Measurement of the Lamb shift in then=4state ofHe+4

The separated oscillatory field technique was used with a 120-keV ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ beam to make a zeromagnetic-field measurement of the $n=4\mathrm{}$ Lamb-shift interval in ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$. Excited ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ions were created by collisions in a cell filled with ${\mathrm{N}}_2$ gas. The $4{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state population was monitored by observing its decay to the $2{}_{}{}^2{}_{}{}^{}P$ state with a large solid-angle NO photoionization detector. The $n=4\mathrm{}$ to 2 light (121.5 nm) decreased when the microwave field induced transitions from the $4{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ to the $4{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state. The value obtained for the Lamb shift was 1769.16mp 0.84 MHz. This result agrees satisfactorily with previous measurements and with the theoretical value for this interval. The experimental uncertainty is dominated by systematic effects attributed to the presence of overlapping cascade signals.