When an atom or molecule with an unpaired electron is placed in a magnetic field, the spin of the unpaired electron can align either in the same direction or in the opposite direction as the field. These two electron alignments have different energies and application of a magnetic field to an unpaired electron lifts the degeneracy of the ±1/2 spins of the electron.
Electron-paramagnetic-resonance (EPR) or electron-spin-resonance (ESR) spectroscopy measures the absorption of microwave radiation by an unpaired electron when it is placed in a strong magnetic field.
Species that contain unpaired electrons:
A klystron tube generates monochromatic microwave radiation (~9500 MHz) in an EPR instrument. The microwave radiation travels down a waveguide to the sample which is held between magnets.
Spectra are obtained by measuring the absorption of the microwave radiation while scanning the magnetic-field strength. EPR spectra are usually displayed in derivative form to improve the signal-to-noise ratio.