Our calculations have shown that the $5f_{5/2}\text{−}7s_{1/2}$ $23131{\text{cm}}^{-1}$ transition from the ground state in the ion ${\text{Th}}^{3+}$ is very sensitive to the temporal variation of the fine-structure constant $\alpha =e^2/\hslash c$ $\left(q=-75300{\text{cm}}^{-1}\right)$. The line is very narrow, the ion has been trapped and laser cooled, and the positive shifter line $5f_{5/2}\text{−}5f_{7/2}$ $4325{\text{cm}}^{-1}$ $\left(q=+2900{\text{cm}}^{-1}\right)$ may be used as a reference. A comparison may also be made with a positive shifter in another atom or ion. This makes ${\text{Th}}^{3+}$ a good candidate to search for the $\alpha$ variation.

• Received 18 October 2009

DOI:https://doi.org/10.1103/PhysRevA.80.064502