The Fine Structure of Singly Ionized Helium

The displacement $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ in singly ionized helium has been determined to be 14,020±100 mc/sec. The method is analogous to that employed for hydrogen by Lamb and Retherford, except that no beam of metastable ions is formed. Instead, helium atoms are bombarded by electrons of a few hundred volts energy. In about one percent of the ionizing collisions, the remaining atomic electron is excited to the metastable $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state of the ion. When the bombardment region is illuminated with microwave radiation of the proper frequency, transitions to the $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state are induced, and the ultraviolet photons emitted in the subsequent decay to the ground state $1{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ are detected by a suitable photoelectric detector. Background effects due to metastable atoms and radiation from HeI are reduced by the use of a collodion film filter. Our experimental value for the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ level shift is about 1.4 percent higher than the present theoretical value.