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  • T. Bredtmann, S. Chelkowski, A.D. Bandrauk: Effect of Nuclear Motion on MHOHG Pump-Probe Spectroscopy, J. Phys. Chem A 116, 11398 (2012). DOI: 10.1021/jp3063977

We study pump–probe schemes for the real time observation of electronic motion on attosecond time scale in the molecular ion H2+ and its heavier isotope T2+ while these molecules dissociate on femtosecond time scale by solving numerically the non-Born–Oppenheimer time-dependent Schrödinger equation. The UV pump laser pulse prepares a coherent superposition of the three lowest lying quantum states and the time-delayed mid-infrared, intense few-femtosecond probe pulse subsequently generates molecular high-order harmonics (MHOHG) from this coherent electron–nuclear wavepacket (CENWP). Varying the pump–probe time delay by a few hundreds of attoseconds, the MHOHG signal intensity is shown to vary by orders of magnitude. Due to nuclear movement, the coherence of these two upper states and the ground state is lost after a few femtoseconds and the MHOHG intensity variations as function of pump–probe delay time are shown to be equal to the period of electron oscillation in the coherent superposition of the two upper dissociative quantum states. Although this electron oscillation period and hence the periodicity of the harmonic spectra are quite constant over a wide range of internuclear distances, a strong signature of nuclear motion is seen in the actual shapes and ways in which these spectra change as a function of pump–probe delay time, which is illustrated by comparison of the MHOHG spectra generated by the two isotopes H2+ and T2+. Two different regimes corresponding roughly to internuclear distances R < 4a0 and R > 4a0 are identified: For R < 4a0, the intensity of a whole range of frequencies in the plateau region is decreased by orders of magnitude when the delay time is changed by a few hundred attoseconds whereas in the cutoff region the peaks in the MHOHG spectra are red-shifted with increasing pump–probe time delay. For R > 4a0, on the other hand, the peaks both in the cutoff and plateau region are red-shifted with increasing delay times with only slight variations in the peak intensities. A time–frequency analysis shows that in the case of a two-cycle probe pulse the sole contribution of one long and associated short trajectory correlates with the attenuation of a whole range of frequencies in the plateau region for R < 4a0 whereas the observed red shift for R > 4a0, even in the plateau region, correlates with a single electron return within one-half laser cycle.

  • P. Kläring, A.-K. Jungton, C. Müller, T. Braun: Synthesis and reactivity of iridium hydrido fluoro complexes, Europ. J. Inorg. Chem. 9, 1430 (2012). DOI: 10.1002/ejic.201100917

The oxidative addition of HF at trans-[Ir(ArF)(η2-C2H4)(PiPr3)2] (1a: ArF = 4-C5NF4; 1b: ArF = 2-C6H3F2) affords the fluorido complexes trans-[Ir(ArF)(F)(H)(PiPr3)2] (2a: ArF = 4-C5NF4; 2b: ArF = 2-C6H3F2). The hydrido fluorido complex 2a is also accessible by means of the reaction of the hydroxido complex trans-[Ir(4-C5NF4)(H)(OH)(PiPr3)2] (3a) with Et3N·3HF. Both compounds 2a and 2b react with CO to give the carbonyl complexes trans-[Ir(4-C5NF4)(F)(H)(CO)(PiPr3)2] (4a: ArF = 4-C5NF4; 4b: ArF = 2-C6H3F2). In the presence of traces of water, a slow reaction of 2a with CO2 yields the hydrogencarbonato complex trans-[Ir(4-C5NF4)(H)(κ2-(O,O)-O2COH)(PiPr3)2] (5a). Upon using 2a or 2b as fluorinating agent, Ph3SiH could be converted into Ph3SiF and CH3C(O)Cl into CH3C(O)F.

  • K. R. Idzik, J. Frydel, R. Beckert, P. Ledwon, M. Lapkowski, C. Fasting, C. Müller, T. Licha: Pyridinylidene-phenoxide in strong electric fields: controlling orientation, conical intersection, and radiation-less decay, J. Phys. Chem. A 116, 11189 (2012). DOI: 10.1016/j.electacta.2012.06.101

A series of various tris(2,2'-bithiophen-5-yl)-aromatic derivatives were synthesized by Stille cross-coupling procedure. Their structures were characterized by 1H NMR, 13C NMR, and elemental analysis. DFT calculations for monomers were also performed. The optical properties of the synthesized materials as well as their energy levels were investigated by UV–vis absorption supported by fluorescence spectra and CV analysis. Oligomers obtained in the process of electropolymerization, possess a tetrathienyl bond with various aromatic and heteroaromatic cores. Electrochemical results confirm that the gained materials can apply successfully for a diversity of organic–electronic devices like organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells.

  • S. Belz, S. Zilberg, M. Berg, T. Grohmann, M. Leibscher: Synthesis and electrochemical properties of tetrathienyl-linked branched polymers with various aromatic cores, Electrochim. Acta 79, 154 (2012). DOI: 10.1021/jp305090b

Strong electric fields open new routes for the control of radiation-less decay in molecules with conical intersections. Here, we present quantum chemical and quantum dynamical simulations which demonstrate that the radiation-less decay and related photoisomerization of pyridinylidene-phenoxide can be effectively manipulated with strong electric fields by shifting the conical intersection. Moreover, we show the effects of the electric field on the orientation of the molecules and on the photoexcitation and discuss the conditions for which the field induced coupling between rotational and vibronic states can be neglected.

  • J. Floß, T. Grohmann, M. Leibscher, and T. Seideman: Nuclear spin selective laser control of rotational and torsional dynamics, J. Chem. Phys. 136, 084309 (2012). DOI: 10.1063/1.3687343

We explore the possibility of controlling rotational-torsional dynamics of non-rigid molecules with strong, non-resonant laser pulses and demonstrate that transient, laser-induced torsional alignment depends on the nuclear spin of the molecule. Consequently, nuclear spin isomers can be manipulated selectively by a sequence of time-delayed laser pulses. We show that two pulses with different polarization directions can induce either overall rotation or internal torsion, depending on the nuclear spin. Nuclear spin selective control of the angular momentum distribution may open new ways to separate and explore nuclear spin isomers of polyatomic molecules.

  • A. Schild, D. Choudhary, V.D. Sambre, B. Paulus: Electron density dynamics in the electronic ground state: motion along the Kekulé mode of benzene, J. Phys. Chem. A 116, 11355 (2012). DOI: 10.1021/jp305735s

If the Born-Oppenheimer approximation is invoked for the description of chemical reactions, the electron density rearranges following the motion of the nuclei. Even though this approach is central to theoretical chemistry, the explicit time dependence of the electron density is rarely studied, especially if the nuclei are treated quantum mechanically. In this article, we model the motion of benzene along the Kekulé vibrational coordinate to simulate the nuclear dynamics and electron density dynamics in the electronic ground state. Details of the change of core, valence, and π electrons are determined and analyzed. We show how the pictures anticipated by drawing Lewis structures of the rearrangement correlate with the time-dependent quantum description of the process.

  • V. Khoromskaia, D. Andrae, B.N. Khoromskij: Fast and accurate 3D tensor calculation of the Fock operator in a general basis, Comp. Phys. Comm. 183, 2392 (2012). DOI: 10.1016/j.cpc.2012.06.007

The present paper contributes to the construction of a “black-box” 3D solver for the Hartree–Fock equation by the grid-based tensor-structured methods. It focuses on the calculation of the Galerkin matrices for the Laplace and the nuclear potential operators by tensor operations using the generic set of basis functions with low separation rank, discretized on a fine N×N×N Cartesian grid. We prove the Ch2 error estimate in terms of mesh parameter, h=O(1/N), that allows to gain a guaranteed accuracy of the core Hamiltonian part in the Fock operator as h→0. However, the commonly used problem adapted basis functions have low regularity yielding a considerable increase of the constant C, hence, demanding a rather large grid-size N of about several tens of thousands to ensure the high resolution. Modern tensor-formatted arithmetics of complexity O(N), or even O(logN), practically relaxes the limitations on the grid-size. Our tensor-based approach allows to improve significantly the standard basis sets in quantum chemistry by including simple combinations of Slater-type, local finite element and other basis functions. Numerical experiments for moderate size organic molecules show efficiency and accuracy of grid-based calculations to the core Hamiltonian in the range of grid parameter N3-1015.

  • E. Voloshina: Local correlation method for metals: Benchmarks for surface and adsorption energies, Phys. Rev. B 83, 045444 (2012). DOI: 10.1103/PhysRevB.85.045444

Highly accurate methods such as coupled cluster (CC) techniques can be used for periodic systems within the framework of the method of increments. Its extension to a low-dimensional conducting system is considered. To demonstrate the presented approach, a clean Mg(0001) surface is selected, where the CC treatment with single and double excitations and perturbative triples is used for calculation of the surface energy. A further example concerns the adsorption energy of Xe on the metal surface. The obtained results can be used to verify the performance of the approximate methods. Along with the computational speed-up at the high level of accuracy, application of the method of increments provides for a possibility to analyze the influence of individual correlation energy increments on the studied property.

  • A. Kenfack, S. Banerjee, and B. Paulus: Probing electron correlation in molecules via quantum fluxes, Phys. Rev. A 85, 032501 (2012). DOI: 10.1103/PhysRevA.85.032501

We present quantum simulations of a vibrating hydrogen molecule H2 and address the issue of electron correlation. After appropriately setting the frame and the observer plane, we were able to determine precisely the number of electrons and nuclei which actually flow by evaluating electronic and nuclear fluxes. This calculation is repeated for three levels of quantum chemistry, for which we account for no correlation, Hartree-Fock, static correlation, and dynamic correlation. Exciting each of these systems with the same amount of energy, we show that the electron correlation can be revealed with the knowledge of quantum fluxes. This is evidenced by a clear sensitivity of these fluxes to electron correlation. In particular, we find that this correlation remarkably enhances more electronic yield than the nuclear one. It turns out that less electrons accompany the nuclei in Hartree-Fock than in the correlation cases.

  • S. Chelkowski, T. Bredtmann, and A. D. Bandrauk: High-order-harmonic generation from coherent electron wave packets in atoms and molecules as a tool for monitoring attosecond electrons, Phys. Rev. A 85, 033404 (2012). DOI: 10.1103/PhysRevA.85.033404

We study numerically pump-probe schemes for monitoring electron-nuclear motion in a dissociating molecule using a midinfrared, intense few-femtosecond probe laser pulse which generates molecular high-order harmonics (MHOHG) from a coherent superposition of electron-nuclear wave packets prepared by a weak femtosecond UV pump pulse from an initial bound state. We show that by varying the time delay between the intense probe pulse and the UV pump pulse by a few hundred attoseconds one alters the MHOHG signal intensity by many orders of magnitude. The periodicity of the MHOHG intensity variations as function of the time delay is equal to the period of the electron oscillation in the coherent superposition which varies with internuclear distance. We use the strong field approximation (SFA) and three-step model to explain this high sensitivity of the harmonic intensity to pulse delay time and to the overlap of nuclear wave packets. We also solve numerically the three-dimensional (3D) time-dependent Schrödinger equation describing harmonic generation for a hydrogen atom prepared in a superposition of its two lowest atomic states, in order to investigate the dependence of the same effect (in a simpler system but in 3D) on the probe-pulse carrier-envelope phase (CEP) and on the probe duration. We also relate these strong effects in the intensity of harmonics to the correlation between the velocity of the recolliding electron wave packet and the electron velocity in the coherent superposition of the electron bound states.

  • D. J. Diestler, A. Kenfack, J. Manz and B. Paulus: Coupled-Channels Quantum Theory of Electronic Flux Density in Electronically Adiabatic Processes: Application to the Hydrogen Molecule Ion, J. Phys. Chem. A 116, 2736 (2012). Link

This article presents the results of the first quantum simulations of the electronic flux density (je) by the “coupled-channels”(CC) theory, the fundamentals of which are presented in the previous article [Diestler, D. J. J. Phys. Chem. A 2012, DOI: 10.1021/jp207843z]. The principal advantage of the CC scheme is that it employs exclusively standard methods of quantum chemistry and quantum dynamics within the framework of the Born–Oppenheimer approximation (BOA). The CC theory goes beyond the BOA in that it yields a nonzero je for electronically adiabatic processes, in contradistinction to the BOA itself, which always gives je = 0. The CC is applied to oriented H2+ vibrating in the electronic ground state (2Σg+), for which the nuclear and electronic flux densities evolve on a common time scale of about 22 fs per vibrational period. The system is chosen as a touchstone for the CC theory, because it is the only one for which highly accurate flux densities have been calculated numerically without invoking the BOA [Barth et al, Chem. Phys. Lett. 2009, 481, 118]. Good agreement between CC and accurate results supports the CC approach, another advantage of which is that it allows a transparent interpretation of the temporal and spatial properties of je.

  • T. Bredtmann and J. Manz: Optimal control of the initiation of a pericyclic reaction in the electronic ground state, J. Chem. Sci. 124, 121 (2012). Link

Pericyclic reactions in the electronic ground state may be initiated by down-chirped pump-dump sub-pulses of an optimal laser pulse, in the ultraviolet (UV) frequency and sub-10 femtosecond (fs) time domain. This is demonstrated by means of a quantum dynamics model simulation of the Cope rearrangement of Semibullvalene. The laser pulse is designed by means of optimal control theory, with detailed analysis of the mechanism. The theoretical results support the recent experimental initiation of a pericyclic reaction. The present approach provides an important step towards monitoring asynchronous electronic fluxes during synchronous nuclear pericyclic reaction dynamics, with femto-to-attosecond time resolution, as motivated by the recent prediction of our group.

  • L. Hammerschmidt, C. Müller, and B. Paulus: Electron correlation contribution to the physisorption of CO on MgF2(110), J. Chem. Phys. 136, 124117 (2012). Link

We have performed CCSD(T), MP2, and DF-LMP2 calculations of the interaction energy of CO on the MgF2 (110) surface by applying the method of increments and an embedded cluster model. In addition, we performed periodic HF, B3LYP, and DF-LMP2 calculations and compare them to the cluster results. The incremental CCSD(T) calculations predict an interaction energy of Eint = −0.37 eV with a C-down orientation of CO above a Mg2+ ion at the surface with a basis set of VTZ quality. We find that electron correlation constitutes about 50 % of the binding energy and a detailed evaluation of the increments shows that the largest contribution to the correlation energy originates from the CO interaction with the closest F ions on the second layer.

  • C. Müller and B. Paulus: Wavefunction-based electron correlation methods for solids, Phys. Chem. Chem. Phys., 14, 7605 (2012). DOI:10.1039/C2CP24020C

In this article we provide an overview of the most common ways of treating electron correlation effects in 3D-periodic systems with some emphasize on wavefunction-based correlation methods such as the method of increments and the local MP2 method implemented in the CRYSCOR program. We discuss strengths and weaknesses of the different approaches and give examples for their application. Additionally, for the method of increments we discuss recent developments for its application to open shell systems and problems related to the treatment of graphene sheets.

M. J. Molski, D. Mollenhauer, S. Gohr, B. Paulus, M. A. Khanfar, H. Shorafa, S. H. Strauss, K. Seppelt: Halogenated Benzene Cation Radicals, Chem. Eur. J., 18(21), 6644 (2012). Link

The halogenated benzenes C6HF5, 2,4,6-C6H3F3, 2,3,5,6-C6H2F4, C6F6, C6Cl6, C6Br6, and C6I6 were converted into their corresponding cation radicals by using various strong oxidants. The cation-radical salts were isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy and by single-crystal X-ray diffraction. The thermal stability of the cation radicals increased with decreasing hydrogen content. As expected, the cation radicals [C6HF5]+ and 2,3,5,6-[C6H2F4]+ had structures with the same geometry as C6HF5 and 2,3,5,6-[C6H2F4]. In contrast, the cation radicals [C6F6]+, [C6Cl6]+, and possibly also [C6Br6]+ exhibited Jahn–Teller-distorted geometries in the crystalline state. In the case of C6F6+Sb2F11, two low-symmetry geometries were observed in the same crystal. Interestingly, the structures of the cation radicals 2,4,6-[C6H3F3]+ and C6I6+ did not exhibit Jahn–Teller distortions. DFT calculations showed that the explanation for the lack of distortion of these cations from the D3h or D6h symmetry of the neutral benzene precursor was different for 2,4,6-[C6H3F3]+ than for [C6I6]+.

  • E. N. Voloshina, Yu. S. Dedkov, S. Torbrügge, A. Thissen, and M. Fonin: Graphene on Rh(111): Scanning tunneling and atomic force microscopies studies, Appl. Phys. Lett. 100, 241606 (2012). Link
    ☆ The article was selected for the June 25, 2012 issue of Virtual Journal of Nanoscale Science & Technology

The electronic and crystallographic structure of the graphene/Rh(111) moiré lattice is studied via combination of density-functional theory calculations and scanning tunneling and atomic force microscopy (STM and AFM). Whereas the principal contrast between hills and valleys observed in STM does not depend on the sign of applied bias voltage, the contrast in atomically resolved AFM images strongly depends on the frequency shift of the oscillating AFM tip. The obtained results demonstrate the perspectives of application atomic force microscopy/spectroscopy for the probing of the chemical contrast at the surface.

  • E. Voloshina and Yu. S. Dedkov: Graphene on metallic surfaces: problems and perspectives, Phys. Chem. Chem. Phys. 14, 13502 (2012). Link

The present manuscript summarizes the modern view on the problem of the graphene–metal interaction. Presently, the close-packed surfaces of d metals are used as templates for the preparation of highly-ordered graphene layers. Different classifications can be introduced for these systems: graphene on lattice-matched and graphene on lattice-mismatched surfaces where the interaction with the metallic substrate can be either “strong” or “weak”. Here these classifications, with the focus on the specific features in the electronic structure in all cases, are considered on the basis of large amount of experimental and theoretical data, summarized and discussed. The perspectives of the graphene–metal interfaces in fundamental and applied physics and chemistry are pointed out.

  • L. Hammerschmidt, S. Schlecht, B. Paulus: Electronic structure and the ground-state properties of cobalt antimonide skutterudites: Revisited with different theoretical methods, Physical Status Solidi A (2012). Link

We have computed the lattice structure, bulk modulus, electronic structure, and cohesive energies for the CoSb3 skutterudite by performing plane wave and atomic basis set DFT, as well as HF atomic basis set calculations. We find that plane wave and atomic basis set DFT calculations compare almost perfectly well. Band gaps vary significantly, depending on the applied functional and subtle changes of the lattice structure of CoSb3. Where LDA strongly overestimates the binding, cohesive energies are reasonably well described by GGA and hybrid DFT functionals within 2 eV in comparison to experiment. HF results are unreasonably far off compared to DFT and experimental values for all calculated properties, which indicates that correlation effects play an important role in the characterization of skutterudites.

  • T. Bredtmann, E. Hupf, B. Paulus: Electronic fluxes during large amplitude vibrations of single, double and triple bonds, Phys. Chem. Chem. Phys. ‘‘‘14‘‘‘, 15494 (2012). Link
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