Nuclear Deexcitation via the Electronic-Bridge Mechanism

We report the first experimental observation of nuclear deexcitation via an electronic-bridge mechanism. We have studied the deexcitation of the 30.7-keV ($T_{\frac{1}{2}}\sim 13.6$ yr) isomeric level of ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ with the electronic bridge involving initial-state $L$ electrons and final-state $N$ or higher electrons. The relative probability for the inelastic-electronic-bridge effect deexcitation, as compared to direct $\gamma$-ray decay of the isomeric level, is 0.070±0.018. This result is within an order of magnitude of our theoretical prediction of 0.19.