isomer shift in Mössbauer spectroscopy

Measure of the energy difference between the source ( $E s$ ) and the absorber( $E a$ ) transition. The measured Doppler velocity shift, $δ$ , is related to the energy difference by

$E a − E s = δ E γ c$

where $E γ$ is the Mössbauer gamma energy and $c$ is the speed of light in vacuum.

Source:

PAC, 1976, 45, 211 (Nomenclature and Conventions for Reporting Mossbauer Spectroscopic Data) on page 214