Resonance nuclear excitation of the $^{229}$Th nucleus via electronic bridge process in Th~II

Abstract: The 8.4 eV transition in the $^{229}$Th nucleus is the basis for a high-precision nuclear clock with exceptional sensitivity to new physics effects. We have identified several cases in the Th$^+$ ion where electronic excitations closely resonate with the nuclear excitation, with the smallest energy difference being $\Delta = -0.09$ cm$^{-1}$. We investigate the electronic bridge process, in which nuclear excitation is induced via electronic transitions, and demonstrate that a proper selection of laser frequencies can lead to a dramatic enhancement of this effect. Additionally, we show that the interaction with electrons significantly shortens the lifetime of the nuclear excited state.