Two-Photon Resonance Mechanism of Optical Pumping of the 8.3-eV $${}^{\boldsymbol{229m}}$$Th Isomer in Neutral Atoms

The possibility of refinement upon the energy of the 8.36-eV $${}^{229m}$$ Th nuclear isomer, which is the most probable candidate for a nuclear reference value for frequency, by means of resonance optical pumping is discussed. Attention is given primarily to the problem of broadening the resonance with the aim of reducing the time of scanning. The proposed two-photon method uses a sharp broadening of the isomer line owing to mixing with an electron transition. In contrast to internal-conversion-induced resonance broadening or a deliberate extra-broadening of the pumping-laser spectral line, this method is not adversely affected by the reduction of cross section. In the case being considered, it is two orders of magnitude more efficient and is applicable both to ionized and to neutral thorium atoms. The implementation of the method entails the excitation of both the nucleus involved and the electron shell in the final state.