Measurement of the Lamb Shift in then=2State ofC5+12

A beam of hydrogenic ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}_{}^{5+}{}_{}{}^{}$ atoms in the metastable $2S$ state has been produced. By quenching the state in a known motional electric field and measuring the decay length, an indirect measure of the Lamb shift $\mathcal{S}$ is obtained. A preliminary result of $\mathcal{S}=744\mathrm{}$ GHz, with a statistical uncertainty of less than 1%, is reported as compared with a theoretical prediction of $\mathcal{S}=783\mathrm{}$ GHz. Possible systematic effects which may reduce the discrepancy are considered.