Optimizing the scattering outcome by controlling the collision geometry

Our paper in Nature Chemistry discusses the importance of side-on orientations in understanding the mechanism of collisions between atoms and molecules.
Optimizing the scattering outcome by controlling the collision geometry

Our experiments focus on elucidating collision mechanisms of small molecular systems. Small systems, isolated in the gas phase, have the advantage that they can be investigated to a high level of detail and are also tractable theoretically. In our particular study, we have oriented NO molecules in an electric field such that they collided side-on with incoming Ar atoms. During the collision process, the translational energy of the collision partners was converted into rotational excitation of the NO molecules. The initial relative velocity of the NO and the Ar atom, the orientation of the NO bond axis, and the direction of the outgoing molecules after collision (the final relative velocity) were thus known quantities in our measurements. The correlation between these three vectors allowed us to investigate the role that the initial relative orientation of the collision partners has on the scattering outcome.

The experimental apparatus, which we refer to as the ‘Blue Monster’ owing to the blue scaffolding frame on which it is supported, comprises over a dozen complex electronic components that all need to work simultaneously. Overlapping the NO and Ar beams with the laser beams within the orientation region, determining the right voltages to apply to the orientation electrodes and the imaging optics, as well as optimizing the relative timings of the beams required patience and perseverance. After months of refining and tinkering, we finally had the experiment running stably. On a typical day in the lab, we would be able to converge the images for a couple of states. So while setting up the experiment took over a year, the actual data for the current study was collected within about three weeks.

In terms of the data analysis, the main challenge was extending the simulation and fitting programs to account for the asymmetry in the experimental images that arises due to the non-cylindrical symmetry around the relative velocity vector in the side-on geometry (or any geometry other than end-on). To implement this, we had to extend the existing quantum theory for unoriented and end-on-oriented molecules. We went through some fiddly maths to derive the expressions for the differential cross sections for any arbitrary bond orientation. Extraction of the differential cross sections from the experimental images heavily relied on this newly developed theory and would have been difficult to do without it. Additionally, the new theory enabled us to demonstrate very good agreement between experiment and calculation.

Our study shows that the preferred orientation for NO colliding with incoming Ar atoms is nearly side-on in the most intense, forward scattered region, indicating that, by careful selection of the initial relative orientation of the collision partners, the scattering intensity can be largely controlled. We see potential for this method to be applicable to larger molecular systems, including reactive species and processes occurring on surfaces. When such studies are performed, they will provide insights into ‘real-life’ mechanisms ranging from catalysis to biologically and atmospherically relevant reactions.