Special issue on atomic and molecular collision mechanisms

This issue presents papers that were invited from the attendees of the Solvay workshop on Atomic and Molecular Collision Mechanisms (ACME), held in Brussels from 30 March to 2 April 2015. with sponsorship from the Wiener Anspach Foundation, the International Solvay Institutes and this journal. The topics presented in posters and talks by the 60 participants were under the headings ‘half collisions’, ‘cold ions, molecules and collisions’, ‘intramolecular events and control’, ‘planetary atmospheres’ and ‘the interstellar medium’.

The temperature at the Earth surface is about 300 K, and then evolves down to some 150 K at the upper limit of the atmosphere. Even colder, extraterrestrial environments are well identified in the solar system, down to 50 K, e.g. in the atmosphere of Neptune. The temperature further drops to between 10 and 20 K in the coma of comets and may even get lower in the interstellar medium. Molecules are present in all of these media, supporting chemistry, and the origin of life. Today, dedicated, sophisticated instrumental means allow the most extreme of these conditions to be matched and even exceeded in the laboratory. Collision mechanisms and their evolution with decreasing temperature are of primary relevance in the related reactive processes. They need to be better measured and modelled and, for the lowest temperatures, the mechanisms basically still remain to be unravelled. Chemistry at ultra-low temperatures, the formation of dimers and larger aggregates, the understanding and active control of collision processes at the molecular level, and their atmospheric and astrophysical applications defines a very active, challenging and multidisciplinary research field that was the subject of the ACME workshop. Many of the talks reflected recent publications in Molecular Physics such as the roaming mechanism of chemical dynamics [1], the spectroscopy of ammonia and its complexes [2], methods for producing cold molecular samples [3], interstellar molecules [4] and aerosol particles [5].