1,047 research outputs found
Electron-ion coupling in semiconductors beyond Fermi's golden rule
In the present work, a theoretical study of electron-phonon (electron-ion)
coupling rates in semiconductors driven out of equilibrium is performed.
Transient change of optical coefficients reflects the band gap shrinkage in
covalently bonded materials, and thus, the heating of atomic lattice. Utilizing
this dependence, we test various models of electron-ion coupling. The
simulation technique is based on tight-binding molecular dynamics. Our
simulations with the dedicated hybrid approach (XTANT) indicate that the widely
used Fermi's golden rule can break down describing material excitation on
femtosecond time scales. In contrast, dynamical coupling proposed in this work
yields a reasonably good agreement of simulation results with available
experimental data
Cavity-induced modifications of molecular structure in the strong coupling regime
In most theoretical descriptions of collective strong coupling of organic
molecules to a cavity mode, the molecules are modeled as simple two-level
systems. This picture fails to describe the rich structure provided by their
internal rovibrational (nuclear) degrees of freedom. We investigate a
first-principles model that fully takes into account both electronic and
nuclear degrees of freedom, allowing an exploration of the phenomenon of strong
coupling from an entirely new perspective. First, we demonstrate the
limitations of applicability of the Born-Oppenheimer approximation in strongly
coupled molecule-cavity structures. For the case of two molecules, we also show
how dark states, which within the two-level picture are effectively decoupled
from the cavity, are indeed affected by the formation of collective strong
coupling. Finally, we discuss ground-state modifications in the ultra-strong
coupling regime and show that some molecular observables are affected by the
collective coupling strength, while others only depend on the single-molecule
coupling constant.Comment: 12 pages, 8 figure
On-the-fly ab initio semiclassical evaluation of absorption spectra of polyatomic molecules beyond the Condon approximation
To evaluate vibronic spectra beyond the Condon approximation, we extend the
on-the-fly ab initio thawed Gaussian approximation by considering the
Herzberg-Teller contribution due to the dependence of the electronic transition
dipole moment on nuclear coordinates. The extended thawed Gaussian
approximation is tested on electronic absorption spectra of phenyl radical and
benzene: Calculated spectra reproduce experimental data and are much more
accurate than standard global harmonic approaches, confirming the significance
of anharmonicity. Moreover, the extended method provides a tool to quantify the
Herzberg-Teller contribution: we show that in phenyl radical, anharmonicity
outweighs the Herzberg-Teller contribution, whereas in benzene, the
Herzberg-Teller contribution is essential, since the transition is
electronically forbidden and Condon approximation yields a zero spectrum.
Surprisingly, both adiabatic harmonic spectra outperform those of the vertical
harmonic model, which describes the Franck-Condon region better. Finally, we
provide a simple recipe for orientationally averaging spectra, valid beyond
Condon approximation, and a relation among the transition dipole, its gradient,
and nonadiabatic coupling vectors.Comment: Final form available via open access in J. Phys. Chem. Lett.:
https://pubs.acs.org/doi/10.1021/acs.jpclett.8b00827. Last 11 pages contain
the Supporting Informatio
Cold and Ultracold Rydberg Atoms in Strong Magnetic Fields
Cold Rydberg atoms exposed to strong magnetic fields possess unique
properties which open the pathway for an intriguing many-body dynamics taking
place in Rydberg gases consisting of either matter or anti-matter systems. We
review both the foundations and recent developments of the field in the cold
and ultracold regime where trapping and cooling of Rydberg atoms have become
possible. Exotic states of moving Rydberg atoms such as giant dipole states are
discussed in detail, including their formation mechanisms in a strongly
magnetized cold plasma. Inhomogeneous field configurations influence the
electronic structure of Rydberg atoms, and we describe the utility of
corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We
review recent work on large, extended cold Rydberg gases in magnetic fields and
their formation in strongly magnetized ultracold plasmas through collisional
recombination. Implications of these results for current antihydrogen
production experiments are pointed out, and techniques for trapping and cooling
of such atoms are investigated.Comment: 46 pages, 38 figures, to appear in Physics Report
From low dimensions to full configuration space: Generalising models for nonadiabatic molecular dynamics
This thesis aims to bridge the development of nonadiabatic dynamics methods and their application for studies of real molecular systems.
First, this work explores fundamental concepts of photochemistry by investigating two different pictures, arising from the Born-Oppenheimer and the exact factorisation representation. Based on a simplistic model, a photochemical experiment from the excitation up to the formation of photoproducts is simulated. This study then compares the Born-Oppenheimer and exact factorisation representations of the processes. Subsequently, the influence of the Born-Oppenheimer picture for approximate nonadiabatic dynamics is investigated on two-dimensional model systems around conical intersections. The effects of neglected couplings and geometric phase are evaluated for ab initio multiple spawning (AIMS), a method for nonadiabatic molecular dynamics based on classically moving Gaussians.
Afterwards, this work introduces a standardised test set of molecules to connect between tests of newly developed nonadiabatic dynamics methods on one-dimensional model systems and their intended application to full-dimensional molecules. Inspired by the widely used one-dimensional Tully models, three molecules are selected to form the molecular Tully models, which undergo similar photophysical processes, but in a high-dimensional space. In addition, the recently proposed stochastic-selection AIMS framework is also tested on two molecules undergoing ring-opening reactions to explore the strengths and limitations of the method.
Finally, a direct comparison between experimental and computational results is presented. The photochemistry of 2(5H)-thiophenone is probed during and after the initial ring opening using time-resolved photoelectron spectroscopy. Static and dynamic calculations unravel the photoprocesses and identify a variety of photoproducts. Using the computational results, the experimental signal can be translated to insights into the ongoing photochemistry.
Overall, this thesis aims to bring models in nonadiabatic dynamics in a real-world context. This work contributes to facilitating the transfer of new nonadiabatic dynamics methods towards the study of molecules in their full dimensionality
- …