Gerrit C. Groenenboom, Liesbeth M. C. Janssen, and A. van der Avoird
Theoretical Chemistry, Institute for Molecules and Materials, Radboud University Nijmegen.
Open-shell polar molecules can be trapped and manipulated with external electric and magnetic fields. The buffer-gas cooling technique for molecules relies on collision with helium. Evaporative cooling to the ultracold regime depends on collisions between molecules. We show that collisions between molecular radicals are very different from atom-molecule collisions.
We report measurements of state-resolved inelastic scattering cross sections for collisions between Stark-decelerated hydroxyl (OH) radicals and hexapole-focused nitric oxide (NO) radicals, both prepared in a single quantum state . Rotationally and spin-orbit inelastic scattering cross sections were measured on an absolute scale for collision energies between 70 and 300 cm-11.
These cross sections can be understood by quantum coupled-channels calculations taking into account long range electrostatic interactions and nonadiabatic couplings between multiple electronic potential energy surfaces.
We also report a theoretical study of collisions between cold, magneticallytrapped NH radicals. We employ single arrangement coupled channels calculationsinvolving three coupled potential energy surfaces imposing reactive boundary conditions. We find that spin-inelastic transitions occur at very long range, but spin-forbidden chemical reactions occurring at short range can dominate other processes, affecting the prospects for evaporative cooling.
 M. Kirste, X. Wang, H. C. Schewe, G. Meijer, K. Liu, A. van der Avoird, L. M. C. Janssen, K. B. Gubbels, G. C. Groenenboom, and S. Y. T. van de Meerakker, Science, 338, 1060 (2012).
 L. M. C. Janssen, A. van der Avoird, and G. C. Groenenboom, Phys. Rev. Lett. 110, 063201 (2013)