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Dataset for Mapping the complete reaction coordinate of a complex chemical reaction
No full textData associated with paper:
Mapping the complete reaction path of a complex photochemical reaction. / Smith, Adam D.; Warne, Emily M.; Bellshaw, Darren; Horke, Daniel; Tudorovskya, Maria; Springate, Emma; Jones, Alfred; Cacho, Cephise; Chapman, Richard T.; Kirrander, Adam; Minns, Russell S.
In: Physical Review Letters, 120(18), 1-6. [183003].
DOI: 10.1103/PhysRevLett.120.183003</span
Correspondence between electronic structure calculations and simulations:: nonadiabatic dynamics in CS2
Full text linkThe choice of ab initio electronic structure method is an important factor in determining the fidelity of nonadiabatic dynamics simulations. We present an in-depth comparison of two simulations of photodissociation in the CS2 molecule following excitation to the 1 1^B_2 state. The simulations account for nonadiabatic and spin-orbit coupling, and are performed using the SHARC surface-hopping approach combined with state-averaged SA8-CASSCF(8,6)/SVP and SA8-CASSCF(10,8)/SVP {\it{ab initio}} calculations, with additional reference calculations at the MRCI(14,10)/aug-cc-pvTZ level. The relative performance and veracity of the simulations can be assessed by inspection of the potential energy curves along specific coordinates. The simulations demonstrate direct competition between internal conversion and intersystem crossing, with strong correlation between molecular geometry, electronic state density, and dynamics
Effect of probe energy and competing pathways on time-resolved photoelectron spectroscopy signals: the ring-opening of 1,3-cyclohexadiene
Full text linkThe ring-opening dynamics of 1,3-cyclohexadiene (CHD) following UV excitation is studied using a model based on quantum molecular dynamics simulations with the ab-initio multiconfigurational Ehrenfest (AI-MCE) method coupled to the Dyson orbital approach for photoionisation cross sections. Time-dependent photoelectron spectra are calculated for probe photon energies in the range 2-15 eV. The calculations demonstrate the value of universal high-energy probes, capableof tracking the full photochemical dynamics of the molecule, as well as the benefit of more selective, lower-energy probes. The predicted signal, especially with the universal probes, becomes highly convoluted due to the contributions from multiple reaction paths, rendering interpretationdifficult unless complementary measurements and theoretical comparisons are available
The contribution of Compton ionization to ultrafast x-ray scattering
No full textWe investigate the role of Compton ionization in ultrafast non-resonant x-ray scattering using a molecular model system, which includes the ionization continuum via an orthonormalized plane wave ansatz. Elastic and inelastic components of the scattering signal, as well as coherent-mixed scattering that arises from electron dynamics, are calculated. By virtue of a near-quantitative distinction between scattering related to electronic transitions into bound and continuum states, we demonstrate how Compton ionization contributes to the coherent-mixed component. Analogous to inelastic scattering, the contribution to the coherent-mixed signal is significant and particularly manifests at intermediate and high-momentum transfers. Strikingly, for molecules with inversion symmetry, the exclusion of bound or continuum transitions may lead to the prediction of spurious coherent-mixed signals. We conclude that qualitative and quantitative accuracies of predicted scattering signals on detectors without energy resolution require that elements of the two-electron density operator are used. This approach inherently accounts for all accessible electronic transitions, including ionization.</p
Ultrafast chemical dynamics simulations for experiments
Full text linkPhotochemistry is a crucial discipline that governs the interaction of molecules with light, but it presents significant challenges due to the difficulties of coupled nuclear and electronic dynamics. While new light sources have enabled measurements on ultrafast timescales, these experiments often yield results that are difficult to interpret and typically require substantial theoretical support. This thesis utilises electronic structure calculations and nonadiabatic dynamics simulations to shed light on a number of recent experiments.
Three examples of complex chemical systems are explored, focusing on the investigation of potential energy surfaces and photodynamics, with the findings compared to ultrafast experiments. First, trajectory surface hopping simulations are employed to study the photoinduced ring-conversion reaction in cyclopentadiene, revealing that photorelaxation leads to a mixture of hot cyclopentadiene and the photoproduct bicyclo[2.1.0]pentene. Observables for novel ultrafast X-ray scattering experiments are computed and the possibility of retrieving electronic information from the scattering signal is discussed. Next, six azanaphthalene systems are investigated, beginning with static analysis to explain the variations in internal conversion and intersystem crossing rates across the molecular series. This allowed for the interpretation of recent transient absorption spectroscopy experiments and illustrated how subtle changes in the nitrogen positions can significantly impact the photochemical behaviour. Nonadiabatic dynamics simulations are subsequently performed on a representative subset, confirming the qualitative predictions, albeit with some discrepancy in the timescales. Finally, the Rydberg state dynamics in N,N′-dimethylpiperazine are simulated as an example of excited state charge transfer. The time-resolved photoelectron spectrum is predicted and displays how the molecule slowly converts between two excited state structures.
Ultimately, this thesis demonstrates how computational simulations allow greater insight into the complex photochemical dynamics observed in state-of-the-art experiments
Dynamics of a photoswitch: norbornadiene and quadricyclane
Full text linkThis thesis comprehensively investigates the molecular photoswitch of norbornadiene and quadricyclane, spanning electronic structure, non-adiabatic dynamics and ultrafast experimental observables. The theoretical results are closely linked to state-of-the-art experimental results, both of already-completed and future experiments. These molecules display promise as molecular solar thermal (MOST) systems, which capture, store and release solar energy. Norbornadiene and quadricyclane access the same intersection after photo-excitation, offering a unique opportunity to study a photochemical reaction through the same conical intersection from two different starting geometries.
We show a full multi-configurational exploration of the valence potential energy surfaces, with specific emphasis on the topography of the S1/S0 conical intersection. The surface hopping dynamics of norbornadiene is then performed with four electronic structure models (CASSCF and XMS-CASPT2 with two active spaces), and the dynamics is compared to the CASSCF(2,2) surface hopping of quadricyclane, affording a greater understanding of the potential energy surfaces and the dynamics itself. Both molecules display a rapid (sub-100 fs) decay, and different electronic structure methods show notably different quantum yields. The pre-conical intersection motion is the principal arbiter of the dynamical outcome, with minor contributions from conical intersection topography, a feature we posit is common in photoswitches. These dynamics are used to predict x-ray scattering patterns and time-resolved photo-electron spectra, allowing for suggestions for realistic potential experiments and critical examination of the experimental methods.
A complete, novel assignment of the static absorption and photo-electron spectra of the two molecules is presented, and finally, the results of an ultrafast time-resolved photo-electron spectroscopy experiment are explained via surface hopping/RMS-CASPT2 simulations. The dynamics simultaneously excites Rydberg and valence states, with notably faster dynamics on the latter, and fully explains the observed bipartite nature of the experimental signal.
This study provides a computationally feasible model that entirely elucidates the ultrafast dynamics of the norbornadiene/quadricyclane system, offering a framework to improve the design for MOST and photoswitch applications. Additionally, this system remains a target of future ultrafast experiments, inevitably requiring correct and reliable simulations for their interpretation. More generally, this system is an excellent example of photoswitch systems, showing quick, coherent, and simple dynamics with complex outcomes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Theory of elastic and inelastic X-ray scattering
Full text linkX-rays have been widely exploited to unravel the structure of matter since
their discovery in 1895. Nowadays, with the emergence of new X-ray sources
with higher intensity and very short pulse duration, notably X-ray Free Electron
Lasers, the number of experiments that may be considered in the X-ray
regime has increased dramatically, making the characterization of gas phase
atoms and molecules in space and time possible. This thesis explores in the
theoretical analysis and calculation of X-ray scattering atoms and molecules,
far beyond the independent atom model.
Amethod to calculate inelastic X-ray scattering from atoms and molecules
is presented. The method utilizes electronic wavefunctions calculated using
ab-initio electronic structure methods. Wavefunctions expressed in Gaussian
type orbitals allow for efficient calculations based on analytical Fourier
transforms of the electron density and overlap integrals. The method is validated
by extensive calculations of inelastic cross-sections in H, He+, He,
Ne, C, Na and N2. The calculated cross-sections are compared to cross-sections
from inelastic X-ray scattering experiments, electron energy-loss
spectroscopy, and theoretical reference values.
We then begin to account for the effect of nuclear motion, in the first instance
by predicting elastic X-ray scattering from state-selected molecules.
We find strong signatures corresponding to the specific vibrational and rotational
state of (polyatomic) molecules.
The ultimate goal of this thesis is to study atomic and molecular wavepackets
using time-resolved X-ray scattering. We present a theoretical framework
based on quantum electrodynamics and explore various elastic and
inelastic limits of the scattering expressions. We then explore X-ray scattering
from electronic wavepackets, following on from work by other groups,
and finally examine the time-resolved X-ray scattering from non-adiabatic
electronic-nuclear wavepackets in the H2 molecule, demonstrating the importance
of accounting for the inelastic effects
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