Direct probe of anisotropy in atom-molecule collisions via quantum scattering resonances

TL;DR

This study directly measures how anisotropy affects atom-molecule quantum resonances by manipulating the molecular rotational state, revealing anisotropy's role in quantum scattering processes at ultra-cold temperatures.

Contribution

It demonstrates experimental control over anisotropy in atom-molecule interactions through internal molecular rotation states, linking anisotropy to observable quantum resonances.

Findings

Quantum resonance appears only when H2 is rotationally excited.

Anisotropy influences effective interaction only in excited rotational states.

Control of molecular rotation state enables switching anisotropy on or off.

Abstract

Anisotropy is a fundamental property of particle interactions. It occupies a central role in cold and ultra-cold molecular processes, where long range forces have been found to significantly depend on orientation in ultra-cold polar molecule collisions. Recent experiments have demonstrated the emergence of quantum phenomena such as scattering resonances in the cold collisions regime due to quantization of the intermolecular degrees of freedom. Although these states have been shown to be sensitive to interaction details, the effect of anisotropy on quantum resonances has eluded experimental observation so far. Here, we directly measure the anisotropy in atom-molecule interactions via quantum resonances by changing the quantum state of the internal molecular rotor. We observe that a quantum scattering resonance at a collision energy of $k_B$ x 270 mK appears in the Penning ionization of molecular hydrogen with metastable helium only if the molecule is rotationally excited. We use state of the art ab initio and multichannel quantum molecular dynamics calculations to show that the anisotropy contributes to the effective interaction only for $H_2$ molecules in the first excited rotational state, whereas rotationally ground state $H_2$ interacts purely isotropically with metastable helium. Control over the quantum state of the internal molecular rotation allows us to switch the anisotropy on or off and thus disentangle the isotropic and anisotropic parts of the interaction. These quantum phenomena provide a challenging benchmark for even the most advanced theoretical descriptions, highlighting the advantage of using cold collisions to advance the microscopic understanding of particle interactions.