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Field Control of Cold Molecular Collisions

Periodic Reporting for period 2 - FICOMOL (Field Control of Cold Molecular Collisions)

Reporting period: 2020-09-01 to 2022-02-28

In this project, we develop new methods to study molecular collisions at very low temperatures (well below 1 K). At these temperatures, the quantum mechanical nature of molecules and their interactions start dominating the collisions, leading to exotic phenomena that very often have been predicted by theory decades ago, but that have been elusive to experimental observation. One of these phenomena is the occurrence of quantum scattering resonances, where molecules become temporarily trapped leading to a sudden and dramatic change in the cross sections. The experimental observation of these resonances would probe molecular interactions with unprecedented detail, and has been one of the Holy Grails in the community for years.
We have developed the methods to study resonances for collisions between NO radicals and He atoms at energies down to 0.2 cm-1. We achieved this by using the combination of Stark deceleration, low angle scattering, and velocity map imaging. We observe beautifully resolved individual resonances, that can be directly attributed to single quantum mechanical waves. Resonances have been manipulated using optical pumping techniques that add a controlled amount of angular momentum to the system, which evolution during the collision can be followed. Preparations to start controlling the resonances using external fields are underway; a new velocity map imaging detector in which it is possible to apply high fields to the molecules while maintaining good focusing properties has been designed and built. Similar experiments have now been also performed using a new type of Zeeman decelerator, that can manipulate atoms and molecules that have a magnetic dipole moment.
The observation of scattering resonances for NO-He at energies down to 0.2 cm-1 has been a true breakthrough. It required theory at the CCSDT(Q) level, which significantly goes beyond the state of the art. At higher energies, we have discovered a new scattering mechanism, which we call Hard Collision Glory Scattering, that explains the observed forward scattering in bimolecular collisions, where common wisdom would predict backscattering instead. We expect more of such results in the coming years, as we are probing molecular collisions in an energy window that has been unexplored thus far.