Observation of Lamb Shifts by Quantum Beats

The technique which we used previously to observe quantum beats due to coherent electron-impact excitation of Zeeman magnetic sublevels is extended to the observation of the Lamb shift in the $n=4\mathrm{}$ state of ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$. The quantum beats in our new experiment are due to temporal variations of the fluorescent light. We extend Series' theory and report the first experimental observation of the effect predicted by Series.