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discussed in the following articles:   • major reference
reaction mechanism: Kinetic isotope effects   • atomic spectra
spectroscopy: Perturbations of levels     ...of nuclei and the
distribution of the electron clouds near nuclei. Systematic
changes in level positions are seen as the number of neutrons in
a nucleus is increased. These effects are known as isotope
shifts and form the basis for laser isotope separation. For
light atoms, the isotope shift is primarily due to differences
in the finite mass of the nucleus. For heavier atoms, the
main...   • description and properties      isotope: Physical
properties associated with isotopes   • hydrogen isotopes and
reaction rate      hydrogen (H): Isotopes of hydrogen   •
hyperfine structure      hyperfine structure (HFS)     A similar
effect of line splitting is caused by mass differences
(isotopes) of atoms in an element and is called isotope
structure, or isotope shift. These spectral lines