Electron Paramagnetic Resonance, or EPR, deals with the resonant absorption of microwave radiation by a paramagnetic substance within a static magnetic field. Molecules have electrons moving around them in some kind of orbit, where each orbit is characterised by a stransition level energy, (ie. the energy an electorn requires to be in that orbit), and a wavefunction describing the shape of the orbit. These orbits, and hence the transition level energy and the wavefunction are affected by the strength of the applied magnetic field. Microwave radiation will be absorbed only if the energy of the radiation exactly matches the difference between the two energy level. The energy of microwave radiation is related to its frequency by Planck's Law, E chemical bounds H f (where H is Planck's constant).
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