Isotope Shift in the (2S01−2P11) Line of Atomic Helium

A measurement of the isotope shift in the ($2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-2{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$) line of atomic helium has been made and is in excellent agreement with theory. Both ${\mathrm{He}}^4$ and ${\mathrm{He}}^3$ $2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ metastable beams were irradiated in a magnetic field with 20 582-Å ($2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-2{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$) radiation from ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ resonance lamps, respectively. In each case, a magnetic sublevel of the $2{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ state was Zeeman shifted so as to scan the resonance-lamp profile. The 584-Å ($2{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$)-decay photons served to monitor the excitation. A shift $\nu \left({\mathrm{He}}^3\right)-\nu \left({\mathrm{He}}^4\right)=0.1408\mathrm{}\pm 0.0019$ ${\mathrm{cm}}^{-1\mathrm{}}$ was obtained.