resonance fluorescence

Fluorescence from the primary excited atomic or molecular species at the wavelength of the exciting radiation (no relaxation within the excited manifold). This term is also used to designate the radiation emitted by an atom of the same wavelength as the longest one capable of exciting its fluorescence, e.g. 122.6 nm in the case of the hydrogen atom, and 253.7 nm in the case of the mercury atom.
See also: resonance line in photochemistry
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2271