Refined observations of parity nonconservation in the $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}-7\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ transition in ${}_{81}{}^{203,205}{}_{}{}^{}\mathrm{Tl}$ are reported. Absorption of circularly polarized 293-nm photons by $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ atoms in an $E$ field results in polarization of the $7{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ state, through interference of the Stark $E1\mathrm{}$ amplitude with $M1\mathrm{}$ and parity-nonconserving $E1\mathrm{}$ amplitudes. Detection of this polarization yields the circular dichroism $\delta =+(2.8\mathrm{}\pm \left\{1.0\right\}\left\{0.9\right\})\times {10}^{-3\mathrm{}}$, which agrees with theoretical estimates based on the Weinberg-Salam model, for ${{sin}^2\theta }_W=0.23\mathrm{}$.

• Received 21 November 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.640