In our group we combine cold molecular beams with state-of-the-art molecular control techniques, femtosecond laser pulses and velocity-map imaging of ions and electrons. A large part of our work is to develop novel approaches that enable us to explore dynamics in chemical systems in ever-greater detail, or to extend time-resolved methods to processes that we currently cannot study at ultrafast time-scales. This development of new methodologies is driven by scientific curiosity as well as technological advances.
Click on one of the images to read more:
Isomer Effects in ultrafast photochemistry
Dynamics of electron-driven processes
Photoelectron-Photoion coincidence imaging of UV-induced photochemistry
Imaging chriality using photoelectron circular dichroism (PECD)
You can also check out some recent posters from the group in the gallery!
Isomer Effects in ultrafast photochemistry
We know that in nature small structural changes – the rotation around a single bond or the relocation of a hydrogen atom – can significantly alter chemical functionality. But actually studying the structure-function relationship on this level in the gas-phase is very challenging, as it is often impossible to separate these slight different molecular structures within a molecular beam. Yet this is exactly what we aim to do in this project by combining the electrostatic deflection technique with time-resolved photoelectron imaging. The electrostatic deflector allow us to select a single structural isomer (such as a conformer or tautomer) on the basis of its dipole moment. The isomer-selected molecular beam will then enter a velocity-map imaging setup, where we will perform ultrafast time-resolved photoelectron imaging experiments. This combination allows us to investigate how small structural changes, such as isomerism, influence the underlying photochemistry. For example, we can begin to address questions such as the influence of structural isomerism on UV photostability.
@article{Teschmit:Angew.Chem.Int.Ed.57:13775,
title = {Spatially Separating the Conformers of a Dipeptide},
author = {Nicole Teschmit and Daniel A. Horke and Jochen K\"{u}pper},
url = {https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201807646},
doi = {10.1002/anie.201807646},
year = {2018},
date = {2018-08-01},
journal = {Angew. Chem. Int. Ed.},
volume = {57},
pages = {13775--13779},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
@article{Horke:Phys.Rev.Lett.117:163002,
title = {Hydrogen Bonds in Excited State Proton Transfer},
author = {D A Horke and H M Watts and A D Smith and E Jager and E Springate and O Alexander and C Cacho and R T Chapman and R S Minns},
url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.163002},
doi = {10.1103/PhysRevLett.117.163002},
year = {2016},
date = {2016-10-01},
journal = {Phys. Rev. Lett.},
volume = {117},
number = {16},
pages = {163002},
abstract = {Hydrogen bonding may safeguard biomolecules against the damaging effects of UV light.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
@article{Chang:Int.Rev.Phys.Chem.34:557,
title = {Spatially-Controlled Complex Molecules and Their Applications},
author = {Yuan-Pin Chang and Daniel A Horke and Sebastian Trippel and Jochen K\"{u}pper},
url = {http://www.tandfonline.com/doi/full/10.1080/0144235X.2015.1077838},
doi = {10.1080/0144235X.2015.1077838},
year = {2015},
date = {2015-10-01},
journal = {Int. Rev. Phys. Chem.},
volume = {34},
number = {4},
pages = {557--590},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Studying the ultrafast dynamics of photon-driven processes (i.e. photochemistry!) is now a well established method. However, there is more to chemistry than photon-driven processes and reactions. In this new project we aim to ‘transfer’ many of the techniques we have developed to study photon-driven processes towards studying electron-driven processes (i.e. electrochemistry!). We aim to establish time-resolved electron-pump photon-probe spectroscopy to directly follow chemical processes initiated by electrons. In particular, we are interested in (dissociative) electron attachment reactions. These are at the heart of many biological damage processes, both detrimental (ionising radiation that destroys DNA) and beneficial (radiation therapy as a cancer treatment).
@article{Horke:NatureChem.5:711,
title = {Ultrafast Above-Threshold Dynamics of the Radical Anion of a Prototypical Quinone Electron-Acceptor},
author = {Daniel A Horke and Quansong Li and Llu A Blancafort and Jan R R Verlet},
url = {http://dx.doi.org/10.1038/nchem.1705},
doi = {10.1038/nchem.1705},
year = {2013},
date = {2013-09-01},
journal = {Nature Chem.},
volume = {5},
number = {8},
pages = {711--717},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
@article{Horke:J.Phys.Chem.A115:8369,
title = {Excited States in Electron-Transfer Reaction Products: Ultrafast Relaxation Dynamics of an Isolated Acceptor Radical Anion},
author = {Daniel A Horke and Gareth M Roberts and Jan R R Verlet},
url = {http://dx.doi.org/10.1021/jp2038202},
doi = {10.1021/jp2038202},
year = {2011},
date = {2011-01-01},
journal = {J. Phys. Chem. A},
volume = {115},
number = {30},
pages = {8369--8374},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Photoelectron-Photoion coincidence imaging of UV-induced photochemistry
At the heart of photochemistry is unravelling the competition between the different pathways a molecule can follow after excitation by a photon, such as dissociation or internal conversion. A powerful method to follow these processes and gain a complete understanding of the photochemistry is coincidence imaging, where we record both the produced photoion and photoelectron follow ionisation of a molecule. Furthermore, we record them ‘in coincidence’, such that we can precisely correlate which electron and ion belong together. Combined with femtosecond pump-probe spectroscopy this yields an extremely detailed view of how molecules deal with excess energy following excitation, and how this energy is redistributed through the molecule and on which timescales.
In collaboration with the group of Russell Minns (Southampton, UK) we are furthermore working on using VUV radiation produced through high-harmonic generation as a probe. The energy of a single VUV photon is typically sufficient to directly ionise any molecular species. This approach therefore allow us to really follow the dynamics of a chemical reaction – from reactants through intermediates to products – with femtosecond time resolution.
@article{Caballo:J.Phys.Chem.A125:9060,
title = {Femtosecond 2 + 1 Resonance-Enhanced Multiphoton Ionization Spectroscopy of the C-State in Molecular Oxygen},
author = {Ana Caballo and Anders J. T. M. Huits and Arno Vredenborg and Michiel Balster and David H. Parker and Daniel A. Horke},
url = {https://doi.org/10.1021/acs.jpca.1c05541},
doi = {10.1021/acs.jpca.1c05541},
issn = {1089-5639},
year = {2021},
date = {2021-10-01},
journal = {J. Phys. Chem. A},
volume = {125},
number = {41},
pages = {9060--9064},
publisher = {American Chemical Society},
abstract = {Coincidence electron-cation imaging is used to characterize the multiphoton ionization of O2 via the v = 4,5 levels of the 3s(3$Pi$g) Rydberg state. A tunable 100 fs laser beam operating in the 271\textendash 263 nm region is found to cause a nonresonant ionization across this wavelength range, with an additional resonant ionization channel only observed when tuned to the 3$Pi$g(v = 5) level. A distinct 3s textrightarrow p wave character is observed in the photoelectron angular distribution for the v = 5 channel when on resonance.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Coincidence electron-cation imaging is used to characterize the multiphoton ionization of O2 via the v = 4,5 levels of the 3s(3$Pi$g) Rydberg state. A tunable 100 fs laser beam operating in the 271– 263 nm region is found to cause a nonresonant ionization across this wavelength range, with an additional resonant ionization channel only observed when tuned to the 3$Pi$g(v = 5) level. A distinct 3s textrightarrow p wave character is observed in the photoelectron angular distribution for the v = 5 channel when on resonance.
@article{Warne:J.Chem.Phys.154:034302,
title = {Time Resolved Detection of the S(1D) Product of the UV Induced Dissociation of CS2},
author = {Emily M. Warne and Adam D. Smith and Daniel A. Horke and Emma Springate and Alfred J. H. Jones and Cephise Cacho and Richard T. Chapman and Russell S. Minns},
url = {http://aip.scitation.org/doi/10.1063/5.0035045},
doi = {10.1063/5.0035045},
issn = {0021-9606, 1089-7690},
year = {2021},
date = {2021-01-01},
urldate = {2021-06-21},
journal = {J. Chem. Phys.},
volume = {154},
number = {3},
pages = {034302},
abstract = {The products formed following the photodissociation of UV (200 nm) excited CS2 are monitored in a time resolved photoelectron spectroscopy experiment using femtosecond XUV (21.5 eV) photons. By spectrally resolving the electrons, we identify separate photoelectron bands related to the CS2 + h$nu$ textrightarrow S(1D) + CS and CS2 + h$nu$ textrightarrow S(3P) + CS dissociation channels, which show different appearance and rise times. The measurements show that there is no delay in the appearance of the S(1D) product contrary to the results of Horio et al. [J. Chem. Phys. 147, 013932 (2017)]. Analysis of the photoelectron yield associated with the atomic products allows us to obtain a S(3P)/S(1D) branching ratio and the rate constants associated with dissociation and intersystem crossing rather than the effective lifetime observed through the measurement of excited state populations alone.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The products formed following the photodissociation of UV (200 nm) excited CS2 are monitored in a time resolved photoelectron spectroscopy experiment using femtosecond XUV (21.5 eV) photons. By spectrally resolving the electrons, we identify separate photoelectron bands related to the CS2 + h$nu$ textrightarrow S(1D) + CS and CS2 + h$nu$ textrightarrow S(3P) + CS dissociation channels, which show different appearance and rise times. The measurements show that there is no delay in the appearance of the S(1D) product contrary to the results of Horio et al. [J. Chem. Phys. 147, 013932 (2017)]. Analysis of the photoelectron yield associated with the atomic products allows us to obtain a S(3P)/S(1D) branching ratio and the rate constants associated with dissociation and intersystem crossing rather than the effective lifetime observed through the measurement of excited state populations alone.
@article{Smith:Phys.Rev.Lett.120:183003,
title = {Mapping the Complete Reaction Path of a Complex Photochemical Reaction},
author = {Adam D. Smith and Emily M. Warne and Darren Bellshaw and Daniel A. Horke and Maria Tudorovskya and Emma Springate and Alfred J. H. Jones and Cephise Cacho and Richard T. Chapman and Adam Kirrander and Russell S. Minns},
url = {https://link.aps.org/doi/10.1103/PhysRevLett.120.183003},
doi = {10.1103/PhysRevLett.120.183003},
year = {2018},
date = {2018-05-01},
urldate = {2019-01-24},
journal = {Phys. Rev. Lett.},
volume = {120},
number = {18},
pages = {183003},
abstract = {We probe the dynamics of dissociating CS2 molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We probe the dynamics of dissociating CS2 molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.
Imaging chirality using photoelectron circular dichroism (PECD)
As part of a large consortium of partners from both academia and industry we are working on developing photoelectron circular dichroism (PECD) spectroscopy as an analytical tool. PECD allows one to distinguish different enantiomers of chiral molecules in the gas-phase. It relies on imaging the photoelectrons produced following ionisation by circular-polarised light, and has been shown to reliably measure enantiomeric excess of species. Working closely together with the start-up MassSpecpecD, we are developing a compact PECD-spectrometer for analytical applications.
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@article{PhysRevApplied.17.044044,
title = {Optical Funnel to Guide and Focus Virus Particles for X-Ray Diffractive Imaging},
author = {Salah Awel and Sebastian Lavin-Varela and Nils Roth and Daniel A. Horke and Andrei V. Rode and Richard A. Kirian and Jochen K\"{u}pper and Henry N. Chapman},
url = {https://link.aps.org/doi/10.1103/PhysRevApplied.17.044044},
doi = {10.1103/PhysRevApplied.17.044044},
year = {2022},
date = {2022-04-01},
journal = {Phys. Rev. Applied},
volume = {17},
number = {4},
pages = {044044},
publisher = {American Physical Society},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
@article{Zhuang:IUCrJ9:204,
title = {Unsupervised Learning Approaches to Characterizing Heterogeneous Samples Using X-ray Single-Particle Imaging},
author = {Yulong Zhuang and Salah Awel and Anton Barty and Richard Bean and Johan Bielecki and Martin Bergemann and Benedikt J. Daurer and Tomas Ekeberg and Armando D. Estillore and Hans Fangohr and Klaus Giewekemeyer and Mark S. Hunter and Mikhail Karnevskiy and Richard A. Kirian and Henry Kirkwood and Yoonhee Kim and Jayanath Koliyadu and Holger Lange and Romain Letrun and Jannik L\"{u}bke and Abhishek Mall and Thomas Michelat and Andrew J. Morgan and Nils Roth and Amit K. Samanta and Tokushi Sato and Zhou Shen and Marcin Sikorski and Florian Schulz and John C. H. Spence and Patrik Vagovic and Tamme Wollweber and Lena Worbs and P. Lourdu Xavier and Oleksandr Yefanov and Filipe R. N. C. Maia and Daniel A. Horke and Jochen K\"{u}pper and N. Duane Loh and Adrian P. Mancuso and Henry N. Chapman and Kartik Ayyer},
url = {https://scripts.iucr.org/cgi-bin/paper?S2052252521012707},
doi = {10.1107/S2052252521012707},
issn = {2052-2525},
year = {2022},
date = {2022-03-01},
urldate = {2022-03-14},
journal = {IUCrJ},
volume = {9},
number = {2},
pages = {204--214},
abstract = {One of the outstanding analytical problems in X-ray single-particle imaging (SPI) is the classification of structural heterogeneity, which is especially difficult given the low signal-to-noise ratios of individual patterns and the fact that even identical objects can yield patterns that vary greatly when orientation is taken into consideration. Proposed here are two methods which explicitly account for this orientation-induced variation and can robustly determine the structural landscape of a sample ensemble. The first, termed common-line principal component analysis (PCA), provides a rough classification which is essentially parameter free and can be run automatically on any SPI dataset. The second method, utilizing variation auto-encoders (VAEs), can generate 3D structures of the objects at any point in the structural landscape. Both these methods are implemented in combination with the noise-tolerant expand\textendash maximize\textendash compress ( EMC ) algorithm and its utility is demonstrated by applying it to an experimental dataset from gold nanoparticles with only a few thousand photons per pattern. Both discrete structural classes and continuous deformations are recovered. These developments diverge from previous approaches of extracting reproducible subsets of patterns from a dataset and open up the possibility of moving beyond the study of homogeneous sample sets to addressing open questions on topics such as nanocrystal growth and dynamics, as well as phase transitions which have not been externally triggered.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
One of the outstanding analytical problems in X-ray single-particle imaging (SPI) is the classification of structural heterogeneity, which is especially difficult given the low signal-to-noise ratios of individual patterns and the fact that even identical objects can yield patterns that vary greatly when orientation is taken into consideration. Proposed here are two methods which explicitly account for this orientation-induced variation and can robustly determine the structural landscape of a sample ensemble. The first, termed common-line principal component analysis (PCA), provides a rough classification which is essentially parameter free and can be run automatically on any SPI dataset. The second method, utilizing variation auto-encoders (VAEs), can generate 3D structures of the objects at any point in the structural landscape. Both these methods are implemented in combination with the noise-tolerant expand– maximize– compress ( EMC ) algorithm and its utility is demonstrated by applying it to an experimental dataset from gold nanoparticles with only a few thousand photons per pattern. Both discrete structural classes and continuous deformations are recovered. These developments diverge from previous approaches of extracting reproducible subsets of patterns from a dataset and open up the possibility of moving beyond the study of homogeneous sample sets to addressing open questions on topics such as nanocrystal growth and dynamics, as well as phase transitions which have not been externally triggered.
@article{Lubke:J.Phys.Chem.C125:25794,
title = {Charge-State Distribution of Aerosolized Nanoparticles},
author = {Jannik L\"{u}bke and Nils Roth and Lena Worbs and Daniel A. Horke and Armando D. Estillore and Amit K. Samanta and Jochen K\"{u}pper},
url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.1c06912},
doi = {10.1021/acs.jpcc.1c06912},
issn = {1932-7447, 1932-7455},
year = {2021},
date = {2021-11-01},
urldate = {2022-01-03},
journal = {J. Phys. Chem. C},
volume = {125},
number = {46},
pages = {25794--25798},
abstract = {In single-particle imaging experiments, beams of individual nanoparticles are exposed to intense pulses of X-rays from free-electron lasers to record diffraction patterns of single, isolated molecules. The reconstruction for structure determination relies on signals from many identical particles. Therefore, well-defined-sample delivery conditions are desired in order to achieve sample uniformity, including avoidance of charge polydispersity. We have observed charging of 220 nm polystyrene particles in an aerosol beam created by a gas-dynamic virtual nozzle focusing technique, without intentional charging of the nanoparticles. Here, we present a deflection method for detecting and characterizing the charge states of a beam of aerosolized nanoparticles. Our analysis of the observed charge-state distribution using optical light-sheet localization microscopy and quantitative particle trajectory simulations is consistent with previous descriptions of skewed charging probabilities of triboelectrically charged nanoparticles.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In single-particle imaging experiments, beams of individual nanoparticles are exposed to intense pulses of X-rays from free-electron lasers to record diffraction patterns of single, isolated molecules. The reconstruction for structure determination relies on signals from many identical particles. Therefore, well-defined-sample delivery conditions are desired in order to achieve sample uniformity, including avoidance of charge polydispersity. We have observed charging of 220 nm polystyrene particles in an aerosol beam created by a gas-dynamic virtual nozzle focusing technique, without intentional charging of the nanoparticles. Here, we present a deflection method for detecting and characterizing the charge states of a beam of aerosolized nanoparticles. Our analysis of the observed charge-state distribution using optical light-sheet localization microscopy and quantitative particle trajectory simulations is consistent with previous descriptions of skewed charging probabilities of triboelectrically charged nanoparticles.
