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We report on spectroscopic measurements on the 4f⁷6s²⁸S_7/2^∘−4f⁷(⁸S^∘)6s6p(¹P^∘)⁸P_5/2,7/2transitions at 466.32 nm and 462.85 nm, respectively, in neutral europium-151 and europium-153. The center of gravity frequencies for the 151 and 153 isotopes for both transitions are reported for the first time using saturated absorption spectroscopy. For the 6s6p(¹P^∘)⁸P_5/2state, the center of gravity frequencies were found to be 642,894,493.3(4) MHz and 642,891,693.3(9) MHz for the 151 and 153 isotopes, respectively. The hyperfine constants for the upper state were found to beA(151)=−157.01(3)MHz,B(151)=74.5(4)MHz andA(153)=−69.43(14)MHz,B(153)=191.0(26)MHz. These hyperfine values are all consistent with previously published results except forB(151) that has a small discrepancy. The isotope shift was found to be 2799.54(20) MHz, a small discrepancy with previously published results. For the 6s6p(¹P^∘)⁸P_7/2state, the center of gravity frequencies were found to be 647,708,930.6(6) MHz and 647,705,958.4(26) MHz for the 151 and 153 isotopes, respectively. The hyperfine constants for the upper state were found to beA(151)=−218.66(4)MHz,B(151)=−293.4(8)MHz andA(153)=−97.15(13)MHz,B(153)=−750(3)MHz. These values are all consistent with previously published results except forA(151) that has a small discrepancy. The isotope shift was found to be 2972.8(5) MHz, a small discrepancy with previously measured results. 

We report absolute frequency measurements on the three excited-state transitions connecting the 2p2 (3P)3s 2P1/2,3/2 and 2p2 (3P)3p 2D◦ 3/2,5/2 states in neutral 14N using saturated absorption spectroscopy in a glass cell filled with a mixture of nitrogen and argon gas and driven with a radio-frequency electric field. The absolute transition frequencies are found to be 319 288 834(150), 319 085 445(125), and 316 795 808(140) MHz for the 1/2–3/2, 3/2–5/2, and 3/2–3/2 transitions, respectively. This work represents the saturated absorption measurements for these transitions. These results are approximately a factor of 5 improvement over previous results, despite being conservative estimates. We found that the overlapping hyperfine structure prevented fitting individual spectral features, presenting difficulties for possible future tests of quantum electrodynamics in seven-electron systems. A better understanding of the amplitude behavior of a merged hyperfine structure will allow the extrapolation of the center of gravity frequencies for these transitions to sub-MHz precision using our collected data. 

We report on spectroscopic measurements on the 4 f 7 6 s 2 8 S 7 / 2 ∘ → 4 f 7 ( 8 S ∘ ) 6 s 6 p ( 1 P ∘ ) 8 P 9 / 2 transition in neutral europium-151 and europium-153 at 459.4 nm. The center of gravity frequencies for the 151 and 153 isotopes, reported for the first time in this paper, to our knowledge, were found to be 652,389,757.16(34) MHz and 652,386,593.2(5) MHz, respectively. The hyperfine coefficients for the 6 s 6 p ( 1 P ∘ ) 8 P 9 / 2 state were found to be A ( 151 ) = − 228.84 ( 2 ) M H z , B ( 151 ) = 226.9 ( 5 ) M H z and A ( 153 ) = − 101.87 ( 6 ) M H z , B ( 153 ) = 575.4 ( 1.5 ) M H z , which all agree with previously published results except for A(153), which shows a small discrepancy. The isotope shift is found to be 3163.8(6) MHz, which also has a discrepancy with previously published results.