Photodissociation and reaction dynamics

Andrew J. Orr-Ewing
School of Chemistry, University of Bristol, Bristol BS8 1TS, UK. E-mail:

Received 10th June 2019 , Accepted 10th June 2019
The Photodissociation and reaction dynamics themed issue of Physical Chemistry Chemical Physics contains 59 papers on a wide range of topics, reflecting both the diversity and broad impact of international research in this interdisciplinary field. It is a research area in which chemistry and physics meet to explore chemical reactivity at a molecular level of understanding. It is also a field in which cutting-edge developments in experimental and theoretical methods drive ever-deeper new insights. Indeed, many of the papers in this special issue combine laboratory measurements with computational simulations to generate rigorous and profound descriptions of chemical dynamics.

Modern researchers have many powerful techniques at their disposal to dissect reaction mechanisms. From an experimental perspective, these include ultrafast laser pulses to track changes in electronic structure and associated molecular motions in real time, and velocity map imaging (VMI) methods to take snapshots of the products of reactions that capture the ways in which they form. Theorists can exploit the ever-increasing accuracy and efficiency of electronic structure calculations to compute the energy landscapes over which reactions occur, and simulate the quantum-mechanical behaviour of both photochemical and collisional dynamics to predict the outcomes. With this greater understanding comes the possibility of manipulating chemistry using external forces, such as intense electric fields from laser beams. Traditionally the domain of studies of isolated molecules in the gas-phase, photodissociation and reaction dynamics research is now extending to interfaces and condensed phases. All of these advances are reflected in the high-quality papers making up this themed issue.

Photochemistry is driven by absorption of light, which raises molecules to electronically excited states lying high enough in energy to promote rearrangement or cleavage of chemical bonds. The optically bright state accessed is not always the one responsible for the photochemical changes, and non-adiabatic crossings at conical intersections between electronic states are increasingly recognized to control dynamical pathways. Several of the papers in this themed issue explore the nature of electronically excited states of molecules (DOI: 10.1039/C8CP06952B and 10.1039/C8CP02757A) including ionized forms (DOI: 10.1039/C8CP06751A, 10.1039/C8CP06640J, 10.1039/C8CP07032F, and 10.1039/C9CP02385B), the ultrafast internal conversion dynamics at conical intersections (DOI: 10.1039/C8CP04426K, 10.1039/C8CP05693E, 10.1039/C8CP06895J, 10.1039/C8CP07189F, 10.1039/C8CP05264F, and 10.1039/C8CP03273D) and the fates of the energized molecules (DOI: 10.1039/C8CP06354K, 10.1039/C8CP07061J, 10.1039/C8CP06412A, 10.1039/C8CP07620K, 10.1039/C8CP07292B, 10.1039/C8CP06482B, 10.1039/C8CP04130J, 10.1039/C8CP04270E, 10.1039/C8CP06738D, 10.1039/C8CP07079B, 10.1039/C8CP06818F, and 10.1039/C9CP00113A). Time resolution in the femtosecond regime, and advances in two-dimensional spectroscopy, allow these dynamical pathways to be observed experimentally (DOI: 10.1039/C8CP04426K, 10.1039/C8CP06895J, and 10.1039/C8CP05264F), while the application of strong external fields can influence – or even catalyse – the photochemical outcomes (DOI: 10.1039/C8CP06107F and 10.1039/C8CP07248E). The photoinduced dynamics are also sensitive to the environment of the molecules, and the effects of explicit intermolecular interactions are explored in clusters and in bulk liquids (DOI: 10.1039/C8CP06656F, 10.1039/C8CP06536E, 10.1039/C8CP05305G, and 10.1039/C9CP01226E). Although much of this research is fundamental in nature, the wider impact is illustrated in studies of UV photochemical pathways in nucleobases (DOI: 10.1039/C8CP07189F and 10.1039/C8CP07864E), solar energy conversion (DOI: 10.1039/C8CP06658B), and protection from solar UV damage (DOI: 10.1039/C8CP06536E and 10.1039/C8CP06794E). In place of photons, collisions with an electron can activate chemical reactions, and techniques such as VMI are now successfully being applied to study electron–molecule interactions (DOI: 10.1039/C8CP06660D and 10.1039/C8CP06682E), which have importance in planetary atmospheres or in technological plasmas used for materials deposition and processing.

In contrast, molecular collisions, leading to exchange of energy, ionization or chemical reactions (DOI: 10.1039/C8CP06690F, 10.1039/C8CP07572G, 10.1039/C8CP06810K and 10.1039/C8CP06351F), mostly occur in the ground electronic states of the colliding species, although excited states can play a role (DOI: 10.1039/C8CP06666C, 10.1039/C8CP06598E, 10.1039/C8CP06917D and 10.1039/C8CP07015F). The level of detail now accessible to researchers exploring collisional events is illustrated by several papers in the special issue, for example through control over collision geometries (DOI: 10.1039/C8CP06225K and 10.1039/C8CP06892E), or correlated measurements of the quantum states and scattering directions of the product molecules (DOI: 10.1039/C8CP07109H and 10.1039/C8CP03443E). Again, studies are no longer restricted to the gas phase, and comparably detailed measurements and simulations are now possible in molecular clusters (DOI: 10.1039/C8CP05959D), bulk liquids (DOI: 10.1039/C8CP06686H), and at interfaces (DOI: 10.1039/C8CP06856A), with importance for our understanding of both homogeneous and heterogeneous catalysis. Bimolecular reactions play a vital role in the chemistry of the atmospheres of Earth and other planets (DOI: 10.1039/C8CP07572G), as well as interstellar space (DOI: 10.1039/C8CP03921F), and experimental and computational techniques deriving from fundamental studies of reaction dynamics are now enriching these fields (DOI: 10.1039/C8CP06810K), as well as others including combustion, pyrolysis (DOI: 10.1039/C8CP06669H), and plasma science.

Molecular spectroscopy lies at the heart of many of the experimental studies reported in the themed issue, because it is the provider of quantum-state resolved information (DOI: 10.1039/C8CP06493H). Moreover, advanced spectroscopic techniques can identify the structures and properties of conformationally flexible molecules (DOI: 10.1039/C9CP00140A) and weakly interacting molecular complexes (DOI: 10.1039/C8CP05995K, 10.1039/C8CP05934A and 10.1039/C8CP06581K), and can resolve reactive intermediates with transient existence along the pathways of textbook reactions, thereby testing the received wisdom for reaction mechanisms (DOI: 10.1039/C8CP06593D).

The publication of the Photodissociation and reaction dynamics themed issue of PCCP coincides with the 65th birthday of Michael Ashfold, who is internationally recognized as a world-leader in the study of photochemical dynamics. Mike's distinguished scientific contributions in the field date back over 40 years, and include many ground-breaking papers published in PCCP. His co-authored review article on photoinduced C–H bond fission is typically broad in its scope and significance, and is a fitting centrepiece for the themed issue (DOI: 10.1039/C8CP07454B). In addition to his considerable scientific achievements, Mike played a key role in establishing PCCP as a premier international journal through his service as Founding Chair of the Editorial Board in 1999, and again as Chair of the Editorial Board from 2003–2005. It is therefore appropriate that this themed issue celebrates the occasion of Mike's 65th birthday. I am sure all of his friends and colleagues in the scientific community will join me in wishing him well.

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