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Probes: (a) Energy levels in fluorescence probes, (b) A: EPR: Spin up and spin down (Ms = ± 1/2) energy levels in a magnetic field B0, which become more widely separated the stronger the field, and a microwave source on the right that can flip the spins. Inset (B), the microwave absorption spectrum (top) from the spin flip in A and its first derivative (below). For details see text.
