Velocity map imaging

In 2011, we started a new project in which we combine Stark deceleration and crossed beam scattering techniques with the velocity map imaging technique (VMI). VMI was invented at the Radboud University Nijmegen in 1997 by David Parker and coworkers, and allows one to efficiently record the recoil velocities of scattered molecules. Differential scattering cross sections are obtained from the measured scattering images.

machine-Nijmegen

One of the crossed beam scattering machines that employ a combination of Stark deceleration and velocity map imaging detection that are operational in our laboratory.

Using the Stark decelerator, we are able to record differential cross sections of molecular scattering processes as a function of the collision energy, and with an unprecedented angular resolution. This allows us to resolve details of the scattering process that remain hidden in conventional crossed beam scattering experiments.

Recently, we have conducted the first crossed beam scattering experiment using the combination of Stark deceleration and velocity map imaging. The high resolution afforded by the decelerator allowed us to fully resolve quantum diffraction oscillations in the state-to-state differential cross sections for NO-rare gas atom collisions. These results were published in Nature Chemistry in 2014.

New approaches that make use of counterpropagating beam geometries resulted in even higher resolutions, revealing structures in the cross sections that gauge the quality of theoretical calculations. These results were published in Physical Review Letters in 2014.

A small beam crossing angle allows us to reach relatively low collision energies. In combination with the high image resolution, this allows us to image differential cross sections at collision energies where scattering resonances occur. This was demonstrated recently for NO + He collisions; we observed a striking variation of the DCS when the collision energy was scanned in small steps over scattering resonances. These results were published in Science in 2015.