We report spectroscopic results on the 2s2p^{1}P_{1} state in neutral atomic beryllium-9. The absolute frequency for the center of gravity is determined to be 42 565.4501(13)  cm^{-1}, a factor of 130 more precise than the previous experimental measurement. The result is in agreement with and a factor of 8 more precise than the current best theoretical estimate of 42 565.441(11)  cm^{-1}, which was calculated including the effects of quantum electrodynamics. Because of the large natural linewidth of the transition, the hyperfine constants were not able to be extracted to high precision.

We report on the calibration of a temperature stabilized ultra-low expansion (ULE) cavity using previously measured molecular tellurium and atomic cesium lines. By means of a dual frequency modulation technique, the frequency dependence of the free spectral range of the ULE cavity is measured and was found to vary by less than 60 Hz over the ∼55 THz range of the calibration. This method of calibration enables the ULE cavity to measure absolute frequencies to better than 1.5 MHz.