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Direct laser excitation of the lowest known nuclear excited state in 229Th has been a long-standing objective. It is generally assumed that reaching this goal would require a considerably reduced uncertainty of the isomer's excitation energy compared to the presently adopted value of (7.8±0.5) eV. Here we present a direct laser excitation scheme for 229mTh, which circumvents this requirement. The proposed excitation scheme makes use of already existing laser technology and therefore paves the way for nuclear laser spectroscopy. In this concept, the recently experimentally observed internal-conversion decay channel of the isomeric state is used for probing the isomeric population. A signal-to-background ratio of better than 104 and a total measurement time of less than three days for laser scanning appear to be achievable. [ABSTRACT FROM AUTHOR]

The first excited isomeric state of 229Th possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today's laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground state (internal conversion, γ decay, and bound internal conversion), whose strengths depend on the charge state of 229mTh. We report on the measurement of the internal-conversion decay half-life of neutral 229mTh. A half-life of 7±1 μs has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of ≈104 s of the photonic decay channel, gives further support for an internal conversion coefficient of ≈109, thus constraining the strength of a radiative branch in the presence of internal conversion. [ABSTRACT FROM AUTHOR]

The excitation of the 8 eV 229mTh isomer through the electronic bridge mechanism in highly charged ions is investigated theoretically. By exploiting the rich level scheme of open 4f orbitals and the robustness of highly charged ions against photoionization, a pulsed high-intensity optical laser can be used to efficiently drive the nuclear transition by coupling it to the electronic shell. We show how to implement a promising electronic bridge scheme in an electron beam ion trap starting from a metastable electronic state. This setup would avoid the need for a tunable vacuum ultraviolet laser. Based on our theoretical predictions, determining the isomer energy with an uncertainty of 10-5 eV could be achieved in one day of measurement time using realistic laser parameters. [ABSTRACT FROM AUTHOR]