The spin-exchange optical polarization method was used to determine the sign and the magnitude of the hyperfine interaction constant in the ground ${\mathrm{}\left(3p\right)\mathrm{}}^3{}_{}{}^4{}_{}{}^{}S_{\frac{3}{2}}^{}{}_{}{}^{}$ state of atomic ${\mathrm{P}}^{31}$. The measurements were made by monitoring the transmission of circularly polarized rubidium $D_1$ ($5P_{\frac{1}{2}}\to 5S_{\frac{1}{2}}$) resonance radiation through a flask containing rubidium, atomic phosphorus, and a buffer gas as a function of the frequency of an external radio-frequency field. The value for the hyperfine interaction constant of ${\mathrm{P}}^{31}$ obtained by extrapolation to zero pressure in a helium buffer gas is $A\left({\mathrm{P}}^{31}\right)=+(55\mathrm{}055691\pm 8) \mathrm{cps}/sec.$ The pressure shifts in cycles/sec/mm Hg of the phosphorus hyperfine interaction constant for helium and neon buffer gases are +3.61±0.23 and +6.73±0.15, respectively.

• Received 31 May 1962

DOI:https://doi.org/10.1103/PhysRev.128.198