Isomeric shift

The isomeric shift (also called isomer shift) is the shift on atomic spectral lines and gamma spectral lines, which occurs as a consequence of replacement of one nuclear isomer by another. It is usually called isomeric shift on atomic spectral lines and Mössbauer isomeric shift respectively. If the spectra have also hyperfine structure the shift refers to the center of gravity of the spectra. 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 isomeric shift on atomic spectral lines is the energy or frequency shift in atomic spectra, which occurs when one replaces one nuclear isomer by another. The effect was predicted by Richard M. Weiner in 1956 whose calculations showed that it should be measurable by atomic (optical) spectroscopy (cf. also). It was observed experimentally for the first time in 1958. The theory of the atomic isomeric shift developed in is also used in the interpretation of the Mössbauer isomeric shift.

The notion of isomer appears also in other fields such as chemistry and meteorology. Therefore, in the first papers devoted to this effect the name nuclear isomeric shift on spectral lines was used. Before the discovery of the Mössbauer effect, the isomeric shift referred exclusively to atomic spectra; this explains the absence of the word atomic in the initial definition of the effect. Subsequently, the isomeric shift was also observed in gamma spectroscopy through the Mössbauer effect and was called Mössbauer isomeric shift. For further details on the history of the isomeric shift and the terminology used cf. Refs.,

Atomic spectral lines are due to transitions of electrons between different atomic energy levels E, followed by emission of photons. Atomic levels are a manifestation of the electromagnetic interaction between electrons and nuclei. The energy levels of two atoms the nuclei of which are different isotopes of the same element are shifted one with respect to the other, despite the fact that the electric charges Z of the two isotopes are identical. This is so because isotopes differ by the number of neutrons and therefore the masses and volumes of two isotopes are different; these differences give rise to the isotopic shift on atomic spectral lines.

...
Wikipedia