Second-order processes in physics is a research topic focusing attention from several fields worldwide including, for example, non-linear quantum electrodynamics with high-power lasers, neutrinoless double-β decay, and stimulated atomic two-photon transitions. For the electromagnetic nuclear interaction, the observation of the competitive double-γ decay from ^137mBa has opened up the nuclear structure field for detailed investigation of second-order processes through the manifestation of off-diagonal nuclear polarisability. Here, we confirm this observation with an 8.7σ significance, and an improved value on the double-photon versus single-photon branching ratio as 2.62 × 10⁻⁶(30). Our results, however, contradict the conclusions from the original experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small quadrupole-quadrupole component in the decay, hindered due to an evolution of the internal nuclear structure. The implied strongly hindered double-photon branching in ^137mBa opens up the possibility of the double-photon branching as a feasible tool for nuclear-structure studies on off-diagonal polarisability in nuclei where this hindrance is not present.

物理学中的二阶过程是一个研究主题，引起了全球多个领域的关注，例如，高功率激光的非线性量子电动力学，无中微双β衰变和受激原子双光子跃迁。对于电磁核相互作用，对^137m Ba的竞争性双γ衰变的观察已经打开了核结构领域，可通过对角核极化率的表现来详细研究二阶过程。在这里，我们以8.7σ的显着性确认了这一观察结果，双光子与单光子支化比的改进值为2.62×10 ^-6（30）。但是，我们的结果与原始实验的结论相矛盾，原始实验的衰减被解释为由四极-四极分量主导。在这里，我们发现能量分布得到了显着增强，这与主要的八极-偶极子特征和衰减中相当小的四极-四极子分量一致，这是由于内部核结构的演化而受到阻碍的。隐含在^137m Ba中的强受阻双光子分支开辟了将双光子分支作为研究不存在该障碍的核中非对角极化率的核结构的可行工具的可能性。