Periodic Reporting for period 4 - FICOMOL (Field Control of Cold Molecular Collisions)
Periodo di rendicontazione: 2023-09-01 al 2025-02-28
In this project, we aimed to 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. At temperatures below 1 K, scattering cross sections are predicted to respond sensitively to external electric or magnetic fields, yielding the thrilling perspective to provide “control knobs” to steer the outcome of a collision. Measurements (and control!) of exotic collision phenomena at low temperatures provide excellent tests for quantum theories of molecular interactions, and pave the way towards the engineering of novel quantum structures, or the collective properties of interacting molecular systems. This is not only important from a pure fundamental point of view, but a full understanding of cold molecular collisions are also required to advance other research fields such as astrochemistry and quanum matter physics.
One of the collision phenomena that can occur at low energies is the occurrence of quantum scattering resonances, where molecules become temporarily trapped leading to a sudden and dramatic change in the cross sections. Within this project, we have made one of the first experimental observations of scattering resonances using collisions between NO radicals and He atoms as a benchmark system, and have found that improved and more accurate theoretical calculations are needed to explain them. Resonances were also observed for the more complicated ND3-H2 system, where the large dipole moment of the ammonia molecule allowed for the first tuning of these resonances in external fields. For the scattering of two molecules each having a dipole moment, we have discovered a new collision mechanism based on the mutual self-polarization of both colliders. This unexpected behavior has major implications on the scattering cross sections, featuring a local maximum at we well-defined collision energy determined by the properties of the molecules involved in the collision. We have been able to observe this feature in ammonia-ammonia collisions.
In conclusion, during this project we have developed the methods to reduce the attainable collision energy to values well below 100 mK, and have experimentally studied various new collision mechanisms that only occur in this low energy regime. The first tuning of low-energy collision cross sections using externally applied electric fields was accomplished.
One of the collision phenomena that can occur at low energies is the occurrence of quantum scattering resonances, where molecules become temporarily trapped leading to a sudden and dramatic change in the cross sections. Within this project, we have made one of the first experimental observations of scattering resonances using collisions between NO radicals and He atoms as a benchmark system, and have found that improved and more accurate theoretical calculations are needed to explain them. Resonances were also observed for the more complicated ND3-H2 system, where the large dipole moment of the ammonia molecule allowed for the first tuning of these resonances in external fields. For the scattering of two molecules each having a dipole moment, we have discovered a new collision mechanism based on the mutual self-polarization of both colliders. This unexpected behavior has major implications on the scattering cross sections, featuring a local maximum at we well-defined collision energy determined by the properties of the molecules involved in the collision. We have been able to observe this feature in ammonia-ammonia collisions.
In conclusion, during this project we have developed the methods to reduce the attainable collision energy to values well below 100 mK, and have experimentally studied various new collision mechanisms that only occur in this low energy regime. The first tuning of low-energy collision cross sections using externally applied electric fields was accomplished.
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 (Science 368, 626 (2020)). The resonances could subsequently be manipulated using optical pumping techniques that add a controlled amount of angular momentum to the system, which evolution during the collision could be followed (Nature Chemistry 14, 538 (2022)). For the more complicated ammonia-H2 system, observation of resonances was thus far hampered by the lack of a suitable high-resolution detection scheme for ammonia. We developed a new recoil-free REMPI scheme for ammonia using a VUV laser (J. Phys. Chem. A 128, 10993- 11004 (2024)), which resulted in the first measurements of scattering resonances in both the intergral and differential cross sections in low-energy ammonia-H2 collisions (Nature Communications 6, 7181 (2025). Measurements to control the resonances using external fields are currently underway. Similar experiments have also been also performed using a new type of Zeeman decelerator, that resulted in the obervation of scattering resonances for collisions between C atoms and H2 molecules (J. Phys. Chem. Lett. 15, 4602 (2024)).
For the scattering of two state-selected molecules, we discovered new mechanisms for glory scattering in low-energy collisions (Nature Chemistry 14, 664 (2022)). Using the merged beam approach, we embarked on a series of measurements to study the collision behavior of bi-molecular systems, in which each collision partner has a dipole moment. We studied collisions between NO radicals and ammonia molecules at energies down to 100 mK, and disclosed a new (universal) scattering mechanism based on the self-polarization of the colliders that results in an intricate maximum in the collision cross section at a well defined collision energy (Science 379, 1031 (2023)). New merged beam approaches were developed (Rev. Sci. Instrum. 95, 093201 (2024) to experimentally observe this maximum for ammonia-ammonia collisions.
For the scattering of two state-selected molecules, we discovered new mechanisms for glory scattering in low-energy collisions (Nature Chemistry 14, 664 (2022)). Using the merged beam approach, we embarked on a series of measurements to study the collision behavior of bi-molecular systems, in which each collision partner has a dipole moment. We studied collisions between NO radicals and ammonia molecules at energies down to 100 mK, and disclosed a new (universal) scattering mechanism based on the self-polarization of the colliders that results in an intricate maximum in the collision cross section at a well defined collision energy (Science 379, 1031 (2023)). New merged beam approaches were developed (Rev. Sci. Instrum. 95, 093201 (2024) to experimentally observe this maximum for ammonia-ammonia collisions.
The observation of scattering resonances for NO-He and ND3-H2 at energies down to 0.2 cm-1 has been true breakthroughs. 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. The large dipole moment of ND3 allowed for the tuning of the scattering resonances using externally applied fields.
For bimolecular scattering focusing on the dipole-dipole interaction using two state-selected and controlled molecules, the application of the merged beam approach resulted in record-low collision energies and unexpected discoveries of collision mechanisms. A new self-polarization mechanism was found, which appears ubiquitous in many systems involving polar molecules. This mechanism was validated experimentally, and offers unique perspectives to further unlock (and control!) cold molecular collisions in the years to come.
For bimolecular scattering focusing on the dipole-dipole interaction using two state-selected and controlled molecules, the application of the merged beam approach resulted in record-low collision energies and unexpected discoveries of collision mechanisms. A new self-polarization mechanism was found, which appears ubiquitous in many systems involving polar molecules. This mechanism was validated experimentally, and offers unique perspectives to further unlock (and control!) cold molecular collisions in the years to come.