Fine Structure of the3P2and4P2States of Lithium

The fine-structure intervals of the $3{}_{}{}^2{}_{}{}^{}P$ state of ${\mathrm{Li}}^6$ and ${\mathrm{Li}}^7$ and the $4{}_{}{}^2{}_{}{}^{}P$ state of ${\mathrm{Li}}^7$ have been measured by observing the level crossing signals from the ${P_{\frac{3}{2}}}^{-\frac{3}{2}}$ and ${P_{\frac{1}{2}}}^{\frac{1}{2}}$ levels. Results for the magnetic fields at which the center of the crossing signals occur are the following, in terms of the proton NMR frequency in kHz for a mineral oil sample: $3{}_{}{}^2{}_{}{}^{}P\left({\mathrm{Li}}^7\right)$, 3897.163(22); $3{}_{}{}^2{}_{}{}^{}P\left({\mathrm{Li}}^6\right)$, 3896.89(20); $4{}_{}{}^2{}_{}{}^{}P\left({\mathrm{Li}}^7\right)$, 1621.71(15). By using a theory of the Zeeman effect that includes estimates of the normal and specific mass effects and relativistic effects, we find the following values for the zero-field fine-structure intervals: $\Delta W({\mathrm{Li}}^7-3P)=2882.903\mathrm{}\left(18\right)\mathrm{}$ MHz; $\Delta W({\mathrm{Li}}^7-4P)=1199.65\mathrm{}\left(11\right)\mathrm{}$ MHz. The uncertainties are three times the standard error for the measurement.