How do we measure Rb Zeeman resonance frequency?
To detect the EPR resonance, we rely on the fact that during optical pumping the polarization of Rb vapor is very high. Most of the Rb atoms are in the F=3, M=-3 state (or M=+3 state for oppositely polarized light). Laser light can penetrate quite far into the cell because most Rb atoms are in the state which cannot absorb circularly polarized photons. Among the atoms that do absorb photons and are excited to P state, most of them will be radiationlessly quenched to ground state by Nitrogen filled into the cell. A small fraction (3~5%) will decay by emitting fluorescence photons at either D1 (wavelength 795 nm) or D2 (wavelength 785 nm) line. Lasers scattered by the cell are tuned to D1 transition frequency, so the D1 light which one could measure is dominated by the scattered laser instead of fluorescence. Only D2 light is directly proportional to fluorescence and is what should be measured during EPR measurement. The intensity of fluorescence is proportional to the rate of photon abs
