Optimal control of open quantum systems applied to the photochemistry of surfaces

A quantum system in a condensed phase undergoes strong dissipative processes. The last decades have seen the rise of experimental and theoretical approaches for gaining control over dissipative phenomena. From a theoretical viewpoint it is important to model such processes in a rigorous way. An efficient and accurate method to find control fields is optimal control theory (OCT). In this Letter, a control scheme relying on OCT with time-dependent targets is employed to minimize dissipation, modeled within the surrogate Hamiltonian approach, on adsorbate-surface systems.     

Closed timelike curves via postselection: theory and experimental test of consistency

Closed timelike curves (CTCs) are trajectories in spacetime that effectively travel backwards in time: a test particle following a CTC can interact with its former self in the past. A widely accepted quantum theory of CTCs was proposed by Deutsch. Here we analyze an alternative quantum formulation of CTCs based on teleportation and postselection, and show that it is inequivalent to Deutsch's. The predictions or retrodictions of our theory can be simulated experimentally: we report the results of an experiment illustrating how in our particular theory the "grandfather paradox" is resolved.     

Evaluation of the Performance of Small Diode Pumped UV Solid State (DPSS) Nd:YAG Lasers as New Radiation Sources for Atmospheric Pressure Laser Ionization Mass Spectrometry (APLI-MS)

The performance of a KrF^* bench top excimer laser and a compact diode pumped UV solid state (DPSS) Nd:YAG laser as photo-ionizing source in LC-APLI MS is compared. The commonly applied bench-top excimer laser, operating at 248 nm, provides power densities of the order of low MW/cm² on an illuminated area of 0.5 cm² (8 mJ/pulse, 5 ns pulse duration, beam waist area 0.5 cm², 3 MW/cm²). The DPSS laser, operating at 266 nm, provides higher power densities, however, on a two orders of magnitude smaller illuminated area (60 μJ/pulse, 1 ns pulse duration, beam waist area 2 × 10^–3 cm², 30 MW/cm²). In a common LC-APLI MS setup with direct infusion of a 10 nM pyrene solution, the DPSS laser yields a significantly smaller ion signal (0.9%) and signal to noise ratio (1.4%) compared with the excimer laser. With respect to the determined low detection limits (LODs) for PAHs of 0.1 fmol using an excimer laser, LODs in DPSS laser LC-APLI MS in the low pmol regime are expected. The advantages of the DPSS laser with respect to applicability (size, cost, simplicity) may render this light source the preferred one for APLI applications not focusing on ultimately high sensitivities. Furthermore, the impact of adjustable ion source parameters on the performance of both laser systems is discussed in terms of the spatial sensitivity distribution described by the distribution of ion acceptance (DIA) measurements. Perspectives concerning the impact on future APLI-MS applications are given.     

Dehydrogenation of dodecahydro-N-ethylcarbazole on Pd/Al2O3 model catalysts

To elucidate the dehydrogenation mechanism of dodecahydro-N-ethylcarbazole (H(12)-NEC) on supported Pd catalysts, we have performed a model study under ultra high vacuum (UHV) conditions. H(12)-NEC and its final dehydrogenation product, N-ethylcarbazole (NEC), were deposited by physical vapor deposition (PVD) at temperatures between 120 K and 520 K onto a supported model catalyst, which consisted of Pd nanoparticles grown on a well-ordered alumina film on NiAl(110). Adsorption and thermally induced surface reactions were followed by infrared reflection absorption spectroscopy (IRAS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) in combination with density functional theory (DFT) calculations. It was shown that, at 120 K, H(12)-NEC adsorbs molecularly both on the Al(2)O(3)/NiAl(110) support and on the Pd particles. Initial activation of the molecule occurs through C-H bond scission at the 8a- and 9a-positions of the carbazole skeleton at temperatures above 170 K. Dehydrogenation successively proceeds with increasing temperature. Around 350 K, breakage of one C-N bond occurs accompanied by further dehydrogenation of the carbon skeleton. The decomposition intermediates reside on the surface up to 500 K. At higher temperatures, further decay to small fragments and atomic species is observed. These species block most of the absorption sites on the Pd particles, but can be oxidatively removed by heating in oxygen at 600 K, fully restoring the original adsorption properties of the model catalyst.     

Calculation of quantitative shunt values using photoluminescence imaging

A proof of concept study for a method of determining quantitative shunt values in silicon solar cells from photoluminescence images is presented. The method is based on interpretation of the luminescence intensity around a local shunt or recombination‐active defect in terms of the extracted current. The theoretical relationship between the photoluminescence signal and the shunt current is derived. Experimental results on specifically prepared test structures show good agreement with known shunt resistance values. Experimental data on diffused wafers are presented. The effect of the front metallisation in complete cells on the appearance and interpretation of shunts in photoluminescence images is investigated experimentally. The limitations of the method are discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Zeeman effect in the N.Q.R. of 81Br in single crystals of 2,4-dibromoanisole and 2,4,6-tribromoanisole

The influence of the intramolecular degrees of freedom on the vapour–liquid equilibrium properties of ammonia is studied for vapour pressure, saturated densities and enthalpy of vaporization. Molecular force fields with and without intramolecular degrees of freedom, keeping all other parameters unchanged, show significantly different phase envelopes. For ammonia, the angle potential is particularly important, because the hydrogen sites are more aligned in the liquid than in the vapour, leading to a significantly enhanced molecular dipole moment in the condensed phase. Based on a rigid force field for ammonia from prior work of our group [Eckl et al., Mol. Phys. 106, 1039 (2008)], a new accurate force field with intramolecular degrees of freedom is developed.     

Improving Cooperative Transmission Feasibility by Network Reconfiguration in Limited Backhaul Networks

For coordinated multi-point (CoMP), sets of base stations (BSs) have to be selected to jointly serve user equipments (UEs). These sets are typically selected based on wireless characteristics only. However, using CoMP also poses strict capacity and latency requirements on the backhaul network. Hence, these requirements additionally need to be taken into account when selecting BSs for CoMP. We have developed a mixed integer linear program and a BSs selection heuristic for CoMP that takes into account both aspects: the wireless channels and the backhaul network status. This heuristic can also identify which bottlenecks in the backhaul network make a desired BSs selection infeasible. We exploit this to dynamically reconfigure the backhaul network to the wireless requirements. Our simulations show that the heuristic’s solution quality is close to the optimum while execution time and memory consumption are reduced by multiple orders of magnitude compared to solving the problem via mathematical optimization. In addition, we simulate the network reconfiguration in different backhaul network scenarios. The results illustrate how our approach helps to better exploit available backhaul resources.