The standard electron spin resonance, also known as electron paramagnetic resonance (EPR), is due to the coupling of electron magnetic moment to the external magnetic field B through the Hamiltonian H = − − μ μ ⋅ ⋅ B describing its Larmor precession. The magnetic moment is related to electron angular momentum S as μ μ = − − g μ μ B S / ℏ ℏ , where g is the g-factor and ℏ ℏ is the reduced Planck constant. For a free electron in vacuum g ≈ ≈ 2 . As the electron is a spin-1/2 particle, the spin operator can take only two values: S = ± ± ℏ ℏ / 2 . So, Larmor interaction has quantized energy levels in a time-independent magnetic field as the energy is equal to ± ± 1 2 g μ μ B B . In the same way, under a resonant AC magnetic field B ~ ~ ( t ) at the frequency ω ω S = g μ μ B B / ℏ ℏ , results in electron paramagnetic resonance, that is, the signal gets absorbed strongly at this frequency as it produces transitions between spin values.