Molecular collision dynamics

Molecular collision dynamics has proved to be a central and fruitful field of research for physical chemists/chemical physicists since its very beginning in the 1950s. The field has recorded many significant achievements, in particular with the awards of the Nobel prizes in chemistry in 1986 and 1999. It has continuously provided major breakthroughs, in no small part facilitated by the synergy which exists between theory and experiment. The goal of this themed issue is to highlight the present state-of-the-art and outline the future prospects for the subject. We are delighted to see that so many leading scientists in the field have contributed to this task.

Exciting experimental progress, accompanied by significant advances in theory, have allowed us to compare very detailed experimental observables with the results of exact quantum scattering calculations on fully ab initio potential energy surfaces for simple triatomic prototype reactions. These include H + H₂, F + H₂ and S(¹D) + H₂ (Perspective by Yang (DOI: 10.1039/c1cp00005e) and papers by Zare (DOI: 10.1039/c0cp02460k), Connor (DOI: 10.1039/c0cp01354d), Aldegunde & Aoiz (DOI: 10.1039/c0cp02457k), De Fazio (DOI: 10.1039/c0cp02738c), Costes (DOI: 10.1039/c0cp02705g), Lara Garrido (DOI: 10.1039/c0cp02091e)), where exquisite details (including resonances) have been revealed, deepening our basic understanding of chemical reactivity. Detailed theoretical work is also reported on other tri-atomic and four-atomic reactions (Guo & Xie (DOI: 10.1039/c0cp02116d), Larrégaray (DOI: 10.1039/c0cp02662j), Petrongolo (DOI: 10.1039/c0cp02233k), Gonzáles-Lézana, Aoiz & Hankel (DOI: 10.1039/c0cp02619k), Varandas (DOI: 10.1039/c0cp02598d)). Rigorous theory of reactive scattering is moving from tri- and four-atomic reactions towards penta-atomic reactions (Kuppermann (DOI: 10.1039/c0cp02907f)). Great progress is being documented in the development of potential energy surfaces for gas phase bimolecular and unimolecular reactions, as well as for gas-surface reactions (Perspective by Bowman (DOI: 10.1039/c0cp02722g) and paper by Collins (DOI: 10.1039/c0cp01843k), respectively). Stereocontrol of attosecond electron dynamics is being tackled theoretically (Levine (DOI: 10.1039/c1cp20094a)).

The aforementioned experimental progress has also permitted researchers to begin exploring even more complex, polyatomic (six-atomic) reactions, such as S(¹D) + CH₄ (Le Picard & Casavecchia (DOI: 10.1039/c0cp02813d)), O(¹D) + CH₄ (Suzuki (DOI: 10.1039/c0cp01760d)), F + CH₄ and Cl + CH₄ in which bond selectivity and mode specificity are explored in great detail, but whose theoretical treatment still poses considerable challenges (Czakó (DOI: 10.1039/c0cp02456b)). Methods for increasing resolution in crossed molecular beam (CMB) experiments down to very low energies are also being developed (van de Meerakker (DOI: 10.1039/c0cp02405h)). In addition, soft-ionization detection methods, based on VUV tuneable synchrotron radiation (Lee (DOI: 10.1039/c0cp02439b)) or low-energy electron-impact (Casavecchia (DOI: 10.1039/c0cp01623c)), greatly facilitate the investigation of the dynamics of multichannel polyatomic reactions, as well as of photodissociation processes. Reactions of molecular radicals such as BO are also tackled by the CMB method (Kaiser (DOI: 10.1039/c0cp02458a)) and particularly important reactions, such as Cl + O₃, are treated theoretically (Castillo & Aoiz (DOI: 10.1039/c0cp02793f)). Ion imaging methods are used for studying H-atom abstraction reactions (Suits (10.1039/c1cp20137a)) and transition metal reactions (Honma (DOI: 10.1039/c0cp02256j)). Radical + radical reactions in CMB are now within reach using mass spectrometric (Casavecchia (DOI: 10.1039/c0cp01623c)) and laser induced fluorescence (Choi (DOI: 10.1039/c0cp02392b)) detection methods. Radical-radical reactions are also being treated theoretically (Honvault (DOI: 10.1039/c0cp02538k)). These kinds of processes, besides their fundamental relevance, are important in a host of gas-phase environments—flames, terrestrial and planetary atmospheres, the interstellar medium as well as plasmas and chemical vapour deposition.

Exciting progress in H-atom Rydberg tagging, ion-imaging techniques and ultra-fast laser methods have also started to allow us to explore molecular photodissociation (half-reaction) dynamics in deeper detail (Bañares (DOI: 10.1039/c0cp02515a)), with the “complete experiment” on the horizon (Janssen (DOI: 10.1039/c0cp02671a)). Examples of great progress are reported in the photodissociation of prototypical systems such as methane (Gauyacq (DOI: 10.1039/c0cp02627a)), atmospherically important species such as ClOOCl (Lin (DOI: 10.1039/c0cp02453h), Yang & Parker (DOI: 10.1039/c1cp00032b)), organic molecules (Perspective by Ashfold (DOI: 10.1039/c0cp02390f)) and radicals (Zhang (DOI: 10.1039/c1cp20310j), Neumark (DOI: 10.1039/c0cp02461a)). Rotational depolarization of photofragments is also being explored in detail (Brouard (DOI: 10.1039/c0cp02560g), Rakitzis (DOI: 10.1039/c0cp02451a)) as well as vector correlations in photodissociation (Vasyutinskii (DOI: 10.1039/c0cp01375g)). It is shown that ultra cold atoms can be produced by photodissociation (Softley (DOI: 10.1039/c0cp02472d)), whilst dissociative double photoionization of polyatomic molecules has been studied by synchrotron radiation (Vecchiocattivi (DOI: 10.1039/c0cp02678f)).

Importantly, the methods and concepts developed for gas-phase studies are also the basis of the molecular approach of investigations of photodissociation dynamics in liquids (Bradforth (DOI: 10.1039/c1cp20252a)) and reaction dynamics at interfaces, both gas-liquid (Nathanson (DOI: 10.1039/c0cp02540b), McKendrick (DOI: 10.1039/c0cp02734k)) and gas-solid surface (Wodtke (DOI: 10.1039/c0cp01418d), Gonzáles-Ureña (DOI: 10.1039/c0cp01869d)). Energy transfer studies are being extended (in the gas-phase) to systems of interstellar interest (Lique (DOI: 10.1039/c0cp02436h), Faure (DOI: 10.1039/c0cp02591g)), highly vibrationally excited molecules (Ni (DOI: 10.1039/c0cp02442b)), and also to gas-surface collisions (Farias (DOI: 10.1039/c0cp02425b)).

Cluster formation and dynamics are explored theoretically (Albertí & Huarte-Larrañaga (DOI: 10.1039/c0cp02426k)) and intermolecular potentials are developed for aggregate dynamics (Albertí (DOI: 10.1039/c0cp01763a)).

This themed issue is particularly timely, in view of the numerous upcoming International Conferences that will take place during summer 2011 and in which “Collision Dynamics” has traditionally been a central theme: (a) The XXIV International Symposium on Molecular Beams, Bordeaux, FRANCE, 23–26 May 2011, (b) 2011 Dynamics of Molecular Collisions Conference, Snowbird, Utah, USA, 10–15 July 2011, (c) International Symposium on Free Radicals, Port Douglas, AUSTRALIA, 24–29 July 2011, (d) COMET XXII (International Conference on Molecular Energy Transfer), Oxford, UK, 11–16 September 2011.

Although this issue collects work on a wide range of molecular collision phenomena, the papers share a common thread and language, as well as experimental and theoretical methodologies.

We thank all contributors for their important work, which highlights the current exciting progress in collision dynamics. We hope that the community will appreciate viewing their work collected.

We thank the PCCP staff for handling this themed issue in such a professional manner.

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