铀-235同位素在尿素形式下的化学效应
Chemical effects on nuclear decay of $^{235}$U isomer in the uranyl form
AI总结：
研究发现尿素形式下铀-235同位素的半衰期受配体电负性影响，首次揭示分子轨道变化对核衰变的影响。
摘要：
铀-235（²³⁵U）的核具有极低能级的同位素态²³⁵mU。与大多数放射性核素不同，²³⁵mU的半衰期会随其化学环境变化而变化，这是因为内部转换（IC）过程中的外层电子相互作用。然而，这种半衰期变化的机制，特别是分子键合中除简单电子密度效应外的作用，仍不清楚。本文研究了不同卤素配体的尿素（UO₂²+）化合物中²³⁵mU半衰期的变化以及相应的IC-电子能谱。测量到尿素氟化物、氯化物、溴化物和碘化物的半衰期分别为25.32(4)、26.05(8)、25.84(3)和25.44(3)分钟，表明半衰期随配体电负性的增加而增加，除了尿素氟化物。尿素氟化物最短的半衰期归因于占据键轨道的6p电子最少，这由IC-电子能谱和量子化学计算所指示。这项工作首次观察到由分子轨道形成变化驱动的核衰变过程的显著变化，为深入理解核与化学键中电子相互作用提供了途径。
英文摘要：
The nucleus of uranium-235 ($^{235}$U) possesses an exceptionally low-energy isomeric state, $^{235m}$U. Unlike most radioactive nuclides, whose nuclear-decay half-lives are constant, the half-life of $^{235m}$U varies with its chemical environment$^{1,2}$ owing to interactions with outer-shell electrons in the internal-conversion (IC) process. However, the mechanism underlying this half-life variation, particularly the role of molecular bonding beyond simple electron-density effects$^{1,2}$, remains unresolved. Here, we investigate variations in the half-lives of $^{235m}$U and the corresponding IC-electron energy spectra for uranyl (UO2$^{2+}$) compounds with different halide ligands. The half-lives of $^{235m}$U are measured to be 25.32(4), 26.05(8), 25.84(3), and 25.44(3) min for uranyl fluoride, chloride, bromide, and iodide, respectively, indicating that the half-life increases with increasing ligand electronegativity, with the exception of uranyl fluoride. The shortest half-life observed for uranyl fluoride is attributed to the smallest number of 6p electrons occupying bonding orbitals, as indicated by the IC-electron energy spectra and quantum chemical calculations. This work provides the first observation of a significant variation in a nuclear decay process driven by changes in molecular orbital formation, paving the way toward a deeper understanding of interactions between a nucleus and electrons involved in chemical bonding.
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