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

Abstract 

Anisotropy is a fundamental property of particle interactions. It occupies a central 
role in cold and ultracold molecular processes, where orientation-dependent long-range
forces have been studied in ultracold polar molecule collisions. In the cold 
collisions regime, quantization of the intermolecular degrees of freedom leads to 
quantum scattering resonances. Although these states have been shown to be sensitive 
to details of the interaction potential, the effect of anisotropy on quantum 
resonances has so far eluded experimental observation. 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 kB x 270mK 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 theory to show that control over the 
rotational state effectively switches the anisotropy on or off, disentangling the 
isotropic and anisotropic parts of the interaction.