High-precision anisotropy measurement of the Lamb shift inHe+

A high-precision measurement of the 2$s_{1/2}$-2$p_{1/2}$ Lamb shift in ${\mathrm{He}}^{+}$ is obtained by means of the quenching anisotropy method. The measured value of 14 042.22±0.35 MHz is in excellent agreement with Mohr’s electron self-energy calculation, but disagrees with Erickson’s by more than 8 standard deviations. The accuracy of the measurement provides the most stringent available test of the order-α(Zα${)}^6$ ${\mathrm{mc}}^2$ contribution to the calculated electron self-energy.