123-CO · Spying on Ultrafast Structural Changes Through Three Sets of Eyes

Spying on Ultrafast Structural Changes Through Three Sets of Eyes

Chemical reactions driven by the absorption of light (“photochemistry”) are of great importance to modern society. For example, they allow us to efficiently and sustainably convert the sun’s energy into electricity. Understanding the detailed mechanisms of photochemical reactions is important in modeling the evolving chemical makeup of our atmosphere, particularly given the profound influence of manmade emissions and in the wake of catastrophic climate change. The most-detailed level of understanding comes from state-of-the-art experimental tools which can study how individual molecules evolve after interacting with light. The molecular motion that underpins photochemistry involves the rearrangement of atomic nuclei on a timescale of femtoseconds (millionths of a billionth of a second), that is caused by the rearrangement of a molecule’s electrons, and is governed by the complex laws of quantum mechanics. Direct observation of these motions has traditionally been extremely challenging, and traditionally we have had to study such photochemistry using indirect experimental observables, which can leave substantial ambiguity in their interpretation and result in profound gaps in our photochemical understanding. This project aims to advance our understanding of the photochemistry of a particular class of organic molecules (“carbonyls”), which are continuously emitted into the atmosphere in vast quantities from both natural and anthropogenic sources. To understand their impact on the overall chemical makeup of the atmosphere, which is constantly being affected by man-made emissions, we must understand the chemistry of these molecules after absorbing light. Ultimately, a better understanding of this fundamental chemistry can be invaluable for building important, but incredibly complex, models of our atmosphere. In particular, we will use three emerging experimental methods that have been recently developed, and can directly image the nuclear motions occurring during photochemical reactions. Importantly, the three techniques (depicted in Fig. 1) offer complementary structural information, and so by targeting the same chemistry with all methods we hope to achieve a fundamental level of chemical understanding which could not be obtained through previous approaches (or by using one of these techniques alone). All of these experimental methods have been enabled by recent developments in accelerator technology. The first two, ultrafast electron diffraction and ultrafast X-ray diffraction involve scattering a high-energy pulse of electrons or X-rays off a molecule, and recording the pattern of the scattered particles, which directly relates to the nuclear structure (i.e. shape) of the molecule. Taking many such structural snapshots at different times after initiating a photochemical reaction (with an ultrashort laser pulse) allows us to record so-called “molecular movies”. The final technique, Coulomb explosion imaging, uses an intense X-ray pulse to strip the molecule of many of its electrons. Without these electrons, which glue the molecule together, the molecule violently explodes. Measuring the relative velocities of these repelling fragments allows us to reconstruct the molecular structure. The work planned in this project will advance the state-of-the-art with these emerging experimental tools, and inform us on how best to use these methods to study chemistry of societal importance at the smallest length and time scales. This will be achieved through a series of experimental campaigns at international accelerator facilities where these unique studies can be carried out. The experimental results will be compared to the most advanced quantum mechanical simulations available - fueling the development of new theoretical tools with which we can tackle photochemistry of ever increasing relevance. We hope that the specific insights into the chemistry of carbonyl molecules will allow us to better understand the behavior of this broad class of molecules in the atmosphere. Additionally, our work will help inform the field about how best to use these newly-developed experimental tools to understand a range of physical and chemical phenomena of isolated molecules.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз