An atomic-beam resonance apparatus has been used to measure the dynamic Stark shift of the ground-state hyperfine transition frequency in cesium $\left(\approx 9.2\mathrm{GHz}\right)$ due to the electric field of blackbody radiation. The shift was measured as a function of the temperature of heated surfaces surrounding the atomic beam. The observed dependence of the transition frequency on temperature is in good agreement with theoretical predictions. At room temperature, the expected relative frequency shift is $-16.9\mathrm{}\times {10}^{-15\mathrm{}}$, whereas the experimental result is $-16.6\mathrm{}\left(2.0\right)\times {10}^{-15\mathrm{}}$.

• Received 11 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.622