Parity nonconservation is observed in the $6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}-7\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ transition in thallium. 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 Stark $E1\mathrm{}$ amplitudes with $M1\mathrm{}$ and parity-nonconserving $E1\mathrm{}$ amplitudes $\mathfrak{M}$ and ${\mathcal{E}}_p$. Detection of this polarization yields the circular dichroism $\delta =+(5.2\mathrm{}\pm 2.4)\times {10}^{-3\mathrm{}}$, which agrees in sign and magnitude with theoretical estimates based on the Weinberg-Salam model.

• Received 20 November 1978

DOI:https://doi.org/10.1103/PhysRevLett.42.343