Nuclear deexcitation via an inelastic electronic bridge

The first experimental observation of nuclear deexcitation via an electronic-bridge mechanism has been reported recently by us. We observed deexcitation of the 30.7-keV ($T_{1/2}$∼13.6 yr) isomeric level of ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ via an inelastic electronic-bridge mechanism involving initial-state L electrons and final-state $N_5$, or higher, electrons. In this work we report a general theoretical analysis of nuclear deexcitation via the inelastic electronic-bridge mechanism. Our estimate of the matrix element is based on solving the inhomogeneous Dirac equation for the external nuclear Coulomb field. Relativistic effects are allowed for and the external field is included in all orders. Specific calculations for ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ agree, within a factor of 4, with the experimental result.