metastable state

in spectrochemistry
https://doi.org/10.1351/goldbook.M03876
Any @E02257@ which in principle, by virtue of the selection rules, cannot radiatively combine with any lower state. It usually has a considerably longer @L03515@ than an ordinary @E02257@.
Source:
PAC, 1985, 57, 1453. (Nomenclature, symbols, units and their usage in spectrochemical analysis - V: Radiation sources (Recommendations 1985)) on page 1463 [Terms] [Paper]