Testing quantum electrodynamics in the lowest singlet state of neutral beryllium-9

We performed high precision spectroscopy on the 2s2p J=1 singlet state in neutral beryllium-9. This result serves as a test of quantum electrodynamics and as an assessment of theoretical methods used to predict the energy levels of beryllium. A frequency quadrupled titanium sapphire laser at 235 nm was used to probe a beam of atomic beryllium. The frequency was measured to high precision by stabilizing the 470 nm light out of the frequency doubler to an ultra-low expansion cavity that was calibrated using molecular tellurium lines. This experimental method allowed us to improve the precision of the energy level to which the singlet state is known by more than 3 orders of magnitude.