Recently, the 1.4 feV ultranarrow nuclear transition at 12.4 keV energy in 45Sc was resonantly excited for the first time using radiation from the self-seeded EuXFEL laser [Y. Shvyd’ko et al., Resonant x-ray excitation of the nuclear clock isomer 45Sc, Nature (London) 622, 471 (2023)], establishing 45Sc as a promising candidate for a future Mössbauer nuclear clock. While this experiment demonstrated a high potential of x-ray free-electron laser sources for resonantly exciting nuclear isomers in the hard x-ray range, it also highlighted a severe limitation in the achievable excitation level caused by their extremely large spectral bandwidth ∼1  eV. In this Letter, we propose a method to enhance the spectral flux of x-ray free-electron laser radiation using a resonant absorber with a longitudinal gradient in the nuclear transition frequency. A portion of the incident pulse can be absorbed into a nuclear collective excitation, and converted back into x-ray radiation by reversing the sign of the frequency gradient. Spectral narrowing and flux enhancement of this re-emitted x-ray field is achieved by using a reversed frequency gradient with a smaller magnitude than the initial one. About a hundredfold of such spectral flux enhancement is feasible in 45Sc2⁢O3 single crystal, rendering a more efficient source for nuclear excitation and facilitating the experimental observation of resonant fluorescence and coherent forward scattering at the 12.4 keV transition, both of which are essential for realizing a nuclear clock.

中文翻译：

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用于解决超窄核跃迁的 X 射线光谱通量增强


最近，使用自种子 EuXFEL 激光器的辐射首次共振激发了 45Sc 中 12.4 keV 能量的 1.4 feV 超窄核跃迁 [Y. Shvyd'ko et al.， Resonant X-ray excitation of the nuclear clock isomer 45Sc， Nature （London） 622， 471 （2023）]，确立了 45Sc 作为未来 Mössbauer 核钟的有前途的候选者。虽然该实验证明了 X 射线自由电子激光源在硬 X 射线范围内对共振激发核异构体的巨大潜力，但它也强调了由于它们的极大光谱带宽 ∼1eV 导致可实现的激发水平的严重限制。在这封信中，我们提出了一种使用具有核跃迁频率纵向梯度的谐振吸收器来增强 X 射线自由电子激光辐射的光谱通量的方法。一部分入射脉冲可以被吸收到核集体激发中，并通过反转频率梯度的符号转换回 X 射线辐射。这个重新发射的 X 射线场的光谱变窄和通量增强是通过使用比初始幅度更小的反向频率梯度来实现的。在 45Sc2O3 单晶中，大约一百倍的这种光谱通量增强是可行的，为核激发提供了更有效的源，并促进了 12.4 keV 跃迁处共振荧光和相干前向散射的实验观察，这两者都是实现核钟所必需的。