Figure 1.

Optical pumping and spin exchange. (a) Simplified conceptual diagram of the optical-pumping and spin-exchange process. In optical pumping (left), circularly-polarized laser light at the appropriate wavelength polarizes the spins of the valence electrons (shown in green) of rubidium atoms by preferentially populating one of the two spin states for the valence electron. In this diagram, electron-spin polarization is illustrated by a change in the electron-spin orientation (symbolized by the small black arrow through the electron) from down to up. Spin-exchange collisions (right) transfer this polarization from rubidium electrons to noble-gas nuclei. After spin exchange, the spins of the valence electrons of rubidium atoms are polarized again and participate in subsequent spin-exchange collisions, further increasing the noble-gas polarization. (Diagram adapted from Fig. 1 of ref. (99).) (b) Photograph of an early prototype commercial gas-polarization system (model IGI 9600, Magnetic Imaging Technologies, Inc. [MITI], Durham, NC), showing some of the system’s major functional aspects. This system, the first manufactured by MITI, has an exchangeable central “cartridge” to permit operation with either ¹²⁹Xe or ³He.