Set of one or more stationary quantum states of a free atom, ion, or molecule having a particular internal energy.
Note:
SI unit: \(\pu{J}\). Common unit: electron volt, symbol \(\pu{eV}\), where \(\pu{1 eV = 1.602176634E-19 J}\). Energy levels are also expressed as \(\pu{kJ mol-1}\).
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 658 (https://doi.org/10.1515/pac-2019-0203)