When a molecule is subjected to a sufficiently energetic photon it can break apart into fragments conserving momentum and energy in a process called “photodissociation”. For over 70 years this simple chemical reaction has served as a vital experimental tool for inferring information about molecular structure, since the character of the photodissociative transition, and through it the geometry of the molecular transition moment, can be related to the (measurable) 3D angular distribution of the resulting photofragments.
While theoretical understanding of this process has gradually evolved from classical considerations to a fully quantum approach, experiments to date have not yet been able to fully reveal the quantum nature of the process, i.e. by controlling all quantum mechanical degrees of freedom of both the input state and output channels and comparing with a fully quantum mechanical theory.
In my talk I will discuss our recent experiments involving the photodissociation of ultracold, optical lattice-trapped, and fully quantum state-resolved 88Sr2 molecules. Detection of the photodissociation products via optical absorption imaging reveals the hallmarks of ultracold quantum chemistry: resonant and nonresonant barrier tunneling, the importance of quantum statistics, the presence of forbidden reaction pathways, and matter wave interference of the reaction products. In particular, this interference results in photofragment angular distributions with a strong breaking of cylindrical symmetry, so far unobserved in photodissociation. We definitively show that quasiclassical models of photodissociation fail in this regime. Instead, a fully quantum mechanical model accurately reproduces the results and yields new intuition about chemistry in the ultracold regime.