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

We propose a new approach to excite the isomeric ${}^{229}\mathrm{Th}$ 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.

Figure 1

Schematic illustration of our approach. When exposed to an intense laser pulse, a ${}^{229}\mathrm{Th}$ atom (or ion) loses an electron due to strong-field ionization. The emitted electron may later be driven back to recollide with and excite the nucleus from the ground state to the isomeric state.

Figure 2

(a) Isomeric excitation cross section of ${}^{229}\mathrm{Th}$ by electrons. $E2$ denotes the cross section from the electric quadrupole channel and $M1$ denotes that from the magnetic dipole channel. (b) Relation between the recollision time $t_r$ and the ionization time $t_i$ within a laser cycle, given in degrees starting from a field peak. (c) Kinetic energy of the electron at the time of recollision as a function of the ionization time $t_i$. $E_r$ is in units of the ponderomotive energy $U_p$.

Figure 3

Fourier component $|\tilde{V}({\omega }_0){|}^2$ as a function of $R_0$, with ${\omega }_0$ the energy of the isomeric state.

Figure 4

Probabilities of ionization (a),(c) and of nuclear excitation (b),(d). The laser pulse is Gaussian with duration 30 fs (FWHM), as shown in each figure as background. Two intensities are used, namely, ${10}^{14}$ (a),(b) and ${10}^{15}\mathrm{W}/{\mathrm{cm}}^2$ (c),(d). The ionization probabilities of individual electrons and their contributions to the nuclear excitation are also separately labeled. For both cases, the first-emitted electron does not have enough recollision energy to excite the nucleus.

Figure 5

Absorption potential seen by the colliding electron for the ${}^{229}{\mathrm{Th}}^{2+}$ ion, calculated using the code elsepa, for incoming energies of 20 eV (blue dashed line), 50 eV (red dotted line), and 100 eV (black solid line).