TY - JOUR A1 - Kramer, Tobias A1 - Läuter, Matthias T1 - Outgassing induced acceleration of comet 67P/Churyumov-Gerasimenko JF - Astronomy & Astrophysics N2 - Cometary activity affects the orbital motion and rotation state due to sublimation induced forces. The availability of precise rotation-axis orientation and position data from the Rosetta mission allows one to accurately determine the outgassing of comet Churyumov-Gerasimenko/67P (67P). We derive the observed non-gravitational acceleration of 67P directly from the Rosetta spacecraft trajectory. From the non-gravitational acceleration we recover the diurnal outgassing variations and study a possible delay of the sublimation response with respect to the peak solar illumination. This allows us to compare the non-gravitational acceleration of 67P with expectations based on empirical models and common assumptions about the sublimation process. We use an iterative orbit refinement and Fourier decomposition of the diurnal activity to derive the outgassing induced non-gravitational acceleration. The uncertainties of the data reduction are established by a sensitivity analysis of an ensemble of best-fit orbits for comet 67P. We find that the Marsden non-gravitational acceleration parameters reproduce part of the non-gravitational acceleration but need to be augmented by an analysis of the nucleus geometry and surface illumination to draw conclusions about the sublimation process on the surface. The non-gravitational acceleration follows closely the subsolar latitude (seasonal illumination), with a small lag angle with respect to local noon around perihelion. The observed minor changes of the rotation axis do not favor forced precession models for the non-gravitational acceleration. In contrast to the sublimation induced torques, the non-gravitational acceleration does not put strong constraints on localized active areas on the nucleus. We find a close agreement of the orbit deduced non-gravitational acceleration and the water production independently derived from Rosetta in-situ measurement. Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201935229 VL - 630 SP - A4 ER - TY - JOUR A1 - Rodríguez, Mirta A1 - Kramer, Tobias T1 - Machine Learning of Two-Dimensional Spectroscopic Data JF - Chemical Physics N2 - Two-dimensional electronic spectroscopy has become one of the main experimental tools for analyzing the dynamics of excitonic energy transfer in large molecular complexes. Simplified theoretical models are usually employed to extract model parameters from the experimental spectral data. Here we show that computationally expensive but exact theoretical methods encoded into a neural network can be used to extract model parameters and infer structural information such as dipole orientation from two dimensional electronic spectra (2DES) or reversely, to produce 2DES from model parameters. We propose to use machine learning as a tool to predict unknown parameters in the models underlying recorded spectra and as a way to encode computationally expensive numerical methods into efficient prediction tools. We showcase the use of a trained neural network to efficiently compute disordered averaged spectra and demonstrate that disorder averaging has non-trivial effects for polarization controlled 2DES. Y1 - 2019 U6 - https://doi.org/10.1016/j.chemphys.2019.01.002 VL - 520 SP - 52 EP - 60 ER - TY - JOUR A1 - Kramer, Tobias A1 - Läuter, Matthias A1 - Hviid, Stubbe A1 - Jorda, Laurent A1 - Keller, Horst Uwe A1 - Kührt, Ekkehard T1 - Comet 67P/Churyumov-Gerasimenko rotation changes derived from sublimation induced torques JF - Astronomy & Astrophysics N2 - Context. The change of the rotation period and the orientation of the rotation axis of comet 67P/Churyumov-Gerasimenko (67P/C-G) is deducible from images taken by the scientific imaging instruments on-board the Rosetta mission with high precision. Non gravitational forces are a natural explanation for these data. Aims. We describe observed changes for the orientation of the rotation axis and the rotation period of 67P/C-G. For these changes we give an explanation based on a sublimation model with a best-fit for the surface active fraction (model P). Torque effects of periodically changing gas emissions on the surface are considered. Methods. We solve the equation of state for the angular momentum in the inertial and the body- fixed frames and provide an analytic theory of the rotation changes in terms of Fourier coefficients, generally applicable to periodically forced rigid body dynamics. Results. The torque induced changes of the rotation state constrain the physical properties of the surface, the sublimation rate and the local active fraction of the surface. Conclusions. We determine a distribution of the local surface active fraction in agreement with the rotation properties, period and orientation, of 67P/C-G. The torque movement confirms that the sublimation increases faster than the insolation towards perihelion. The derived relatively uniform activity pattern is discussed in terms of related surface features. Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201834349 VL - 630 SP - A3 ER - TY - JOUR A1 - Läuter, Matthias A1 - Kramer, Tobias A1 - Rubin, Martin A1 - Altwegg, Kathrin T1 - Surface localization of gas sources on comet 67P/Churyumov-Gerasimenko based on DFMS/COPS data JF - Monthly Notices of the Royal Astronomical Society N2 - We reconstruct the temporal evolution of the source distribution for the four major gas species H2O, CO2, CO, and O2 on the surface of comet 67P/Churyumov-Gerasimenko during its 2015 apparition. The analysis applies an inverse coma model and fits to data between August 6th 2014 and September 5th 2016 measured with the Double Focusing Mass Spectrometer (DFMS) of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the COmet Pressure Sensor (COPS). The spatial distribution of gas sources with their temporal variation allows one to construct surface maps for gas emissions and to evaluate integrated productions rates. For all species peak production rates and integrated productions rates per orbit are evaluated separately for the northern and the southern hemisphere. The nine most active emitting areas on the comet’s surface are defined and their correlation to emissions for each of the species is discussed. Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty3103 VL - 483 SP - 852 EP - 861 PB - Monthly Notices of the Royal Astronomical Society ER - TY - JOUR A1 - Kreisbeck, Christoph A1 - Kramer, Tobias A1 - Molina, Rafael T1 - Time-dependent wave packet simulations of transport through Aharanov–Bohm rings with an embedded quantum dot JF - Journal of Physics: Condensed Matter N2 - We have performed time-dependent wave packet simulations of realistic Aharonov-Bohm (AB) devices with a quantum dot embedded in one of the arms of the interferometer. The AB ring can function as a measurement device for the intrinsic transmission phase through the quantum dot, however, care has to be taken in analyzing the influence of scattering processes in the junctions of the interferometer arms. We consider a harmonic quantum dot and show how the Darwin–Fock spectrum emerges as a unique pattern in the interference fringes of the AB oscillations. Y1 - 2017 U6 - https://doi.org/10.1088/1361-648X/aa605d VL - 29 IS - 15 SP - 155301 ER - TY - JOUR A1 - Kramer, Tobias A1 - Rodriguez, Mirta A1 - Zelinskyi, Yaroslav T1 - Modeling of Transient Absorption Spectra in Exciton Charge-Transfer Systems JF - Journal of Physical Chemistry B N2 - Time-resolved spectroscopy provides the main tool for analyzing the dynamics of excitonic energy transfer in light-harvesting complexes. To infer time-scales and effective coupling parameters from experimental data requires to develop numerical exact theoretical models. The finite duration of the laser-molecule interactions and the reorganization process during the exciton migration affect the location and strength of spectroscopic signals. We show that the non-perturbative hierarchical equations of motion (HEOM) method captures these processes in a model exciton system, including the charge transfer state. Y1 - 2017 U6 - https://doi.org/10.1021/acs.jpcb.6b09858 VL - 121 SP - 463 EP - 470 ER - TY - JOUR A1 - Kramer, Tobias A1 - Noack, Matthias T1 - On the origin of inner coma structures observed by Rosetta during a diurnal rotation of comet 67P/Churyumov-Gerasimenko JF - The Astrophysical Journal Letters N2 - The Rosetta probe around comet 67P/Churyumov–Gerasimenko (67P) reveals an anisotropic dust distribution of the inner coma with jet-like structures. The physical processes leading to jet formation are under debate, with most models for cometary activity focusing on localized emission sources, such as cliffs or terraced regions. Here we suggest, by correlating high-resolution simulations of the dust environment around 67P with observations, that the anisotropy and the background dust density of 67P originate from dust released across the entire sunlit surface of the nucleus rather than from few isolated sources. We trace back trajectories from coma regions with high local dust density in space to the non-spherical nucleus and identify two mechanisms of jet formation: areas with local concavity in either two dimensions or only one. Pits and craters are examples of the first case; the neck region of the bi-lobed nucleus of 67P is an example of the latter case. The conjunction of multiple sources, in addition to dust released from all other sunlit areas, results in a high correlation coefficient (~0.8) of the predictions with observations during a complete diurnal rotation period of 67P. Y1 - 2016 U6 - https://doi.org/10.3847/2041-8205/823/1/L11 VL - 823 IS - 1 SP - L11 ER - TY - JOUR A1 - Reimers, Jeffrey R. A1 - Biczysko, Malgorzata A1 - Bruce, Douglas A1 - Coker, David F. A1 - Frankcombe, Terry J. A1 - Hashimoto, Hideki A1 - Hauer, Jürgen A1 - Jankowiak, Ryszard A1 - Kramer, Tobias A1 - Linnanto, Juha A1 - Mamedov, Fikret A1 - Müh, Frank A1 - Rätsep, Margus A1 - Renger, Thomas A1 - Styring, Stenbjörn A1 - Wan, Jian A1 - Wang, Zhuan A1 - Wang-Otomo, Zheng-Yu A1 - Weng, Yu-Xiang A1 - Yang, Chunhong A1 - Zhang, Jian-Ping A1 - Freiberg, Arvi A1 - Krausz, Elmars T1 - Challenges facing an understanding of the nature of low-energy excited states in photosynthesis JF - BBA Bioenergetics N2 - While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding. Y1 - 2016 U6 - https://doi.org/10.1016/j.bbabio.2016.06.010 VL - 1857 IS - 9 SP - 1627 EP - 1640 ER - TY - JOUR A1 - Kramer, Tobias A1 - Kreisbeck, Christoph A1 - Riha, Christian A1 - Chiatti, Olivio A1 - Buchholz, Sven A1 - Wieck, Andreas A1 - Reuter, Dirk A1 - Fischer, Saskia T1 - Thermal energy and charge currents in multi-terminal nanorings JF - AIP Advances N2 - We study in experiment and theory thermal energy and charge transfer close to the quantum limit in a ballistic nanodevice, consisting of multiply connected one-dimensional electron waveguides. The fabricated device is based on an AlGaAs/GaAs heterostructure and is covered by a global top-gate to steer the thermal energy and charge transfer in the presence of a temperature gradient, which is established by a heating current. The estimate of the heat transfer by means of thermal noise measurements shows the device acting as a switch for charge and thermal energy transfer. The wave-packet simulations are based on the multi-terminal Landauer-Büttiker approach and confirm the experimental finding of a mode-dependent redistribution of the thermal energy current, if a scatterer breaks the device symmetry. Y1 - 2016 U6 - https://doi.org/10.1063/1.4953812 VL - 6 SP - 065306 ER - TY - JOUR A1 - Kramer, Tobias A1 - Noack, Matthias A1 - Baum, Daniel A1 - Hege, Hans-Christian A1 - Heller, Eric J. T1 - Homogeneous dust emission and jet structure near active cometary nuclei: the case of 67P/Churyumov-Gerasimenko N2 - We compute trajectories of dust grains starting from a homogeneous surface activity-profile on a irregularly shaped cometary nucleus. Despite the initially homogeneous dust distribution a collimation in jet-like structures becomes visible. The fine structure is caused by concave topographical features with similar bundles of normal vectors. The model incorporates accurately determined gravitational forces, rotation of the nucleus, and gas-dust interaction. Jet-like dust structures are obtained for a wide range of gas-dust interactions. For the comet 67P/Churyumov-Gerasimenko, we derive the global dust distribution around the nucleus and find several areas of agreement between the homogeneous dust emission model and the Rosetta observation of dust jets, including velocity-dependent bending of trajectories. Y1 - 2015 ER -