Dominant Secondary Nuclear Photoexcitation with the X-Ray Free-Electron Laser

The new regime of resonant nuclear photoexcitation rendered possible by x-ray free-electron laser beams interacting with solid state targets is investigated theoretically. Our results unexpectedly show that secondary processes coupling nuclei to the atomic shell in the created cold high-density plasma can dominate direct photoexcitation. As an example, we discuss the case of ${}^{93m}\mathrm{Mo}$ isomer depletion for which nuclear excitation by electron capture as a secondary process is shown to be orders of magnitude more efficient than the direct laser-nucleus interaction. General arguments revisiting the role of the x-ray free-electron laser in nuclear experiments involving solid-state targets are further deduced.

Figure 1

${}^{93m}\mathrm{Mo}$ excitation induced by NEEC into the $L$ shell (right) with subsequent decay to the nuclear ground state (long blue solid and dashed arrows in the left panel). The nuclear levels are labeled with their total angular momentum, parity, and energy (in keV).

Figure 2

(a) NEEC resonance cross sections ${\sigma }_{\mathrm{NEEC}}$ for captures into the $L$, $M$, $N$, and $O$ shell (left axis) together with the electronic energy distribution (right axis). (b) Partial NEEC rate ${\lambda }_{\mathrm{NEEC}}^q$ (left axis) together with the corresponding CSD (right axis). Results are presented for two plasma temperatures, 350 eV and 500 eV.