Directly probing anisotropy in atom–molecule collisions through quantum scattering resonances

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 1,2 . 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 3-8 . 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 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 molecules in the first excited rotational state, whereas rotationally ground state 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.

A buffer gas beam source for short, intense and slow molecular pulses

“…The starting point for many experiments with cold molecules is the pulsed molecular beam. For example, beams of cold molecules are used to test fundamental physics (7)(8)(9)(10)(11)(12)(13)(14) and to study collisions and reactions (16)(17)(18).…”

Cold Collisions in a Molecular Synchrotron

“…Second, the sensitivity to collisions is enhanced by storing the ammonia molecules for many round-trips. Our approach thus combines the low collision energies obtained in experiments that use merged molecular beams [16][17][18][19][20] with the high sensitivity of experiments that monitor trap loss [21][22][23][24].…”

“…Second, the sensitivity to collisions is enhanced by storing the ammonia molecules for many round-trips. Our approach thus combines the low collision energies obtained in experiments that use merged molecular beams [16][17][18][19][20] PHYSICAL REVIEW LETTERS 120, 033402 (2018) 0031-9007=18=120(3)=033402 (5) 033402-1 …”

“…Even lower temperatures can be obtained by using magnetic or electric guides to merge two molecular beams into a single beam. This technique has been used to study Penning ionization reactions of various atoms and molecules with metastable helium [16][17][18][19] and collisions between ground state hydrogen molecules and hydrogen molecules in high Rydberg states [20].…”

“…Even lower temperatures can be obtained by using magnetic or electric guides to merge two molecular beams into a single beam. This technique has been used to study Penning ionization reactions of various atoms and molecules with metastable helium [16][17][18][19] and collisions between ground state hydrogen molecules and hydrogen molecules in high Rydberg states [20].In a different approach, trapped ions at millikelvin temperatures are monitored while slow, velocity-selected beams of molecules pass through the trap [21,22]. The ions are stored for a long time and their number can be accurately determined, which allows the study of reactions with rates as small as one per minute.…”