Transition in an atom, ion, or molecule from an electronic energy level
\(E_{1}\) to another energy level
\(E_{2}\).
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 657 (https://doi.org/10.1515/pac-2019-0203)