2026 Theses Doctoral

Spectroscopy of Sr₂ for Metrology, Quantum Chemistry, and Tests of Fundamental Physics

Iritani, Brandon

The interaction between light and matter provides a window into the intricate structure of atoms and molecules. From the first observation of the Balmer series in hydrogen, to modern atomic clocks, spectroscopy has been at the forefront of atomic, molecular, and optical (AMO) physics. As the field of AMO progresses towards increasingly complex systems, a natural next step from atoms is diatomic molecules, which have a rich internal structure that makes them attractive for applications in quantum simulation, quantum information and precision measurement.

Here, optical lattice clock technology is combined with molecular physics through precision spectroscopy of Sr₂ molecules. As a benchmark, the systematic uncertainty of a clock transition spanning the ground-state potential of 88Sr₂ is evaluated to 4.6 × 10−14, and the absolute frequency of the transition is measured to ∼ 5 Hz uncertainty. In pursuit of reduced systematic uncertainty, a novel method for evaluating the notoriously difficult blackbody radiation (BBR) shift is presented, by comparing a near-infrared (IR) polarizability measurement to ab initio theory. A promising direction for the molecular clock is an isotope shift measurement to constrain non-Newtonian Yukawa-type internuclear interactions.

To this end, stable samples of 86Sr₂ are produced, and precision spectroscopy is performed on its long-range halo state. The binding energies of all 63 vibrational states in the ground-state potential of 86Sr₂ are measured, and a completely ab initio calculation of the ground-state potential is tested against these measurements. These results lay the groundwork for refining quantum chemistry methods, as well as combining them with experimental techniques in the search for new physics.