Electron Spin Resonance (ESR) is a branch of absorption spectra in which radiation having frequency in microwave region is absorbed by paramagnetic substances to induce transitions between magnetic energy levels of electrons with unpaired spins.

ESR also called Electronic Paramagnetic Resonance is a spectroscopic technique confined to study of those species having one or more unpaired electrons.

Phenomenon of ESR was invented by Zaveiskii in 1904.


When we consider an unpaired electron it is associated with spin. When a magnetic field is applied, the magnetic moment of electron interacts with field and results in splitting of otherwise degenerate field. Now the energy difference between the levels falls in microwave region.So when radiation in microwave range equal to this energy difference is made to incident on substance, transitions occur between these levels absorbing quanta of energy leading to a absorption peak.

Only spin moment contributes towards the magnetic behaviour of electrons.

Consider that system has only spin magnetic moment

μ= -gμBS ....................................(1)

'μ' & 's' are in opposite directions.

For an electron of spin S=1/2, the spin angular momentum quantum number will have values
ms = ±1/2 ..................................(2)

In absence of magnetic field, the two values of 'ms' i.e. +1/2 and -1/2 will give rise to a doubly degenerate spin energy state.

When magnetic field is applied this degeneracy is removed and thus leads to two non degenerate energy levels.

Now the interaction energy is given by

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