Spin Exchange

The spin-exchange apparatus is quite simple: a cell filled with several 100 kPA (several bar) of ³He gas (indicated by the spectrum), 10 kPa (100 mbar) of nitrogen gas, and a fraction of a gram of Rb metal are placed in the magnetic field generated by a pair of Helmholtz coils. An oven (not shown) heats the cell to drive some of the rubidium into the vapor phase. 795 nm circularly polarized laser light incident on the cell optically pumps the Rb into one of its magnetic sub-states, as shown below. A "bare" Rb atom would require an average of three photons to become fully polarized. Because of collisional mixing between the two excited states, however, only two photons are required. A small amount of nitrogen in the cell prevents radiative decay of the upper state, another source of polarization loss.

During binary collisions with ³He atoms, the spin stored in the Rb electrons is transferred to the ³He nucleus via the hyperfine interaction.

Inquiries or comments: david.gilliam@nist.gov
Online: November 1998