Relaxation to the m s = 0 state necessarily results in a decrease in visible wavelength fluorescence (as the emitted photon is in the infrared range). Microwave pumping at a resonant frequency of ν ν = 2.87 G H z places the centre in the degenerate m s = ± ± 1 state. The application of a magnetic field lifts this degeneracy, causing Zeeman splitting and the decrease of fluorescence at two resonant frequencies, given by h ν ν = g e μ μ B B 0 , where h is the Planck constant, g e is the electron g-factor and μ μ B is the Bohr magneton. Sweeping the microwave field through these frequencies results in two characteristic dips in the observed fluorescence, the separation between which enables determination of the strength of the magnetic field B 0 .