Forbidden atomic transitions provide some of the most stringent low-energy tests of physics beyond the Standard Model, with sensitivity set by the interplay between the sought-for signals and systematics suppressed by symmetry. Here we identify the previously unobserved $4f^{6}6s^{2}\,{}^{5}$ D $_{0}$ level of neutral samarium at $14\,564.90(2)\,\mathrm{cm}^{-1}$ , opening the ${}^{7}$ F $_{0}\rightarrow{}^{5}$ D $_{0}$ inner-shell transition for precision spectroscopy. Candidate lines extracted from dual-comb absorption spectra were assigned using double-resonance population-depletion and sequential-excitation measurements. The observed pressure broadening, $0.12(2)\,\mathrm{MHz/torr}$ , and pressure shift, $-0.145(4)\,\mathrm{MHz/torr}$ , indicate an inner-shell $4f$ -transition shielded from external perturbations. Many-body calculations predict a metastable lifetime in the range $120$ - $200\,\mathrm{ms}$ , corresponding to a quality factor of order $\mathcal{Q}\sim10^{14}$ , large sensitivity coefficients for variation of the fine-structure constant, and a nuclear-spin-dependent parity-violation amplitude comparable to that of cesium. Crucially, the $J=0\rightarrow J=0$ selection rule suppresses by symmetry both the nuclear-spin-independent parity-violation channel and the M1 and E2 backgrounds that complicated previous heavy-atom experiments, yielding a uniquely clean window onto the nuclear anapole moment. The two stable spin- $7/2$ isotopes of samarium provide a remarkable opportunity to largely cancel atomic-structure uncertainties by measuring the ratio of parity-violation effects in the two isotopes. These results establish neutral samarium as a platform for inner-shell precision spectroscopy and tests of physics beyond the Standard Model.Forbidden atomic transitions provide some of the most stringent low-energy tests of physics beyond the Standard Model, with sensitivity set by the interplay between the sought-for signals and systematics suppressed by symmetry. Here we identify the previously unobserved $4f^{6}6s^{2}\,{}^{5}$D$_{0}$ level of neutral samarium at $14\,564.90(2)\,\mathrm{cm}^{-1}$, opening the ${}^{7}$F$_{0}\rightarrow{}^{5}$D$_{0}$ inner-shell transition for precision spectroscopy. Candidate lines extracted from dual-comb absorption spectra were assigned using double-resonance population-depletion and sequential-excitation measurements. The observed pressure broadening, $0.12(2)\,\mathrm{MHz/torr}$, and pressure shift, $-0.145(4)\,\mathrm{MHz/torr}$, indicate an inner-shell $4f$-transition shielded from external perturbations. Many-body calculations predict a metastable lifetime in the range $120$-$200\,\mathrm{ms}$, corresponding to a quality factor of order $\mathcal{Q}\sim10^{14}$, large sensitivity coefficients for variation of the fine-structure constant, and a nuclear-spin-dependent parity-violation amplitude comparable to that of cesium. Crucially, the $J=0\rightarrow J=0$ selection rule suppresses by symmetry both the nuclear-spin-independent parity-violation channel and the M1 and E2 backgrounds that complicated previous heavy-atom experiments, yielding a uniquely clean window onto the nuclear anapole moment. The two stable spin-$7/2$ isotopes of samarium provide a remarkable opportunity to largely cancel atomic-structure uncertainties by measuring the ratio of parity-violation effects in the two isotopes. These results establish neutral samarium as a platform for inner-shell precision spectroscopy and tests of physics beyond the Standard Model.

physics.atom-phCross-listedarXiv:2605.22318May 22, 2026

The effect of field strength on the resonance structure of three-photon ionization spectra of the samarium atom

Aleksandr I. Gomonai, Eugene Yu. Remeta

Amplitude ratios of resonance maxima caused by two-photon transitions from different levels of the ground term of the samarium atom are studied experimentally and theoretically as functions of the laser-radiation field strength. The results of these studies are shown to be promising for identifying the resonance structure of three-photon ionization spectra of the samarium atom. A not yet observed excited even-parity state with energy E ≈ 36789.1 cm −1 and hypothetical values of total angular momentum J = 5, 6 has been discovered.Amplitude ratios of resonance maxima caused by two-photon transitions from different levels of the ground term of the samarium atom are studied experimentally and theoretically as functions of the laser-radiation field strength. The results of these studies are shown to be promising for identifying the resonance structure of three-photon ionization spectra of the samarium atom. A not yet observed excited even-parity state with energy E ≈ 36789.1 cm −1 and hypothetical values of total angular momentum J = 5, 6 has been discovered.

Optics and SpectroscopyMar 1, 20131 citation

4f^{6}6s^{2}\,{}^{5}

_{0}

level of neutral samarium at

14\,564.90(2)\,\mathrm{cm}^{-1}

, opening the

{}^{7}

_{0}\rightarrow{}^{5}

_{0}

inner-shell transition for precision spectroscopy. Candidate lines extracted from dual-comb absorption spectra were assigned using double-resonance population-depletion and sequential-excitation measurements. The observed pressure broadening,

0.12(2)\,\mathrm{MHz/torr}

, and pressure shift,

-0.145(4)\,\mathrm{MHz/torr}

, indicate an inner-shell

4f

-transition shielded from external perturbations. Many-body calculations predict a metastable lifetime in the range

120

200\,\mathrm{ms}

, corresponding to a quality factor of order

\mathcal{Q}\sim10^{14}

, large sensitivity coefficients for variation of the fine-structure constant, and a nuclear-spin-dependent parity-violation amplitude comparable to that of cesium. Crucially, the

J=0\rightarrow J=0

selection rule suppresses by symmetry both the nuclear-spin-independent parity-violation channel and the M1 and E2 backgrounds that complicated previous heavy-atom experiments, yielding a uniquely clean window onto the nuclear anapole moment. The two stable spin-

7/2

isotopes of samarium provide a remarkable opportunity to largely cancel atomic-structure uncertainties by measuring the ratio of parity-violation effects in the two isotopes. These results establish neutral samarium as a platform for inner-shell precision spectroscopy and tests of physics beyond the Standard Model.