Measurement of the Electric Quadrupole Moment of the4dD5/22Level inSr+88

The quadrupole moment of the $4d{}^{2}D_{5/2}$ level in ${}^{88}\mathrm{S}\mathrm{r}^{+}$ has been measured to be $2.6(3)e{a}_0^2$, where $a_0$ is the Bohr radius and $e$ the elementary charge. A single laser-cooled strontium ion was confined in an end cap trap with a variable dc quadrupole potential, and measurements were made on the $5s{}^{2}S_{1/2}$–$4d{}^{2}D_{5/2}$ transition at 674 nm using a femtosecond optical frequency comb. This work shows that measurements of the unperturbed ${}^{88}\mathrm{S}\mathrm{r}^{+}$ transition frequency with sub-Hz uncertainty are possible and is important in understanding the reproducibility of ion trap optical frequency standards.