where γ γ e and γ γ n are the gyromagnetic ratio of the electron and the nucleus respectively. B 1 is the magnetic field of the observer which is microwave radiation while B 2 is the magnetic field of the pump which is radio frequency radiation. T 1 e and T 1 n are the spin-lattice relaxation time for the electron and the nucleus respectively. T 2 e and T 2 n are the spin-spin relaxation time for the electron and the nucleus respectively.