 Xe

We present a comprehensive theory of nuclear spin polarization of He and Xe gases by spin-exchange collisions with optically pumped alkali-metal vapors. The most important physical processes considered are ~1! spin-conserving spin-exchange collisions between like or unlike alkali-metal atoms; ~2! spin-destroying collisions of the alkali-metal atoms with each other and with buffer-gas atoms; ~3! electron-nuclear spin-exchange collisions between alkali-metal atoms and He or Xe atoms; ~4! spin interactions in van der Waals molecules consisting of a Xe atom bound to an alkali-metal atom; ~5! optical pumping by laser photons; ~6! 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 He or Xe gas are assumed, namely, atmospheres of gas pressure and nearly complete quenching of the optically excited alkali-metal atoms by collisions with N2 or H2 gas. Some of the more important results of this work are as follows: ~1! 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. ~2! 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. ~3! Expressions given for the radio-frequency ~rf! resonance linewidths and areas can be used to make reliable estimates of the local spin polarization of the alkali-metal atoms. ~4! 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. @S1050-2947~98!07408-3#