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OPTICAL PUMPING

Spin Exchange Optical Pumping (SEOP)

 

Optical Pumping Rb

 

Using the spin exchange optical pumping (SEOP) method, polarization of the 3He gas involves three steps. Enclosed inside the glass cell is ~100 mg of Rubidium (Rb) and Potassium (K). The cell can be pressurized up to 3 atm depending on the instrument usage. Most of the gas is 3He while only a small volume (0.06-0.13 atm) is N2. A uniform magnetic field is maintained around the cell in order to sustain the polarization.

The first step is the polarization of Rubidium (Rb) in the vapor phase. A simple electron state diagram for the unpaired Rb valence electron is shown above. Using a high power infrared diode laser array (\(\lambda\) = 795 nm, specific to the Rb spectrum), angular momentum from right circularly polarized light is passed from photons to Rb valence electrons. Photons, which have a spin magnetic moment ms = +1, are absorbed by the Rb atoms. Under conservation of angular momentum, excited electrons follow the selection rule \(\Delta\)mj= +1. The only allowed transition is from an mj = -½ state to an mj = +½ state, since electrons are spin-½ particles. The electrons are excited from the ground state 5s½, mj = -½ orbital, to the excited state 5p½, mj = +½. The excited electrons are evenly distributed between the spin and states of the 5p orbital through collisional mixing. From the excited state, electrons radiatively decay back to the 5s½ orbital, a process known as collisional de-excitation, with half decaying to the mj = +½ state and half decaying to the mj = -½ state. Electrons in the mj = +½ state remain in that state for two reasons. The selection rules prevent another transition where \(\Delta\)mj= +1. Additionally, the N2 gas prohibits radiative photon emissions with mj= −1 from exciting electrons in the mj = +½ ground state to the mj = -½ excited state. N2 has a large quenching absorption cross section with the ability to transfer the energy emitted from Rb into its own vibrational and rotational motion. Instead, the laser light re-excites electrons which decay to the mj = -½ ground state. This process, known as depopulation pumping, removes electrons from the mj = -½ state in order to fill the mj = +½ state, polarizing the Rb. The second step is the polarization of the Potassium (K). This process occurs through spin-exchange collisions of Rb atoms with K atoms. In the case of K, valence electrons are excited from the ground state 4s½, mj = -½ orbital, to the excited state 4p½, mj = +½. This interaction transfers the Rb polarization to the K.

Polarization Transfer to 3He
Schematic by Earl D. Babcock

The final step is the polarization of the 3He nucleus by both K and Rb through hyperfine interaction. Though, both Rb and K atoms collide with the 3He atoms, the spin-exchange process is more efficient for K-3He collision than for Rb-3He collisions (schematic above). For the spin-exchange to occur, unpaired valence electrons must penetrate the 3He electron cloud and collide with the nucleus. Over time, the 3He gas becomes polarized. Due to low probability of spin-exchange, the 3He polarization process is very slow. The full polarization time or "pump-up time" can be on the order of 1-2 days. The pump-up time is determined by a number of factors and varies from cell to cell. Although, the SEOP process is slow, it is possible to polarize cells with high pressures (1-10 atm) as well as low.

Created May 29, 2018, Updated November 15, 2019