Schematic representations of SEOP.[2b, 4] (a)
SEOP cell containing a noble gas (here, Xe), buffer gases (e.g. N2),
and a small quantity of vaporized alkali metal (here, Rb); the cell is
irradiated by circularly polarized laser light that can be absorbed by the
alkali metal atoms. (b) The first step of SEOP: in order to conserve angular
momentum, photon absorption results in selective population depletion from one
Rb ground electronic state (neglecting Rb nuclear spin for simplicity). Although
gas-phase collisions work to equalize the excited-state populations (and hence,
the ground-state repopulation rates), continuous depletion of one state by the
laser leaves the AM vapor electronically spin-polarized. (c) The second step of
SEOP: Gas-phase collisions occasionally allow spin order to be transferred from
the AM atom electrons to the noble gas nuclei via Fermi contact hyperfine
interactions, thereby hyperpolarizing the noble gas over time. Reproduced with
permission from Ref. [36] © John Wiley & Sons, Ltd., 2015.