resonance line

in photochemistry
https://doi.org/10.1351/goldbook.R05340
The longest @W06659@ capable of exciting @F02453@ in an atom.
See also:
resonance fluorescence
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2271 [Terms] [Paper]