Joint work of Prof Ken McKendrick with collaborators at Radboud University Nijmegen and University of Oxford has been published in Nature Chemistry, (2012, 4, 985). Hydroxyl radicals (OH) are important in many chemical systems, including combustion and atmospheric reactions. However, measuring experimentally the speed and direction of travel of OH with specific forms of internal (rotational, vibrational and electronic) energy, which is a key diagnostic of the mechanism of the process that formed them, has proved difficult. Now, these distributions (the state-to-state differential cross sections) for inelastic scattering of fully state-specified OH with He and Ar have been measured for the first time.  This has been made possible by exploiting the velocity-map imaging technique in a crossed molecular-beam arrangement. The measured speed and angular distributions are shown to compare favourably with theoretical predictions. This confirms the quality of calculated potential energy surfaces that are used to describe the astrochemically relevant collisions of OH with He.