Measurement of the1sLamb shift in hydrogenlike chlorine

The $1s$ Lamb shift in hydrogenlike chlorine has been determined from a precision measurement of the ${2p}_{\frac{3}{2}}-{1s}_{\frac{1}{2}}\left(\mathrm{Ly}{\alpha }_1\right)$ and ${2p}_{\frac{1}{2}}-{1s}_{\frac{1}{2}}\left(\mathrm{Ly}{\alpha }_2\right)$ x-ray transitions using beam-foil excitation. The x rays are emitted from high-velocity chlorine-ion beams at several ion velocities produced by a Van de Graaf accelerator. The $1s$ Lamb shifts obtained from the $\mathrm{Ly}{\alpha }_1$ and $\mathrm{Ly}{\alpha }_2$ measurements are 0.84(12) and 0.90(10) eV, respectively, compared with a calculated value of 0.9384(6) eV. The fine-structure splitting of the $2p$ level was also determined in this experiment and found to be 3.889(30) eV compared with a theoretical value of 3.827 18(2) eV. A precision measurement of the $\mathrm{Ar} K\alpha$ x rays was made in order to establish the energy scale for $\mathrm{Cl} \mathrm{Ly}\alpha$ x rays. The newly determined energies are 2957.813(8), 2955.684(13), and 2977.51(6) eV for the $K{\alpha }_1$, $K{\alpha }_2$, and $K{\alpha }_4$ energies, respectively.