In spectroscopy, the Rydberg constant is a physical constant relating to the electromagnetic spectra of an atom. Its symbol is for heavy atoms or for hydrogen. The constant is named after the Swedish physicist Johannes Rydberg. The constant first arose as an empirical fitting parameter in the Rydberg formula for the hydrogen spectral series. Niels Bohr later showed that its value could be calculated from more fundamental constants via his Bohr model. As of 2018[update], and electron spin g-factor are the most accurately measured physical constants.
The constant is expressed for either hydrogen as , or at the limit of infinite nuclear mass as . In either case, the constant is used to express the limiting value of the highest wavenumber (inverse wavelength) of any photon that can be emitted from an atom, or, alternatively, the wavenumber of the lowest-energy photon capable of ionizing an atom from its ground state. The hydrogen spectral series can be expressed simply in terms of the Rydberg constant for hydrogen and the Rydberg formula.
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