266 research outputs found
Zur Theorie photoinduzierter Dynamik offener Molekularsysteme: Kontrolle von Dissipation durch ultrakurze Laser-Pulse
Zusammenfassung in PostScript In dieser Arbeit wird die photoinduzierte Dynamik offener Molekularsysteme unter dem EinfluĂ intensiver und ultrakurzer Laserpulse untersucht. Die Anregung eines MolekĂŒls durch einen optischen ultrakurzen Laserpuls fĂŒhrt zu ĂbergĂ€ngen zwischen verschiedenen elektronischen ZustĂ€nden. Dieser AnregungsprozeĂ wird begleitet von dissipativen VorgĂ€ngen wie Energie-- und Phasenrelaxation. Die Beschreibung dieser photoinduzierten Dynamik erfolgt mit Hilfe der Methode der Dichtematrixtheorie. Dabei zeigt die Ableitung der Quanten--Master--Gleichung im Rahmen des Projektionsoperator--Formalismus, daĂ die wirkenden Ă€uĂeren Felder einmal direkt im reversiblen Anteil der Bewegungsgleichung auftreten, aber auch einen indirekten EinfluĂ ĂŒber den die Dissipation beschreibenden Dissipations--Superoperator ausĂŒben. In dieser Arbeit wird zum ersten Mal die durch ultrakurze Laserpulse induzierte FeldabhĂ€ngigkeit des Dissipations--Superoperators berĂŒcksichtigt. Im Rahmen der Darstellung der Quanten--Master--Gleichung im Floquetbild kann eine anschauliche Deutung dieses feldabhĂ€ngigen Effektes gegeben werden: die die Dissipation beschreibende frequenzabhĂ€ngige Spektraldichte der Umgebungsmoden wird feldabhĂ€ngig bei verschiedenen Frequenzen abgefragt. Analytische Untersuchungen zum Zwei--Niveau--System zeigen, daĂ die FeldabhĂ€ngigkeit dann relevant wird, wenn die PulslĂ€nge vergleichbar ist mit der Zeitskala, auf der die Autokorrelationsfunktion der Umgebungsfreiheitsgrade abklingt. Um den EinfluĂ auf experimentelle GröĂen zu untersuchen, wird ein zweifarbiges Pump--Test--Experiment zum LaserfarbstoffmolekĂŒl IR 125 betrachtet, bei welchem die spektral und zeitlich aufgelöste Transmission auf einer Femtosekunden-- und Pikosekunden--Zeitskala gemessen wurde. Im Rahmen des Modells einer effektiven Schwingungsmode wird eine Anpassungsrechnung an das Experiment vorgenommen. Dabei wird zunĂ€chst die Standard-Redfield-Theorie verwendet, um ein Referenzmodell zu gewinnen. Es gelingt, eine gute Ăbereinstimmung mit dem Experiment zu erreichen. Die exakte BerĂŒcksichtigung des Einflusses der internen Konversion zwischen den angeregten elektronischen ZustĂ€nden fĂŒhrt zu einem Anstieg der Transmission innnerhalb einer Pikosekunde. Es ist notwendig, die Dichtematrixgleichungen exakt zu lösen, da eine vergleichende Untersuchung mit Hilfe der nichtlinearen SuszeptibilitĂ€t dritter Ordnung eine deutliche Abweichung zum exakten Resultat zeigt. Ausgehend vom Referenzfall feldunabhĂ€ngiger Dissipation wird dann die FeldabhĂ€ngigkeit der Relaxationsraten bestimmt sowie der EinfluĂ auf Observablen wie der relativen Transmission untersucht. In Ăbereinstimmung mit den analytischen Ergebnissen zeigt sich, daĂ der feldabhĂ€ngige Effekt am gröĂen ausgeprĂ€gt ist, wenn die PulslĂ€nge kleiner als die Korrelationszeit der Umgebungsfreiheitsgrade wird und die wirkenden Felder hinreichend intensiv sind.Damit wird eine Kontrolle von Dissipation möglich. Ein EinfluĂ des feldabhĂ€ngigen Effektes auf experimentelle Observablen wird vorhergesagt.abstract in PostScript This thesis investigates the influence of intense and ultrashort laser pulses on the photoinduced dynamics of open molecular systems. The excitation of a molecule by an optical ultrashort laser pulse induces transitions between different electronic states. This excitation process is accompanied by the dissipative processes of energy and vibrational relaxation. This excitation process is described within the method of the density matrix theory. Thereby, the derivation of the quantum master equation in the framework of the projection operator formalism demonstrates that the external fields are present in the reversible part of the equation of motion and also exert an indirect influence by acting on the dissipation superoperator which accounts for dissipation. In this thesis the field--dependency of the dissipation superoperator which is induced by the external fields is considered for the first time. By a representation of the quantum master equation in the Floquet picture, an interpretation of this field--dependent effect can be given: the frequency--dependent spectral density of the environmental modes which describe dissipation is determined at different field--dependent frequencies. Analytical investigations for the two level system demonstrate that the field dependence becomes relevant if the pulse length is comparable with the time scale on which the autocorrelation function of the environmental degrees of freedom decays.To investigate the influence on experimental quantities, a two--color pump--probe experiment for the laser dye molecule IR 125 is considered for which the spectrally and temporally resolved transmission on a femtosecond and picosecond time scale has been measured. Within the model of one effective vibrational mode the experimental data is fitted. The standard Redfield theory is used to provide a reference model. A high degree of concurrence between the theory and the results of the experiment is achieved. The exact treatment of internal conversion between the excited electronic states leads to a rise in transmission within one picosecond. It is necessary to solve the density matrix equations exactly because a comparative investigation with the nonlinear susceptibility of third order leads to a clear viation from the exact result. Starting from the reference case of field--independent dissipation, the field--dependency of the relaxation rates is determined and the influence on observables for example the relative transmission is investigated. The analytical results show that the field--dependent effect is strongest if the pulse length becomes smaller than the correlation time of the environmental modes and if the acting fields are sufficiently strong. Thereby, a control of dissipation becomes possible. An influence of the field--dependent effect on experimental observables is predicted
A large-scale evaluation framework for EEG deep learning architectures
EEG is the most common signal source for noninvasive BCI applications. For
such applications, the EEG signal needs to be decoded and translated into
appropriate actions. A recently emerging EEG decoding approach is deep learning
with Convolutional or Recurrent Neural Networks (CNNs, RNNs) with many
different architectures already published. Here we present a novel framework
for the large-scale evaluation of different deep-learning architectures on
different EEG datasets. This framework comprises (i) a collection of EEG
datasets currently including 100 examples (recording sessions) from six
different classification problems, (ii) a collection of different EEG decoding
algorithms, and (iii) a wrapper linking the decoders to the data as well as
handling structured documentation of all settings and (hyper-) parameters and
statistics, designed to ensure transparency and reproducibility. As an
applications example we used our framework by comparing three publicly
available CNN architectures: the Braindecode Deep4 ConvNet, Braindecode Shallow
ConvNet, and two versions of EEGNet. We also show how our framework can be used
to study similarities and differences in the performance of different decoding
methods across tasks. We argue that the deep learning EEG framework as
described here could help to tap the full potential of deep learning for BCI
applications.Comment: 7 pages, 3 figures, final version accepted for presentation at IEEE
SMC 2018 conferenc
Different Dynamics and Entropy Rates in Quantum-Thermodynamics
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugÀnglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Different dynamics of the non-equilibrium canonical density operator, such as Canonical Dynamics, Linear Projection Dynamics, Generalized Robertson Dynamics, and Contact Time Dynamics, especially for time dependent work variables are derived. For two discrete systems in contact the rate of entropy is non-negative, if the contact time is short, and if one of the two discrete systems is in equilibrium and the compound system composed of both is isolated. The contact temperature is identified with one of the Lagrange parameters of the non-equilibrium canonical density operator of the compound system
Influence of Late Quaternary paleoenvironmental conditions on the distribution of mammals fauna in the Laptev Sea region
Permafrost in arctic and subarctic lowlands
In recent decades permafrost landscapes in the arctic and subarctic lowlands have experienced warming resulting in a clear rising trend of permafrost temperatures despite some inter-annual variability and occasional cooling or stabilization. A widespread general deepening of the active layer, on the other hand, has not been observed. Changes in the carbon cycle of permafrost ecosystems also remain inconclusive due to a limited number of studies and their usually much localized focus. During the Quaternary environmental history of the Arctic, the non-glaciated Siberian lowlands have repeatedly experienced times of permafrost formation and permafrost degradation. Thus, the climate-change related long-term processes of permafrost dynamics can be reconstructed using environmental indicators from permafrost archives. Additionally, the amount of fossil organic material stored in the permafrost documents the permafrost's relevance for the global carbon cycle. In order to determine the current state and extent of permafrost ecosystems with regard to their thermal, hydrological, geomorphological, and carbon gas emission characteristics, a comprehensive standardized monitoring network combining remote sensing, modelling, long-term observations, and detailed process studies is urgently needed. Only if the current state of permafrost is well known, changes can be detected and future trends and developments can be predicted. That prediction in turn requires a thorough understanding of the paleoenvironmental history of permafrost landscapes
Positron emission tomography imaging of coronary atherosclerosis
Inflammation has a central role in the progression of coronary atherosclerosis. Recent developments in cardiovascular imaging with the advent of hybrid positron emission tomography have provided a window into the molecular pathophysiology underlying coronary plaque inflammation. Using novel radiotracers targeted at specific cellular pathways, the potential exists to observe inflammation, apoptosis, cellular hypoxia, microcalcification and angiogenesis in vivo. Several clinical studies are now underway assessing the ability of this hybrid imaging modality to inform about atherosclerotic disease activity and the prediction of future cardiovascular risk. A better understanding of the molecular mechanisms governing coronary atherosclerosis may be the first step toward offering patients a more stratified, personalized approach to treatment
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