Experimental and Theoretical Investigation of the Crossover from the Ultracold to the Quasiclassical Regime of Photodissociation

TL;DR

This study explores the transition from quantum to classical descriptions in ultracold molecule photodissociation, demonstrating how quantum effects persist at high energies for identical particles and comparing experimental results with theoretical models.

Contribution

It provides a combined experimental and theoretical analysis of the quantum-to-classical crossover in ultracold molecule photodissociation, highlighting the limitations of quasiclassical models for identical particles.

Findings

Reaction convergence to axial recoil limit at high energies

Quasiclassical models generally accurate except for identical particles

Quantum statistics influence high-energy reaction dynamics

Abstract

At ultralow energies, atoms and molecules undergo collisions and reactions that are best described in terms of quantum mechanical wave functions. In contrast, at higher energies these processes can be understood quasiclassically. Here, we investigate the crossover from the quantum mechanical to the quasiclassical regime both experimentally and theoretically for photodissociation of ultracold diatomic strontium molecules. This basic reaction is carried out with a full control of quantum states for the molecules and their photofragments. The photofragment angular distributions are imaged, and calculated using a quantum mechanical model as well as the WKB and a semiclassical approximation that are explicitly compared across a range of photofragment energies. The reaction process is shown to converge to its high-energy (axial recoil) limit when the energy exceeds the height of any reaction barriers. This phenomenon is quantitatively investigated for two-channel photodissociation using intuitive parameters for the channel amplitude and phase. While the axial recoil limit is generally found to be well described by a commonly used quasiclassical model, we find that when the photofragments are identical particles, their bosonic or fermionic quantum statistics can cause this model to fail, requiring a quantum mechanical treatment even at high energies.