1,721,006 research outputs found
Ultrafast carriers dynamics in silicon: A joint experimental and theoretical study
No full textWe investigate the carriers dynamics in bulk silicon using pump-probe spectroscopy. The experimental results are compared with theoretical calculations which combine for the first time the non-equilibrium Green’s functions theory with ab initio methods
Disentangling Structural and Dynamical Effects via Multidimensional High Harmonic Spectroscopy
No full textMolecular quantum computing applying optimal control theory: quantum gates and algorithms
No full textComment on “Anharmonic properties of the vibrational quantum computer” [J. Chem. Phys. 126, 204102 (2007)]
No full textMolecular population control including rotation
No full textCoherent control theory has been applied to steer molecular
vibrational dynamics so far disregarding the rotational degree of
freedom. In this paper the complete rovibrational molecular
Hamiltonian is considered, especially its effect on the
controllability. It is shown that suitably shaped pulses can
overcome the difficulties arising from the reorientational motion
of the molecule achieving again a high degree of laser control
Relation between molecular structure and ultrafast photoreactivity with application to molecular switches
Full text linkPhotoinduced ultrafast isomerizations are fundamental reactions in photochemistry and photobiology. This thesis aims for an understanding of the generic forces driving these reactions and a theoretical approach is set up, able to handle realistic systems, whose fast relaxation is mediated by conical intersections. The main focus is on the development of strategies for the prediction and accelerated optimization of conical intersections and their application to artificial compounds with promising physicochemical properties for technical applications as molecular switches. All calculations are based on advanced quantum chemical methods and mixed quantum-classical dynamics.
In the first part of this thesis the two-electron two-orbital theory by Michl and Bonacic-Koutecky used in its original formulation to rationalize the structure of conical intersections in charged polyene systems is extended by including the interactions of the active pair of electrons with the remaining closed-shell electrons that are present in any realistic system. A set of conditions, called resonance and heterosymmetry conditions, for the formation of conical intersections in multielectronic systems are derived and verified by calculations on the basic units ethylene, cis-butadiene and 1,3-cyclohexadiene at various geometries and functionalizational patterns. The quantitative results help to understand the role of geometrical deformations and substituent effects for the formation of conical intersections and to derive rules of thumb for their qualitative prediction in arbitrary polyenes. An extension of the rules of thumb to conical intersection seams is formulated. The strategy pursued is to divide the molecular system into basic units and into functional groups. Each unit and its intersection space are treated independently, thereby reducing the dimensionality of the search space compared to the complete molecule. Subsequently, the interconnectivity of the intersection spaces of the different units is determined and an initial guess for the complete seam is constructed. This guess is then fed into a quantum chemistry package to finalize the optimization. The strategy is demonstrated for two multi-functionalized systems, hemithioindigo-hemistilbene and trifluoromethyl-pyrrolylfulgide.
In the second part of this thesis state-of-the-art quantum chemical calculations and time-resolved transient and infrared spectroscopy are used to reconstruct the complex multi-channel isomerization mechanisms of hemithioindigo-hemistilbene and trifluoromethyl-indolylfulgide. Both the cis-trans isomerization in hemithioindigo-hemistilbene and the electrocyclic ring closure/opening in indolylfulgide are characterized by a charge transfer in the excited state. The ability of each system to stabilize this charge transfer is essential for the returning to the ground state. The relaxation to the ground state through extended regions of the seam is found to be the decisive step determining the reaction speed and the quantum yield.
In the last part of this thesis mixed quantum-classical dynamics simulations at multi-configurational perturbation theory (MS-CASPT2) level, using Tully's fewest switches surface hopping approach, are performed to study the ultrafast photoreactivity of 1,3-cyclohexadiene in the gas-phase. For this purpose a numerical routine for the efficient calculation of non-adiabatic couplings at MS-CASPT2 level is presented. The major part of the excited molecules are found to circumvent the 1B2/2A1 conical intersection and reach the conical intersection seam between the excited state and the ground state instantaneuosly. Time constants for the evolution of the wavepacket on the bright 1B2-state, the relaxation into the 2A1-state and the return to the ground state are extracted. It is demonstrated that the accessibility of the conical intersection seam depends on its energetic and spatial relation to the minimum energy path, as well as on the momentum which is accumulated during relaxation on the excited state potential energy surface
Searching for pathways involving dressed states in optimal control theory
Full text linkSelective population of dressed states has been proposed as an alternative control
pathway in molecular reaction dynamics [Wollenhaupt et al., J. Photochem.
Photobiol. A: Chem., 2006, 180, 248]. In this article we investigate if, and under
which conditions, this strong field pathway is included in the search space of
optimal control theory. For our calculations we used the proposed example of the
potassium dimer, in which the different target states can be reached via dressed
states by resonant transition. Especially, we investigate whether the optimization
algorithm is able to find the route involving the dressed states although the target
state lies out of resonance in the bare state picture
Sub-cycle switching of a photonic bandstructure via ultrastrong light-matter coupling
No full textSub-cycle switching of a photonic bandstructure via ultrastrong light-matter coupling
AB Phase-locked multi-terahertz transients map out the full photonic bandstructure of a one-dimensional photonic crystal while a 12-fs control pulse activates ultrastrong interaction on a sub-cycle time scale with quantized electronic transitions in semiconductor quantum wells. We trace the build-up dynamics of a large vacuum Rabi splitting and observe an unexpected asymmetric formation of the upper and lower polariton bands. The pronounced flattening of the photonic bands causes a slow-down of the group velocity by one order of magnitude on the time scale of the oscillation period of light
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