( ( During excitation ) ( , ( electrons ( ( ( ( emit ( light ( as ( they ( ( fall back ) ( into ( lower ( energy levels ) ) ) ) ) ) ) ) , ) ( producing ( the ( emission spectrum ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN During) (NP (NN excitation))) (, ,) (NP (NNS electrons)) (VP (VBP emit) (ADJP (JJ light) (SBAR (IN as) (S (NP (PRP they)) (VP (VBP fall) (ADVP (RB back)) (PP (IN into) (NP (JJR lower) (NN energy) (NNS levels))))))) (, ,) (S (VP (VBG producing) (NP (DT the) (NN emission) (NN spectrum))))) (. .)))
During excitation, electrons emit light as they fall back into lower energy levels, producing the emission spectrum.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NNS electrons)) (VP (VB return) (PP (TO to) (NP (DT a) (JJR lower) (NN energy) (NN level)))))) (, ,) (NP (PRP they)) (VP (VBP emit) (NP (NP (DT the) (JJ excess) (NN energy)) (PP (IN in) (NP (NP (DT the) (NN form)) (PP (IN of) (NP (NN light))))))) (. .)))