Electron-nuclear correlations for photo-induced dynamics in molecular dimers

Ultrafast photoinduced dynamics of electronic excitation in molecular dimers is drastically affected by the dynamic reorganization of inter- and intra- molecular nuclear configuration modeled by a quantized nuclear degree of freedom [Cina et. al, J. Chem Phys. {118}, 46 (2003)]. The dynamics of the electronic population and nuclear coherence is analyzed by solving the chain of coupled differential equations for %mean coordinate, population inversion, electron-vibrational correlation, etc. [Prezhdo, Pereverzev, J. Chem. Phys. {113} 6557 (2000)]. Intriguing results are obtained in the approximation of a small change of the nuclear equilibrium upon photoexcitation. In the limiting case of resonance between the electronic energy gap and the frequency of the nuclear mode these results are justified by comparison to the exactly solvable Jaynes-Cummings model. It is found that the photoinduced processes in the model dimer are arranged according to their time scales: (i) fast scale of nuclear motion, (ii) intermediate scale of dynamical redistribution of electronic population between excited states as well as growth and dynamics of electron-nuclear correlation, (iii) slow scale of electronic population approach to the quasi-equilibrium distribution, decay of electron-nuclear correlation, and decrease of the amplitude of mean coordinate oscillation. The latter processes are accompanied by a noticeable growth of the nuclear coordinate dispersion associated with the overall nuclear wavepacket width. The demonstrated quantum relaxation features of the photoinduced vibronic dynamics in molecular dimers are obtained by a simple method, applicable to systems with many degrees of freedom

The role of the environment in molecular systems

Die Dissipation von Energie von einem molekularen System in die Umgebung und die damit verbundene Zerstörung der Phasenkohärenz hat einen Einfluss auf mehrere physikalische Prozesse wie Bewegung der Schwingungsmoden eines Moleküls, eines Ions in einer Falle oder einer Strahlungsfeldmode, sowie auf Excitonen- und Elektronentransfer. Elektronrntransfer spielt eine wichtide Rolle in vielen Bereichen der Physik und Chemie. In dieser Arbeit wird die Elektronentransferdynamik mit Bewegungsgleichungen für die reduzierte Dichtematrix beschrieben, deren Herleitung ausgehend von der Liouville- von Neumann Gleichung über die Kumulanten-Entwicklung führt. Durch Ankopplung an ein Wärmebad werden dissipative Effekte Berücksichtigt. Zunächst wird diese Theorie auf Modellsysteme angewendet, um die verschiedene Einflüsse der Umgebung auf Depopulation, Dephasierung und Dekohärenz besser zu verstehen. Dann wird die Dynamik von konkreten intramolekularen Transferreaktionen in realen Molekülen berechnet und die Ergebnisse mit denen von Experimenten und anderer Theorien vergliechen. Zu den untersuchten Systemen zälen die Komplexe H2P-ZnP-Q und ZnPD-H2P

Nonadiabatic Dynamics in Two-Dimensional Perovskites Assisted by Machine Learned Force Fields

An exploration of the “on-the-fly” nonadiabatic couplings (NACs) for nonradiative relaxation and recombination of excited states in 2D Dion–Jacobson (DJ) lead halide perovskites (LHPs) is accelerated by a machine learning approach. Specifically, ab initio molecular dynamics (AIMD) of nanostructures composed of heavy elements is performed with the use of machine-learning force-fields (MLFFs), as implemented in the Vienna Ab initio Simulation Package (VASP). The force field parametrization is established using on-the-fly learning, which continuously builds a force field using AIMD data. At each time step of the molecular dynamics (MD) simulation, the total energy and forces are predicted based on the MLFF and if the Bayesian error estimate exceeds a threshold, an ab initio calculation is performed, which is used to construct a new force field. Model training of MLFF and evaluation were performed for a range of DJ-LHP models of different thicknesses and halide compositions. The MLFF-MD trajectories were evaluated against pure AIMD trajectories to assess the level of discrepancy and error accumulation. To examine the practical effectiveness of this approach, we have used the MLFF-based MD trajectories to compute NAC and excited-state dynamics. At each stage, results based on machine learning are compared to traditional ab initio based electronic dissipative dynamics. We find that MLFF-MD provides comparable results to AIMDs when MLFF is trained in an NPT ensemble

Electronic structure and hot carrier relaxation in ⟨001⟩ anatase TiO₂ nanowire

<div><p>We present an electronic structure and non-adiabatic excited state dynamics study of ⟨001⟩ anatase TiO₂ nanowire (NW) by combining density matrix formalism and <i>ab initio</i> electronic structure calculations. Our results show that quantum confinement increases the energy gap as the dimension of TiO₂ is reduced from the bulk to a NW with a diameter of several nanometres and that the probability of electronic transitions induced by lattice vibrations for the NW follows band gap law. The electron non-radiative relaxation to the bottom of the conduction band is involving Ti 3d orbitals, while the hole non-radiative relaxation of holes to the top of the valence band occurs by subsequent occupation of O 2p orbitals.</p></div

Electron dynamics of solvated titanium hydroxide

<div><p>Increasing interest in the photocatalytic activity of TiO₂ has led to the consideration of using TiO₂ nanoparticles in renewable energy generation. In order to better understand the electronic properties of nanoscale TiO₂ structures, it is important to understand fundamental components that serve as synthesis precursors of TiO₂ nanomaterials. Knowing the properties of basic building blocks of TiO₂ materials, such as titanium hydroxide, Ti(OH)₄, helps in a better understanding of the properties of larger scale structures. Titanium hydroxide is simulated in an explicit aqueous environment by density functional theory with hybrid functionals to model realistic conditions and to gain insight on mechanisms of how the charge carriers relax and recombine. Computed on-the-fly non-adiabatic couplings are determined from the first principles molecular dynamics calculations and allow the application of reduced density matrix formalism to calculate electron dynamics and dissipation times. An ultrafast charge transfer between ligand and metal is facilitated by energy dissipation via non-radiative relaxation occurring within a stepwise cascade thermalisation mechanism. The results show the possibility for design of promising and efficient materials for solar driven water splitting reactions.</p></div