ESR, Electron Spin Resonance measures number of charges occupying deep traps in crystal band gap. Electron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR) is a spectroscopic technique which detects species that have unpaired electrons, generally meaning that the molecule in question is a free radical if it is an organic molecule, or that it has transition metal ions if it is an inorganic complex. Because most stable molecules have a closed-shell configuration without a suitable unpaired spin, the technique is less widely used than nuclear magnetic resonance (NMR). The EPR was first discovered in Kazan State University by Soviet physicist Yevgeniy Zavoyskiy in 1944