The isomeric shift provides important information about the nuclear structure and the physical, chemical or biological environment of atoms.
More recently the effect has also been proposed as a tool in the search for the time variation of fundamental constants of nature.
The effect was predicted by Richard M. Weiner[2] in 1956, whose calculations showed that it should be measurable by atomic (optical) spectroscopy (see also[3]).
While in the case of the isotopic shift the determination of the interaction energy between electrons and nuclei is a relatively simple electromagnetic problem, for isomers the problem is more involved, since it is the strong interaction, which accounts for the isomeric excitation of the nucleus and thus for the difference of charge distributions of the two isomeric states.
As to the experimental observation of this shift, it also had to await the development of a new technique, that permitted spectroscopy with isomers, which are metastable nuclei.
Moreover, combined with the Mössbauer effect, the isomeric shift constitutes at present a unique tool in many other fields besides physics.
[2] Thus the results[4] could be explained[8] within the theory[2] by associating with the odd optical neutron an effective electric charge of Z/A.
This combined shift is the Mössbauer isomeric shift, and it is described mathematically by the same formalism as the nuclear isomeric shift on atomic spectral lines, except that instead of one electron wave function, that in the source ψs, one deals with the difference between the electron wave function in the source ψs and the electron wave function in the absorber ψa: The first measurement of the isomeric shift in gamma spectroscopy with the help of the Mössbauer effect was reported[9] in 1960, two years after its first experimental observation in atomic spectroscopy.
[4] By measuring this shift, one obtains important and extremely precise information, both about the nuclear isomer states and about the physical, chemical or biological environment of the atoms, represented by the electronic wave functions.