Experiments on the2P3State of Helium. I. A Measurement of the2P13−2P23Fine Structure

A measurement, accurate to 3 ppm, is reported of the $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}-2\mathrm{}{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ fine structure in ${\mathrm{He}}^4$. An atomic-beam optical-microwave resonance technique was used. Two sets of measurements were made on the transition $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}(m_J=0)\leftrightarrow 2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}({\mathrm{m}}_J=0)$. The first set was in a magnetic field of about 100 G ("low" field), where the uncertainties arose from field-dependent asymmetries in the line shape. The second set of measurements was in a field of about 500 G ("high" field), and here the main uncertainties were in the field measurements and the calculation of $g$ factors. The results of the two sets were in good agreement: low field: $E\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\right)-E\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\right)=2291.188\mathrm{}\pm 0.015$ MHz; high field: $E\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\right)-\mathrm{E}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\right)=2291.197\mathrm{}\pm 0.008$ MHz; weighted mean: $E\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\right)-E\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\right)=2291.195\mathrm{}\pm 0.007$ MHz. The advantages of the atomic-beam technique and the possible importance of this fine-structure measurement are discussed.