Periodic Reporting for period 1 - 123-CO (Spying on Ultrafast Structural Changes Through Three Sets of Eyes)
Reporting period: 2022-10-01 to 2024-09-30
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.
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.
To study the carbonyl photochemistry mentioned above, we first identified a few model systems: small carbonyl molecules which are well-suited to our experimental tools, and whose behavior can be studied by high-level quantum chemical theory (which becomes computationally infeasible for larger molecules). Time at the large accelerator facilities (“beamtime”) required to carry out this work is competitive, and must be applied for on a beamtime-by-beamtime basis. The project was extremely successful in this regard, being awarded time at multiple world-leading facilities (LCLS in California, MeV-UED in California and European XFEL in Hamburg) to carry out the key experiments outlined. In this section we will focus on the most important experiments which were central to the goals of the proposal, while the project did also support participation in a number of advanced experimental campaigns yielding important scientific results across gas-phase photochemistry and molecular physics.
Firstly at LCLS in October 2023, we performed a highly-successful experiment which used time-resolved hard X-ray scattering to study the photochemistry of two carbonyls: cyclobutanone and cyclopentanone, following the absorption of ultraviolet light. We were able to collect very high quality datasets where scattering patterns from both molecules were recorded at a range of delays after excitation by the ultraviolet pulse. These scattering patterns reveal the ultrafast dissociation dynamics of the molecules. Firstly a bond breaks within the molecule to disrupt the cyclic structure of the molecule, followed by emission of fragments. These ultrafast nuclear dynamics had not been directly observed previously. The analysis of these data is a complex task and is undergoing, as is high-level quantum simulations of the experimental data by a leading group in theoretical chemistry.
Secondly at the SLAC MeV-UED facility, we had a successful experiment studying the photochemistry of cyclobutanone and cyclopentanone using the ultrafast MeV electron diffraction technique. Similar to the LCLS experiment, scattering patterns are recorded at a range of time points within the photochemical reaction. Again high quality time-resolved data was recorded. In contrast to the X-ray scattering data, a higher spatial resolution can in principle be achieved, at the cost of poorer temporal resolution and signal-to-noise. However, one key unique capability offered by ultrafast electron diffraction is that there can be distinct signatures of populated electronic states, offering information beyond the nuclear dynamics. These electronic signatures were observed for both cyclobutanone and cyclopentanone and give key information regarding electronic relaxation processes, which our results show are significantly faster in cyclobutanone than cyclopentanone. Once more, these exciting dynamics are undergoing detailed analysis, and we expect to yield highly-informative insights into the photochemistry of molecules.
Finally, we performed a time-resolved X-ray Coulomb explosion imaging study at the European XFEL facility in Hamburg. The SQS endstation at European XFEL has recently pioneered this technique, described briefly in the previous section. To date, the key results using the technique have primarily focussed on imaging static molecular structure - extending the technique to time-resolved chemistry is a challenge, with our experiment being one of the first to attempt this. Initially the target molecule of the experiment was again cyclobutanone, but due to technical issues related to the ultraviolet laser pulse, this experiment would not have been feasible. Instead we studied another carbonyl molecule, trifluoroacetylacetone (TfAcAc), which absorbs UV light far more strongly, and is expected to exhibit similar photochemistry, involving ultrafast breaking of a cyclic structure. This experiment had to surmount some technical issues due to a hardware failure at SQS. However, in spite of this we were successful in recording time-resolved Coulomb explosion imaging data on TfAcAc. This experiment was performed most recently, and yields the most complex and highly-dimensional dataset, and so analysis is very preliminary. However, we saw signals in the coincident ion momentum distributions that are consistent with ultrafast ring-opening and distortion of the molecule following excitation.
Firstly at LCLS in October 2023, we performed a highly-successful experiment which used time-resolved hard X-ray scattering to study the photochemistry of two carbonyls: cyclobutanone and cyclopentanone, following the absorption of ultraviolet light. We were able to collect very high quality datasets where scattering patterns from both molecules were recorded at a range of delays after excitation by the ultraviolet pulse. These scattering patterns reveal the ultrafast dissociation dynamics of the molecules. Firstly a bond breaks within the molecule to disrupt the cyclic structure of the molecule, followed by emission of fragments. These ultrafast nuclear dynamics had not been directly observed previously. The analysis of these data is a complex task and is undergoing, as is high-level quantum simulations of the experimental data by a leading group in theoretical chemistry.
Secondly at the SLAC MeV-UED facility, we had a successful experiment studying the photochemistry of cyclobutanone and cyclopentanone using the ultrafast MeV electron diffraction technique. Similar to the LCLS experiment, scattering patterns are recorded at a range of time points within the photochemical reaction. Again high quality time-resolved data was recorded. In contrast to the X-ray scattering data, a higher spatial resolution can in principle be achieved, at the cost of poorer temporal resolution and signal-to-noise. However, one key unique capability offered by ultrafast electron diffraction is that there can be distinct signatures of populated electronic states, offering information beyond the nuclear dynamics. These electronic signatures were observed for both cyclobutanone and cyclopentanone and give key information regarding electronic relaxation processes, which our results show are significantly faster in cyclobutanone than cyclopentanone. Once more, these exciting dynamics are undergoing detailed analysis, and we expect to yield highly-informative insights into the photochemistry of molecules.
Finally, we performed a time-resolved X-ray Coulomb explosion imaging study at the European XFEL facility in Hamburg. The SQS endstation at European XFEL has recently pioneered this technique, described briefly in the previous section. To date, the key results using the technique have primarily focussed on imaging static molecular structure - extending the technique to time-resolved chemistry is a challenge, with our experiment being one of the first to attempt this. Initially the target molecule of the experiment was again cyclobutanone, but due to technical issues related to the ultraviolet laser pulse, this experiment would not have been feasible. Instead we studied another carbonyl molecule, trifluoroacetylacetone (TfAcAc), which absorbs UV light far more strongly, and is expected to exhibit similar photochemistry, involving ultrafast breaking of a cyclic structure. This experiment had to surmount some technical issues due to a hardware failure at SQS. However, in spite of this we were successful in recording time-resolved Coulomb explosion imaging data on TfAcAc. This experiment was performed most recently, and yields the most complex and highly-dimensional dataset, and so analysis is very preliminary. However, we saw signals in the coincident ion momentum distributions that are consistent with ultrafast ring-opening and distortion of the molecule following excitation.
As described above, the three key experiments, studying ultrafast carbonyl photochemistry using the aforementioned experimental techniques were all successful to varying degrees. Given the fact that these techniques are state of the art, and in many cases our work has gone beyond what has been done previously, we expect these results to be of great impact to the community interested in gas-phase photochemistry and these emerging probes of ultrafast structural dynamics. We are confident that our results represent the first detailed, direct measurements of the ultrafast nuclear dynamics of photoexcited carbonyl molecules, while they will also give invaluable information about how the relaxation of the initially populated electronic state(s) drive these dynamics.
The key experiments have not yet delivered publications related to the main objectives of the project. In this field this is not surprising: the timeline from performing a complex facility-based experiment to publication is often very extended owing to the complex rich datasets obtained. However, we are confident that several publications reporting these results will be delivered in the coming months and years.
The LCLS X-ray scattering experiment yielded some of the highest quality data recorded with this technique to date, and did so for two target molecules and with two X-ray probe photon energies. Consequently, we expect multiple publications (at least two) to result from this experiment. Given the high quality of the data, the extensive insights into the important photochemistry which was previously inaccessible, and our collaboration with a world-leading theory group, we anticipate these results will be of high impact and so we expect to target a high profile journal with broad readership.
The SLAC MeV-UED experiment’s results will be reported in most-likely two publications. The first of these, focussing solely on the cyclobutanone experiment, is already in the early stages of planning. These results will be of particular importance in the field as this experiment inspired a recent special issue in the Journal of Chemical Physics entitled “Prediction Challenge: Cyclobutanone Photochemistry”. For this special issue, dozens of leading groups in theoretical chemistry submitted their own predictions of the signals recorded in our experiment. Our upcoming report of the experimental data will therefore be highly influential in determining which theoretical approaches were most successful in describing this photochemistry, and why. This will culminate in a conference discussing the different theoretical approaches and their comparison to experiment, as well as a review article.
While the European XFEL results are only recently recorded, again we believe these will form an impactful publication, particularly as the time-resolved application of the X-ray Coulomb explosion imaging technique is in its infancy. The insights into how best to use these emerging experimental techniques will be invaluable for researchers in the field. Part of this will be disseminated in a review article which I am currently writing with Dr Wolf, discussing different methods for ultrafast structural imaging. As other researchers plan their future research goals, it is key to understand the strengths and limitations of these techniques, as well as identify areas for future developments.
The key experiments have not yet delivered publications related to the main objectives of the project. In this field this is not surprising: the timeline from performing a complex facility-based experiment to publication is often very extended owing to the complex rich datasets obtained. However, we are confident that several publications reporting these results will be delivered in the coming months and years.
The LCLS X-ray scattering experiment yielded some of the highest quality data recorded with this technique to date, and did so for two target molecules and with two X-ray probe photon energies. Consequently, we expect multiple publications (at least two) to result from this experiment. Given the high quality of the data, the extensive insights into the important photochemistry which was previously inaccessible, and our collaboration with a world-leading theory group, we anticipate these results will be of high impact and so we expect to target a high profile journal with broad readership.
The SLAC MeV-UED experiment’s results will be reported in most-likely two publications. The first of these, focussing solely on the cyclobutanone experiment, is already in the early stages of planning. These results will be of particular importance in the field as this experiment inspired a recent special issue in the Journal of Chemical Physics entitled “Prediction Challenge: Cyclobutanone Photochemistry”. For this special issue, dozens of leading groups in theoretical chemistry submitted their own predictions of the signals recorded in our experiment. Our upcoming report of the experimental data will therefore be highly influential in determining which theoretical approaches were most successful in describing this photochemistry, and why. This will culminate in a conference discussing the different theoretical approaches and their comparison to experiment, as well as a review article.
While the European XFEL results are only recently recorded, again we believe these will form an impactful publication, particularly as the time-resolved application of the X-ray Coulomb explosion imaging technique is in its infancy. The insights into how best to use these emerging experimental techniques will be invaluable for researchers in the field. Part of this will be disseminated in a review article which I am currently writing with Dr Wolf, discussing different methods for ultrafast structural imaging. As other researchers plan their future research goals, it is key to understand the strengths and limitations of these techniques, as well as identify areas for future developments.