High Precision Spectroscopy of the 2s2p 1P◦ 1 State in Neutral Beryllium-9 inside a Large Magnetic Field

Quantum electrodynamics (QED) is a quantum field theory that describes the interactions between light and charged particles. While incredibly successful, there are problems with the theory including its incompatibility with gravitation theories and baryon asymmetry. This thesis presents a measurement of the absolute energy of the 2s2p 1P◦ 1 state in neutral 9Be, performed using saturated absorption spectroscopy in a 400 G magnetic field to isolate a well-resolved spectral feature. The transition frequency from the ground state was determined to be 1 276 080 105.5(1.5) MHz, corresponding to a center-of-gravity energy of 42 565.45058(5) cm−1. This result improves upon the previous best measurement by a factor of 25 and represents the most precise determination of an excited-state energy in a four-electron system to date. Combined with theoretical predictions, this thesis will serve as the most stringent test to date for quantum electrodynamics in a four-electron system.