Spectroscopic method based on the Zeeman effect. It studies paramagnetic materials and substances with unpaired electrons (e.g. radicals or transition metal complexes). A magnetic field is applied to the probe which results in a splitting of the spin energy states of the unpaired electrons. The splitting is proportional to the strength of the applied magnetic field:
ΔΕ = ms ge μB B0 = h ν
(with ms = magnetic spin quantum number of the electron ±½, ge = g-factor or Landé factor of the electron, μB = Bohr magneton, B0 = magnetic field, h = Planck constant, ν = resonance frequency)
Irradiation of microwaves with the same energy(resonance frequency) results in absorption. Normally, the first derivative of the absorption with respect to the change of the magnetic field is measured.