Hyperfine Structure of the Metastable Hydrogen Atom

The hyperfine separation $\Delta \nu \mathrm{}\left(2S\right)\mathrm{}$ of the metastable $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state has been measured by a new atomic-beam magnetic-resonance method. Rf transitions were detected by utilizing a property peculiar to metastable hydrogen atoms, namely, that atoms with $m_J=-\frac{1}{2}$ decay much more rapidly in passing through a magnetic field of about 575 gauss than do atoms with $m_J=+\frac{1}{2}$. We found that $\Delta \nu \mathrm{}\left(2S\right)=177\mathrm{}556.86\pm 0.05$ kc/sec. From this result and the very accurately known hyperfine separation $\Delta \nu \mathrm{}\left(1S\right)\mathrm{}$ of the ground state, we obtain $R\equiv \frac{\Delta \nu \mathrm{}\left(2S\right)\mathrm{}}{\Delta \nu \mathrm{}\left(1S\right)\mathrm{}}=\frac{1}{8}(1.000\mathrm{}0346\pm 0.0000003)$. The deviation of $R$ from the presently available theoretical value suggests the existence of a higher-order quantum-electrodynamic term.