atomic spectrum vs fine structure

atomic spectrum and fine structure both are nouns.

Word	Noun	Adjective	Verb	Adverb
atomic spectrum	Yes	No	No	No
fine structure	Yes	No	No	No

atomic spectrum (noun)	fine structure (noun)
(physics) a spectrum of radiation caused by electron transitions within an atom; the series of spectrum lines is characteristic of the element	the presence of groups of closely spaced spectrum lines observed in the atomic spectrum of certain elements