The Lamb shift in hydrogenic phosphorus ($Z=15\mathrm{}$) has been measured by the $2s_{\frac{1}{2}}-2\mathrm{}{p}_{\frac{3}{2}}$ laser resonance-quenching method. An efficient synchronization between the light pulse from a tunable dye laser, pumped by a nitrogen laser, and the beam bursts of a cyclotron is used to enhance up to 0.8 the signal-to-background ratio. From the measured $2s_{\frac{1}{2}}-2\mathrm{}{p}_{\frac{3}{2}}$ energy splitting, $\lambda =5556.5\mathrm{}\left(2.5\right)\mathrm{}$ Å, and the calculated fine-structure splitting the Lamb shift $S=20.18\mathrm{}\left(25\right)\mathrm{}$ THz was deduced. This result is confronted with the presently available calculations of QED.

• Received 17 August 1982

DOI:https://doi.org/10.1103/PhysRevLett.49.1762