3,865 research outputs found

    Patterns of remnant discrete symmetries

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    We analyze patterns of remnant discrete symmetries that arise from U(1)^N theories by spontaneous breaking. We describe a simple, geometrical way to understand these patterns and provide methods for identifying the discrete symmetries and bringing them to the simplest possible form. Applications in GUT and string model building are briefly discussed.Comment: 14 pages, 2 figures, a related Mathematica package can be downloaded from http://einrichtungen.physik.tu-muenchen.de/T30e/codes/DiscreteBreaking

    strong field quantum control in K2

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    We demonstrate that the semiclassical field-induced surface hopping (FISH) method (Mitrić et al., Phys. Rev. A: At., Mol., Opt. Phys., 2009, 79, 053416.) accurately describes the selective coherent control of electronic state populations. With the example of the strong field control in the potassium dimer using phase-coherent double pulse sequences, we present a detailed comparison between FISH simulations and exact quantum dynamics. We show that for short pulses the variation of the time delay between the subpulses allows for a selective population of the desired final state with high efficiency. Furthermore, also for pulses of longer time duration, when substantial nuclear motion takes place during the action of the pulse, optimized pulse shapes can be obtained which lead to selective population transfer. For both types of pulses, the FISH method almost perfectly reproduces the exact quantum mechanical electronic population dynamics, fully taking account of the electronic coherence, and describes the leading features of the nuclear dynamics accurately. Due to the significantly higher computational efficiency of FISH as a trajectory-based method compared to full quantum dynamics simulations, this offers the possibility to theoretically investigate control experiments on realistic systems including all nuclear degrees of freedom

    HORTENSIA, a program package for the simulation of nonadiabatic autoionization dynamics in molecules

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    We present a program package for the simulation of ultrafast vibration-induced autoionization dynamics in molecular anions in the manifold of the adiabatic anionic states and the discretized ionization continuum. This program, called HORTENSIA (\underline{Ho}pping \underline{r}eal-time \underline{t}rajectories for \underline{e}lectron-ejection by \underline{n}onadiabatic \underline{s}elf-\underline{i}onization in \underline{a}nions), is based on the nonadiabatic surface-hopping methodology, wherein nuclei are propagated as an ensemble along classical trajectories in the quantum-mechanical potential created by the electronic density of the molecular system. The electronic Schr\"odinger equation is numerically integrated along the trajectory, providing the time evolution of electronic state coefficients, from which switching probabilities into discrete electronic states are determined. In the case of a discretized continuum state, this hopping event is interpreted as the ejection on an electron. The derived diabatic and nonadiabatic couplings in the time-dependent electronic Schr\"odinger equation are calculated from anionic and neutral wavefunctions obtained from quantum chemical calculations with commercially available program packages interfaced with our program. Based on this methodology, we demonstrate the simulation of autoionization electron kinetic energy spectra that are both time- and angle-resolved. In addition, the program yields data that can be interpreted easily with respect to geometric characteristics such as bonding distances and angles, which facilitates the detection of molecular configurations important for the autoionization process. Moreover, useful extensions are included, namely generation tools for initial conditions and input files as well as for the evaluation of output files both through console commands and a graphical user interface

    Photodissociation Dynamics of Propargylene, HCCCH

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    We report a joint theoretical and experimental study on the photodissociation of the C3H2 isomer propargylene, HCCCH, combining velocity map imaging with nonadiabatic trajectory surface hopping calculations. Propargylene loses an H-atom, after laser excitation at around 250 nm, presumably to the T6 state. The photofragment angular distribution exhibits only a very small anisotropy, but the H-atom translational energy distribution extends to high energies and shows an expectation value of 〈fT〉, the fraction of excess energy released as translation, of 48%, outside the range expected for a statistical reaction mechanism. The computations suggest a predissociation in the T4–T6 state and lead to a translational energy distribution and photofragment angular distribution that match the experimentally observed ones very well

    Glucose induced MAPK signalling influences NeuroD1-mediated activation and nuclear localization

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    AbstractThe helix–loop–helix transcription factor NeuroD1 (also known as Beta2) is involved in β-cell survival during development and insulin gene transcription in adults. Here we show NeuroD1 is primarily cytoplasmic at non-stimulating glucose concentrations (i.e. 3 mM) in MIN6 β-cells and nuclear under stimulating conditions (i.e. 20 mM). Quantification revealed that NeuroD1 was in 40–45% of the nuclei at 3 mM and 80–90% at 20 mM. Treatment with the MEK inhibitor PD98059 or substitution of a serine for an alanine at a potential mitogen-activated protein kinase phosphorylation site (S274) in NeuroD1 significantly increased the cytoplasmic level at 20 mM glucose. The rise in NeuroD1-mediated transcription in response to glucose also correlated with the change in sub-cellular localization, a response attenuated by PD98059. The data strongly suggest that glucose-stimulation of the MEK–ERK signalling pathway influences NeuroD1 activity at least partially through effects on sub-cellular localization

    Multiloop World-Line Green Functions from String Theory

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    We show how the multiloop bosonic Green function of closed string theory reduces to the world-line Green function as defined by Schmidt and Schubert in the limit where the string world-sheet degenerates into a ÎŚ3\Phi^3 particle diagram. To obtain this correspondence we have to make an appropriate choice of the local coordinates defined on the degenerate string world sheet. We also present a set of simple rules that specify, in the explicit setting of the Schottky parametrization, which is the corner of moduli space corresponding to a given multiloop ÎŚ3\Phi^3 diagram.Comment: 33 pages, 6 figures. Published versio

    Laser pulse trains for controlling excited state dynamics of adenine in water

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    We investigate theoretically the control of the ultrafast excited state dynamics of adenine in water by laser pulse trains, with the aim to extend the excited state lifetime and to suppress nonradiative relaxation processes. For this purpose, we introduce the combination of our field-induced surface hopping method (FISH) with the quantum mechanical–molecular mechanical (QM/MM) technique for simulating the laser-driven dynamics in the condensed phase under explicit inclusion of the solvent environment. Moreover, we employ parametric pulse shaping in the frequency domain in order to design simplified laser pulse trains allowing to establish a direct link between the pulse parameters and the controlled dynamics. We construct pulse trains which achieve a high excitation efficiency and at the same time keep a high excited state population for a significantly extended time period compared to the uncontrolled dynamics. The control mechanism involves a sequential cycling of the population between the lowest and higher excited states, thereby utilizing the properties of the corresponding potential energy surfaces to avoid conical intersections and thus to suppress the nonradiative decay to the ground state. Our findings provide a means to increase the fluorescence yield of molecules with an intrinsically very short excited state lifetime, which can lead to novel applications of shaped laser fields in the context of biosensing

    Switching from molecular to bulklike dynamics in electronic relaxation of a small gold cluster

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    We have investigated the ultrafast electronic relaxation of Au−7 using time- resolved photoelectron spectroscopy combined with first-principles simulations of the excited-state dynamics. Unlike previous findings, which have demonstrated molecularlike excited-state relaxation in Au−7 at low excitation energy (1.56 eV), we show here that excitation with 3.12 eV leads to bulklike electronic relaxation without a considerable change of geometry. The experimental findings are fully supported by theoretical simulations, which reveal a bulklike electron-hole relaxation mechanism in a far band-gap cluster. Our findings demonstrate that small gold clusters in the sub-nm size range can exhibit either molecularlike or bulklike properties, depending on the excitation energy
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