Isomer production by multiphoton excitation

The multiphoton excitation to the 8-eV nuclear isomeric state $^{229\text{m}}\mathrm{Th}$ in the direct laser-nucleus interaction is investigated theoretically. We solve the time-dependent Schr"odinger equation with a method that allows us to study the $n$-photon absorption in the nuclear excitation in the direct laser-nucleus interaction. Based on the laser facilities available currently or in the near future, we analyze the impact of the laser parameters on the excitation probability of the multiphoton excitation. The possibilities of the two-, three-, and four-photon excitations to the isomeric state $^{229\text{m}}\mathrm{Th}$ from the ground state are discussed in detail. Our results show the strong impact of the laser intensity and pulse duration on the multiphoton excitation probability. The onset of high-order effects in the multiphoton excitation in the direct laser-nucleus interaction is also revealed. Our findings open new possibilities to study the multiphoton laser-nucleus interaction in high-power laser facilities.