Th3+229 as an ionic optical clock for fine-structure-constant variations

In this paper, we propose the 5f5/2→5f7/2 transition in ²²⁹Th³⁺ as an ionic clock with accuracy of 10^-18 level, which can be also used together with the nuclear clock transition for measuring variations of fine-structure constant with common-mode rejection of certain systematic effects. The many-body perturbation theory and Dirac-Fock plus core polarizability method are used to perform theoretical calculations. Calculation results show that several systematic frequency shifts can be suppressed to 10^-18 level or even below. Assuming the accuracy of ionic clock frequency can be achieved in 10^-18 level, ²²⁹Th³⁺ potentially offers a precision of drift of fine-structure constant of ̇ \.α/α in 10^-21-yr^-1 level by measuring the frequency ratio of nuclear clock frequency and ionic clock frequency.