Electronic Bridge Excitation in Highly Charged ^{229}Th Ions

Deep-Learning Approach for the Atomic Configuration Interaction Problem on Large Basis Sets

High-precision atomic structure calculations require accurate modeling of electronic correlations typically addressed via the configuration interaction (CI) problem on a multiconfiguration wave function expansion. The latter can easily become challenging or infeasibly large even for advanced supercomputers. Here, we develop a deep-learning approach which allows us to preselect the most relevant configurations out of large CI basis sets until the targeted energy precision is achieved. The large CI computation is thereby replaced by a series of smaller ones performed on an iteratively expanding basis subset managed by a neural network. While dense architectures as used in quantum chemistry fail, we show that a convolutional neural network naturally accounts for the physical structure of the basis set and allows for robust and accurate CI calculations. The method was benchmarked on basis sets of moderate size allowing for the direct CI calculation, and further demonstrated on prohibitively large sets where the direct computation is not possible.

Isomeric Excitation of ^{229}Th in Laser-Heated Clusters

We consider theoretically isomeric excitation of ^{229}Th in a laser-heated cluster. A ^{229}Th cluster is first radiated by an intense femtosecond laser pulse, causing ionization of the constituting atoms. The cluster will then survive for a time on the order of 1 ps, during which the electrons collide with the nuclei repeatedly and excite them to the isomeric state. Two mechanisms are responsible for the isomeric excitation: nuclear excitation by electron capture (NEEC) and nuclear excitation by inelastic electron scattering (NEIES). By changing the laser intensity, one can tune between NEEC and NEIES continuously. This laser-heated-cluster scheme not only provides a highly efficient means for isomeric excitation, but also provides an approach for the confirmation of the NEEC process, which has been predicted for over forty years without conclusive experimental verifications.

Tunable VUV frequency comb for 229mTh nuclear spectroscopy

Laser spectroscopy of the ^229mTh nuclear clock transition is necessary for the future construction of a nuclear-based optical clock. Precision laser sources with broad spectral coverage in the vacuum ultraviolet are needed for this task. Here, we present a tunable vacuum-ultraviolet frequency comb based on cavity-enhanced seventh-harmonic generation. Its tunable spectrum covers the current uncertainty range of the ^229mTh nuclear clock transition.

Exciting the Isomeric ^{229}Th Nuclear State via Laser-Driven Electron Recollision

We propose a new approach to excite the isomeric ^{229}Th nuclear state, which has attracted much attention recently as a potential "nuclear clock." Our approach is based on a laser-driven electron recollision process, the core process of strong-field atomic physics. Bringing together knowledge of recollision physics and of the related nuclear physics, we calculate the isomeric excitation probability. This new approach does not require precise knowledge of the energy of the isomeric state. The excitation is well timed which may be exploited to control the coherence of the isomeric state. Experimental realization is within reach using tabletop laser systems.

Nuclear Excitation of the ^{229}Th Isomer via Defect States in Doped Crystals

When Th nuclei are doped in CaF_{2} crystals, a set of electronic defect states appear in the crystal band gap which would otherwise provide complete transparency to vacuum-ultraviolet radiation. The coupling of these defect states to the 8 eV ^{229m}Th nuclear isomer in the CaF_{2} crystal is investigated theoretically. We show that although previously viewed as a nuisance, the defect states provide a starting point for nuclear excitation via electronic bridge mechanisms involving stimulated emission or absorption using an optical laser. The rates of these processes are at least 2 orders of magnitude larger than direct photoexcitation of the isomeric state using available light sources. The nuclear isomer population can also undergo quenching when triggered by the reverse mechanism, leading to a fast and controlled decay via the electronic shell. These findings are relevant for a possible solid-state nuclear clock based on the ^{229m}Th isomeric transition.