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
Novel methods for ultrafast 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{abmaSinglecolorIsomerresolvedSpectroscopy2022,
title = {Single-Color Isomer-Resolved Spectroscopy},
author = {Grite L. Abma and Dries Kleuskens and Siwen Wang and Michiel Balster and Andre Roij and Niek Janssen and Daniel A. Horke},
url = {https://doi.org/10.1021/acs.jpca.2c02277},
doi = {10.1021/acs.jpca.2c02277},
year = {2022},
date = {2022-06-01},
urldate = {2022-06-01},
journal = {The Journal of Physical Chemistry A},
volume = {126},
pages = {3811\textendash3815},
abstract = {Structural isomers, such as conformers or tautomers, are
of significant importance across chemistry and biology, as they can have
different functionalities. In gas-phase experiments using molecular
beams, formation of many different isomers cannot be prevented, and
their presence significantly complicates the assignment of spectral lines.
Current isomer-resolved spectroscopic techniques heavily rely on
theoretical calculations or make use of elaborate double-resonance
schemes. We show here that isomer-resolved spectroscopy can also be
performed using a single tunable laser. In particular, we demonstrate
single-color isomer-resolved spectroscopy by utilizing electrostatic
deflection to spatially separate the isomers. We show that for 3-
aminophenol we can spatially separate the syn and anti conformers and
use these pure samples to perform high-resolution REMPI spectroscopy, making the assignment of transitions to a particular isomer trivial, without any additional a priori information. This approach allows one to add isomer specificity to any molecular-beam-based experiment.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Structural isomers, such as conformers or tautomers, are
of significant importance across chemistry and biology, as they can have
different functionalities. In gas-phase experiments using molecular
beams, formation of many different isomers cannot be prevented, and
their presence significantly complicates the assignment of spectral lines.
Current isomer-resolved spectroscopic techniques heavily rely on
theoretical calculations or make use of elaborate double-resonance
schemes. We show here that isomer-resolved spectroscopy can also be
performed using a single tunable laser. In particular, we demonstrate
single-color isomer-resolved spectroscopy by utilizing electrostatic
deflection to spatially separate the isomers. We show that for 3-
aminophenol we can spatially separate the syn and anti conformers and
use these pure samples to perform high-resolution REMPI spectroscopy, making the assignment of transitions to a particular isomer trivial, without any additional a priori information. This approach allows one to add isomer specificity to any molecular-beam-based experiment.
@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}
}
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).
Novel methods for ultrafast 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 XUV radiation produced through high-harmonic generation as a probe. The energy of a single XUV 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{caballoDisentanglingMultiphotonIonization2023,
title = {Disentangling Multiphoton Ionization and Dissociation Channels in Molecular Oxygen Using Photoelectron\textendashPhotoion Coincidence Imaging},
author = {Ana Caballo and Anders J. T. M. Huits and David H. Parker and Daniel A. Horke},
url = {https://pubs.acs.org/doi/10.1021/acs.jpca.2c06707},
doi = {10.1021/acs.jpca.2c06707},
issn = {1089-5639, 1520-5215},
year = {2023},
date = {2023-01-01},
urldate = {2023-01-01},
journal = {J. Phys. Chem. A},
volume = {127},
number = {1},
pages = {92--98},
abstract = {Multiphoton excitation of molecular oxygen in the 392-408 nm region is studied using a tunable femtosecond laser coupled with a double velocity map imaging photoelectron- photoion coincidence spectrometer. The laser intensity is held at $\leqsim$1 TW/cm2 to ensure excitation in the perturbative regime, where the possibility of resonance enhanced multiphoton ionization (REMPI) can be investigated. O2+ production is found to be resonance enhanced around 400 nm via three-photon excitation to the e$'$3$Delta$u(v = 0) state, similar to results from REMPI studies using nanosecond dye lasers. O+ production reaches 7% of the total ion yield around 405 nm due to two processes: autoionization following five-photon excitation of O2, producing O2+(X(v)) in a wide range of vibrational states followed by two- or three-photon dissociation, or six-photon excitation to a superexcited O2** state followed by neutral dissociation and subsequent ionization of the electronically excited O atom. Coincidence detection is shown to be crucial in identifying these competing pathways.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Multiphoton excitation of molecular oxygen in the 392-408 nm region is studied using a tunable femtosecond laser coupled with a double velocity map imaging photoelectron- photoion coincidence spectrometer. The laser intensity is held at $łeqsim$1 TW/cm2 to ensure excitation in the perturbative regime, where the possibility of resonance enhanced multiphoton ionization (REMPI) can be investigated. O2+ production is found to be resonance enhanced around 400 nm via three-photon excitation to the e$'$3$Delta$u(v = 0) state, similar to results from REMPI studies using nanosecond dye lasers. O+ production reaches 7% of the total ion yield around 405 nm due to two processes: autoionization following five-photon excitation of O2, producing O2+(X(v)) in a wide range of vibrational states followed by two- or three-photon dissociation, or six-photon excitation to a superexcited O2** state followed by neutral dissociation and subsequent ionization of the electronically excited O atom. Coincidence detection is shown to be crucial in identifying these competing pathways.
@article{Caballo:J.Phys.Chem.A125:9060,
title = {Femtosecond 2 + 1 Resonance-Enhanced Multiphoton Ionization Spectroscopy of the C-State in Molecular Oxygen},
author = {Ana Caballo and Anders J. T. M. Huits and Arno Vredenborg and Michiel Balster and David H. Parker and Daniel A. Horke},
url = {https://doi.org/10.1021/acs.jpca.1c05541},
doi = {10.1021/acs.jpca.1c05541},
issn = {1089-5639},
year = {2021},
date = {2021-10-01},
journal = {J. Phys. Chem. A},
volume = {125},
number = {41},
pages = {9060--9064},
publisher = {American Chemical Society},
abstract = {Coincidence electron-cation imaging is used to characterize the multiphoton ionization of O2 via the v = 4,5 levels of the 3s(3$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.
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.
Grite is off to the FEMTO15 conference in Berlin, presenting our work on using UV-XUV pump-probe spectroscopy to follow roaming dynamics in acetaldehyde. You can also find the poster in our gallery.
Our latest paper on vaporization of intact neutral biomolecules using laser-based thermal desorption is out now in J. Am. Soc. Mass. Spec. – Well done Yerbolat and Siwen! https://doi.org/10.1021/jasms.3c00194
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@article{molecules28135058,
title = {High-Throughput UV Photoionization and Fragmentation of Neutral Biomolecules as a Structural Fingerprint},
author = {Siwen Wang and Yerbolat Dauletyarov and Daniel A. Horke},
doi = {10.3390/molecules28135058},
issn = {1420-3049},
year = {2023},
date = {2023-06-28},
urldate = {2023-06-28},
journal = {Molecules},
volume = {28},
number = {13},
pages = {5058},
abstract = {We present UV photofragmentation studies of the structural isomers paracetamol, 3-Pyridinepropionic acid (3-PPIA) and (R)-(-)-2-Phenylglycine. In particular, we utilized a new laser-based thermal desorption source in combination with femtosecond multiphoton ionization at 343 nm and 257 nm. The continuous nature of our molecule source, combined with the 50 kHz repetition rate of the laser, allowed us to perform these experiments at high throughput. In particular, we present detailed laser intensity dependence studies at both wavelengths, producing 2D mass spectra with highly differential information about the underlying fragmentation processes. We show that UV photofragmentation produces highly isomer-specific mass spectra, and assign all major fragmentation pathways observed. The intensity-dependence measurements, furthermore, allowed us to evaluate the appearance intensities for each fragmentation channel, which helped to distinguish competing from consecutive fragmentation pathways.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We present UV photofragmentation studies of the structural isomers paracetamol, 3-Pyridinepropionic acid (3-PPIA) and (R)-(-)-2-Phenylglycine. In particular, we utilized a new laser-based thermal desorption source in combination with femtosecond multiphoton ionization at 343 nm and 257 nm. The continuous nature of our molecule source, combined with the 50 kHz repetition rate of the laser, allowed us to perform these experiments at high throughput. In particular, we present detailed laser intensity dependence studies at both wavelengths, producing 2D mass spectra with highly differential information about the underlying fragmentation processes. We show that UV photofragmentation produces highly isomer-specific mass spectra, and assign all major fragmentation pathways observed. The intensity-dependence measurements, furthermore, allowed us to evaluate the appearance intensities for each fragmentation channel, which helped to distinguish competing from consecutive fragmentation pathways.
@article{dauletyarovVaporizationIntactNeutral2023,
title = {Vaporization of Intact Neutral Biomolecules Using Laser-Based Thermal Desorption},
author = {Yerbolat Dauletyarov and Siwen Wang and Daniel A. Horke},
url = {https://pubs.acs.org/doi/10.1021/jasms.3c00194},
doi = {10.1021/jasms.3c00194},
issn = {1044-0305, 1879-1123},
year = {2023},
date = {2023-06-01},
urldate = {2023-06-01},
journal = {J. Am. Soc. Mass Spectrom.},
volume = {34},
pages = {1538},
abstract = {The production of a clean neutral molecular sample is a crucial step in many gas-phase spectroscopy and reaction dynamics experiments investigating neutral species. Unfortunately, conventional methods based on heating cannot be used with most nonvolatile biomolecules due to their thermal instability. In this paper, we demonstrate the application of laser-based thermal desorption (LBTD) to produce neutral molecular plumes of biomolecules such as dipeptides and lipids. Specifically, we report mass spectra of glycylglycine, glycyl-L-alanine, and cholesterol obtained using LBTD vaporization, followed by soft femtosecond multiphoton ionization (fs-MPI) at 400 nm. For all molecules, the signal from the intact precursor ion was observed, highlighting the softness and applicability of the LBTD and fs-MPI approach. In more detail, cholesterol underwent hardly any fragmentation. Both dipeptides fragmented significantly, although mostly through only a single channel, which we attribute to the fs-MPI process.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The production of a clean neutral molecular sample is a crucial step in many gas-phase spectroscopy and reaction dynamics experiments investigating neutral species. Unfortunately, conventional methods based on heating cannot be used with most nonvolatile biomolecules due to their thermal instability. In this paper, we demonstrate the application of laser-based thermal desorption (LBTD) to produce neutral molecular plumes of biomolecules such as dipeptides and lipids. Specifically, we report mass spectra of glycylglycine, glycyl-L-alanine, and cholesterol obtained using LBTD vaporization, followed by soft femtosecond multiphoton ionization (fs-MPI) at 400 nm. For all molecules, the signal from the intact precursor ion was observed, highlighting the softness and applicability of the LBTD and fs-MPI approach. In more detail, cholesterol underwent hardly any fragmentation. Both dipeptides fragmented significantly, although mostly through only a single channel, which we attribute to the fs-MPI process.
@article{D3CP00328K,
title = {High-throughput UV-photofragmentation studies of thymine and guanine},
author = {Siwen Wang and Yerbolat Dauletyarov and Peter Kr\"{u}ger and Daniel A. Horke},
url = {http://dx.doi.org/10.1039/D3CP00328K},
doi = {10.1039/D3CP00328K},
year = {2023},
date = {2023-04-15},
urldate = {2023-01-01},
journal = {Phys. Chem. Chem. Phys.},
volume = {25},
pages = {12322},
publisher = {The Royal Society of Chemistry},
abstract = {High-throughput photofragmentation studies of thymine and guanine were performed at 257 and 343 nm and for a wide range of ionisation laser intensities. Combining a continuous laser-based thermal desorption source with femtosecond multiphoton ionisation using a 50 kHz repetition rate laser allowed us to produce detailed 2D maps of fragmentation as a function of incident laser intensity. The fragmentation was distinctly soft, the parent ions being at least an order of magnitude more abundant than fragment ions. For thymine there was a single dominant fragmentation channel, which involves consecutive HNCO and CO losses. In contrast, for guanine there were several competing ones, the most probable channel corresponding to CH2N2 loss through opening of the pyrimidine ring. The dependence of parent ion abundance on the ionisation laser intensity showed that at 257 nm the ionisation of thymine is a 1 + 1 resonance enhanced process through its open-shell singlet state.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
High-throughput photofragmentation studies of thymine and guanine were performed at 257 and 343 nm and for a wide range of ionisation laser intensities. Combining a continuous laser-based thermal desorption source with femtosecond multiphoton ionisation using a 50 kHz repetition rate laser allowed us to produce detailed 2D maps of fragmentation as a function of incident laser intensity. The fragmentation was distinctly soft, the parent ions being at least an order of magnitude more abundant than fragment ions. For thymine there was a single dominant fragmentation channel, which involves consecutive HNCO and CO losses. In contrast, for guanine there were several competing ones, the most probable channel corresponding to CH2N2 loss through opening of the pyrimidine ring. The dependence of parent ion abundance on the ionisation laser intensity showed that at 257 nm the ionisation of thymine is a 1 + 1 resonance enhanced process through its open-shell singlet state.
