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arXiv2013

Optical and magnetic excitations of metal-encapsulating Si cages: A systematic study by time-dependent density functional theory

Systematic study of the optical and magnetic excitations of twelve MSi$_{12}$ and four MSi$_{10}$ transition metal encapsulating Si cages has been carried out by employing real time time-dependent density functional theory. Criteria for the choice of transition metals (M) are clusters' stability, synthesizability, and diversity. It was found that both the optical absorption and the spin-susceptibility spectra are mainly determined by, in decreasing order of importance: 1) the cage shape, 2) the group in the Periodic Table M belongs to, and 3) the period of M in the Periodic Table. Cages with similar structures and metal species that are close to each other in the Periodic Table possess spectra sharing many similarities, e. g., the optical absorption spectra of the MSi$_{12}$ (M = V, Nb, Ta, Cr, Mo, and W), which are highly symmetric and belong to groups 4 and 5 of the Periodic Table, all share a very distinctive peak at around 4 eV. In all cases, although some of the observed transitions are located at the Si skeleton of the cages, the transition metal specie is always significant for the optical absorption and the spin-susceptibility spectra. Our results provide finger-print data for identification of gas-phase MSi$_{12}$ and MSi$_{10}$ by optical absorption spectroscopy.

physics.atm-clus↗

arXiv2014

A Couped-Qubit Tavis Cummings Scheme for Prolonging Quantum Coherence

Qubit-qubit interactions can significantly boost quantum coherence times for Bell states. The coherence-time-enhancements are however not monotonic and there exists a phase where further increasing the interaction is unhelpful. A resonator in a suggested circuit QED type implementation of the Tavis-Cummings(Dicke) model, is shown to shift this transition point depending on the number of loaded photons. This allows the resonator to amplify the coherence enhancements in certain regimes. The interactions also induce unusual collapse and revival type behavior for the entanglement dynamics. A new and exact open quantum systems formalism -- the quasi-Hamiltonians for the Dicke model thus reveals how a Bell state in a resonator can be protected against $1/f$ noise from randomly fluctuating two level systems. Simple circuit level details are given for flux qubits.

cond-mat.mes-hall↗

arXiv2014

Pick-off annihilation of positronium in matter using full correlation single particle potentials: solid He

We investigate the modeling of positronium (Ps) states and their pick-off annihilation trapped at open volumes pockets in condensed molecular matter. Our starting point is the interacting many-body system of Ps and a He atom because it is the smallest entity that can mimic the energy gap between the highest occupied and lowest unoccupied molecular orbitals of molecules and yet the the many-body structure of the HePs system can be calculated accurately enough. The exact-diagonalization solution of the HePs system enables us to construct a pair-wise full-correlation single-particle potential for the Ps-He interaction and the total potential in solids is obtained as a superposition of the pair-wise potentials. We study in detail Ps states and their pick-off annihilation rates in voids inside solid He and analyse experimental results for Ps-induced voids in liquid He obtaining the radii of the voids. More importantly, we generalize our conclusions by testing the validity of the Tao-Eldrup model, widely used to analyse ortho-Ps annihilation measurements for voids in molecular matter, against our theoretical results for the solid He. Moreover, we discuss the influence of the partial charges of polar molecules and the strength of the van der Waals interaction on the pick-off annihilation rate.

physics.atm-clus↗

arXiv2013

Glassy clusters: Relations between their dynamics and characteristic features of their energy landscape

Based on a recently introduced metric for measuring distances between configurations, we in- troduce distance-energy (DE) plots to characterize the potential energy surface (PES) of clusters. Producing such plots is computationally feasible on the density functional (DFT) level since it re- quires only a set of a few hundred stable low energy configurations including the global minimum. By comparison with standard criteria based on disconnectivity graphs and on the dynamics of Lennard- Jones clusters we show that the DE plots convey the necessary information about the character of the potential energy surface and allow to distinguish between glassy and non-glassy systems. We then apply this analysis to real systems on the DFT level and show that both glassy and non-glassy clusters can be found in simulations. It however turns out that among our investigated clusters only those can be synthesized experimentally which exhibit a non-glassy landscape.

physics.atm-clus↗

arXiv2013

Vibronic speed-up of the excitation energy transfer in the Fenna-Matthews-Olson complex

We show that the efficient excitation energy transfer in the Fenna-Matthews-Olson molecular aggregate under realistic physiological conditions is fueled by underdamped vibrations of the embedding proteins. For this, we present numerically exact results for the quantum dynamics of the excitons in the presence of nonadiabatic vibrational states in the Fenna-Matthews-Olson aggregate employing a environmental fluctuation spectral function derived from experiments. Assuming the prominent 180 cm$^{-1}$ vibrational mode to be underdamped, we observe, on the one hand, besides vibrational coherent oscillations between different excitation levels of the vibration also prolonged electronic coherent oscillations between the initially excited site and its neighbours. On the other hand, however, the underdamped vibrations provide additional channels for the excitation energy transfer and by this increase the transfer speed by up to $30\%$ .

physics.chem-ph↗

arXiv2013

Comment on the paper Photoionization of endohedral atoms using R-matrix methods: application to Xe@C60 by T. W. Gorczyca, M. F. Hasoglu and S. T. Manson, Phys. Rev. A86, 033204 (2012) [1]

We demonstrate serious misrepresentation of existing experimental data and important omissions in the List of references in the paper [1]. We demonstrate that what is called in [1] total photoionization cross-section is in fact a partial photoionization cross-section. We demonstrate that long before [1] and presented there experimental data for photoionization of Xe@C60+ were obtained, a theoretical prediction for Xe@C60 have been published that was in a reasonable agreement with experiment. These data are even not mentioned in [1].

physics.atom-ph↗

arXiv2013

A beamline for time-resolved extreme ultraviolet and soft x-ray spectroscopy

High harmonic generation is a convenient way to obtain extreme ultraviolet light from table-top laser systems and the experimental tools to exploit this simple and powerful light source for time-resolved spectroscopy are being developed by several groups. For these applications, brightness and stability of the high harmonic generation is a key feature. This article focuses on practical aspects in the generation of extreme ultraviolet pulses with ultrafast commercial lasers, namely generation parameters and online monitoring as well as analysis of generation yield and stability.

physics.atm-clus↗

arXiv2014

Gas Storage Potential of Li-decorated ExBox4+

The newly developed compound ExBox4+ is explored to check whether it is a proficient hydrogen storage material. Both exoherdal and endohedral hydrogen adsorption on ExBox4+ are studied. Endohedral hydrogen molecules interact strongly than exohedral ones. The hydrogen adsorption energy is as good as the recently studied charged fullerenes. The hydrogen storage capacity appears to be ~4.3 wt%. The endohedral CO sorption is also analysed with the help of DFT. The first principle DFT calculation and MD simulation are performed to investigate the effect of lithium doping on the gas adsorbing capacity and adsorption enthalpy as well as adsorption energy of ExBox4+.The metal atom interaction with ExBox4+is found to be pretty strong, and the interaction energy appears to be higher than the metal cohesive energy.The thermodynamic parameters suggest that metal doping method is spontaneous in nature. The analysis of adsorption energy, thermodynamic properties and MD simulation results suggest that Li doped ExBox4+ can be a promising gas adsorbing material. Lithium doping increases the gas adsorption capacity of ExBox4+. The Li decorated ExBox4+ is found to adsorb twenty four hydrogen molecules with a gravimetric wt% of 6.23. Adsorption of CO on the metal decorated ExBox4+ is also studied. AIM analysis is performed to obtain a general idea of the bonding interaction between Li-H and Li-C.

physics.atm-clus↗

arXiv2014

Does Confinement Force Marriage between Two Unwilling Partners?: A Case Study of He2@BmNm (m=12, 16)

TThe stability of Ngn@B12N12 and Ngn@B16N16 systems is assessed through a density functional study and ab initio simulation. Although they are found to be thermodynamically unstable with respect to the dissociation of individual Ng atoms and parent cage, ab initio simulation reveals that except Ne2@B12N12 they are kinetically stable to retain their structures intact throughout the simulation time (500 fs). The Ne2@B12N12 cage dissociates and the Ne atoms get separated as the simulation proceeds. The He-He unit undergoes translation, rotation and vibration inside the cavity of B12N12 and B16N16 cages. Electron density analysis (Atoms-in-Molecule (AIM)) shows that there is some degree of covalent character in He-He bond (Wc type) of He2@B12N12. In case of He2@B16N16, the He-He interaction is mostly of noncovalent type (Wn). In many cases, especially for the heavier Ng atoms the Ng-N/B bonds are found to be of covalent type or at least having some degree of covalent character. But Wiberg bond indices show zero bond order in He-He bond and very low bond order in cases of Ng-N/B bonds. Energy decomposition analysis (EDA) provides further insights into the bonding among the Ng atoms and the cage. The change in charge distribution, radius, hardness, electrophilicity and polarizability implies that they possess different properties and reactivity from their parent moieties. The HOMO energies of He2 units in He2@B12N12 and He2@B16N16 are significantly higher than that in free He atoms. Their kinetic stability and different calculated properties imply that the He-He interaction and Ng-N/B interaction may be considered as chemical bonds according to the IUPAC definition.

physics.atm-clus↗

arXiv2014

New Transition Mechanism: The Shaking Effect

It has been observed that the trajectory of a H2 molecular beam is modified as it passes through a homogeneous magnetic field and an oscillating magnetic field. Based on this observation, this work describes a new method to determine the magnetic moment of atoms and molecules with internal angular momentum. This new method is similar to that used in the Molecular Beam Magnetic Resonance (MBMR) but 'it does not employ' the inhomogeneous fields A and B. Results shown here are in agreement with the theory of the motion of systems with internal angular momentum developed by M.Dorado. Furthermore, in this work the transition mechanism given by I.I. Rabi and N.F. Ramsey is reviewed and completed with the introduction of nutation frequency and shaking effect.

physics.atm-clus↗

arXiv2014

Transverse ultrasonic anomaly in La1/3Sr2/3MnO3

The charge ordering (CO) transition in polycrystalline La1/3Sr2/3MnO3 has been studied by measuring the resistivity, magnetization and transverse ultrasonic velocity. At about 235K, a conspicuous increase in resistivity was observed, while the magnetization shows a cusp structure, corresponding to an antiferromagnetic charge ordering transition. Around this transition temperature, dramatic anomaly in transverse sound velocity was observed. The simultaneous occurrence of electron, magnon and phonon anomalous features implies strong spin-phonon coupling and electron-phonon in La1/3Sr2/3MnO3. The analysis suggests that the spin-phonon interaction is due to single-ion magnetostriction, and electron-phonon coupling originates from the Jahn-Teller effect of Mn3+.

physics.atm-clus↗

arXiv2014

Watching the acetylene vinylidene intramolecular reaction in real time

It is a long-standing dream of scientists to capture the ultra-fast dynamics of molecular or chemical reactions in real time and to make a molecular movie. With free-electron lasers delivering extreme ultraviolet (XUV) light at unprecedented intensities, in combination with pump-probe schemes, it is now possible to visualize structural changes on the femtosecond time scale in photo-excited molecules. In hydrocarbons the absorption of a single photon may trigger the migration of a hydrogen atom within the molecule. Here, such a reaction was filmed in acetylene molecules (C2H2) showing a partial migration of one of the protons along the carbon backbone which is consistent with dynamics calculations on ab initio potential energy surfaces. Our approach opens attractive perspectives and potential applications for a large variety of XUV-induced ultra-fast phenomena in molecules relevant to physics, chemistry, and biology.

physics.atm-clus↗

arXiv2014

Finite temperature dynamics of anionic water trimers

Utilising Car-Parrinello molecular dynamics simulations, the finite temperature behaviour of water trimers, (H2O)3, in both neutral and anionic frameworks, has been investigated. A significant structural change in the anionic structure has been observed at temperatures above 100 K where a chain geometry has formed and stabilised entropically. On the other hand, neutral trimers have remained in their ring structure, as predicted theoretically at low temperatures, for long time periods.

physics.atm-clus↗

arXiv2014

Methane Oxidation to Methanol without CO2 Emission: Catalysis by Atomic Negative Ions

The catalytic activities of the atomic Y-, Ru-, At-, In-, Pd-, Ag-, Pt-, and Os- ions have been investigated theoretically using the atomic Au- ion as the benchmark for the selective partial oxidation of methane to methanol without CO2 emission. Dispersion-corrected density-functional theory has been used for the investigation. From the energy barrier calculations and the thermodynamics of the reactions, we conclude that the catalytic effect of the atomic Ag-, At-, Ru-, and Os- ions is higher than that of the atomic Au- ion catalysis of CH4 conversion to methanol. By controlling the temperature around 290K (Os-), 300K (Ag-), 310K (At-), 320K (Ru-) and 325K (Au-) methane can be completely oxidized to methanol without the emission of CO2. We conclude by recommending the investigation of the catalytic activities of combinations of the above negative ions for significant enhancement of the selective partial oxidation of methane to methanol.

physics.chem-ph↗

arXiv2014

Femtosecond photoelectron and photoion spectrometer with vacuum ultraviolet probe pulses

We describe a setup to study ultrafast dynamics in gas-phase molecules using time-resolved photoelectron and photoion spectroscopy. The vacuum ultraviolet (VUV) probe pulses are generated via strong field high-order harmonic generation from infrared femtosecond laser pulses. The band pass characteristic in transmission of thin indium (In) metal foil is exploited to isolate the $9^{\text{th}}$ harmonic of the 800 nm fundamental (H9, 14 eV, 89 nm) from all other high harmonics. The $9^{\text{th}}$ harmonic is obtained with high conversion efficiencies and has sufficient photon energy to access the complete set of valence electron levels in most molecules. The setup also allows for direct comparison of VUV single-photon probe with 800 nm multi-photon probe without influencing the delay of excitation and probe pulse or the beam geometry. We use a magnetic bottle spectrometer with high collection efficiency for electrons, serving at the same time as a time of flight spectrometer for ions. Characterization measurements on Xe reveal the spectral width of H9 to be $190\pm60$ meV and a photon flux of $\sim1\cdot10^{7}$ photons/pulse after spectral filtering. As a first application, we investigate the S$_1$ excitation of perylene using time-resolved ion spectra obtained with multi-photon probing and time-resolved electron spectra from VUV single-photon probing. The time resolution extracted from cross-correlation measurements is $65\pm10$ fs for both probing schemes and the pulse duration of H9 is found to be $35\pm8$ fs.

physics.atm-clus↗

arXiv2014

A single impurity in an ideal atomic Fermi gas: current understanding and some open problems

We briefly review some current theoretical and experimental aspects of the problem of a single spinless impurity in a 3D polarised atomic Fermi gas at zero temperature where the interactions can be tuned using a wide Feshbach resonance. We show that various few-body states in vacuum composed of the impurity and background gas atoms (single impurity, dimer, trimer, tetramer) give rise to corresponding dressed states ({\em polaron}, {\em dimeron}, {\em trimeron}, {\em tetrameron}) in the gas and inherit many of their characteristics. We study the ground state focussing on the choice of wave function and its properties. We raise a few unsolved problems: whether the polaron and dimeron are really separate branches, what other few-body states might exist, the nature of the groundstate for large numbers of particle-hole pairs and why is the polaron ansatz so good. We then turn to the excited states, and to the calculation of the effective mass. We examine the bounds on the effective mass and raise a conjecture about that of composite quasiparticle states.

cond-mat.quant-gas↗

arXiv2014

Extended Wigner function formalism for the spatial propagation of particles with internal degrees of freedom

An extended Wigner function formalism is introduced for describing the quantum dynamics of particles with internal degrees of freedom in the presence of spatially inhomogeneous fields. The approach is used for quantitative simulations of molecular beam experiments involving space-spin entanglement, such as the Stern-Gerlach and the Rabi experiment. The formalism allows a graphical visualization of entanglement and decoherence processes.

quant-ph↗

arXiv2014

Nano cluster dissociation sensitive to the elasticity of metal as a collision target

The sensitive dependence of the cluster ion dissociation behavior on the elastic response of the target metal is experimentally demonstrated. Five types of metal are bombarded with cluster ions consisting of thousands of argon atoms at an incident kinetic energy per atom of less than 10 eV. A mass spectroscopic analysis of dissociated ions such as dimer or trimer is carried out. The dissociation rate is found to be significantly different for each metal. The relationship between the dissociation rate and the impulsive stress at the contact between the cluster ion and the metal is investigated. The impulsive stress is calculated based on the Youngs modulus of the cluster ion and the metal, under the assumption that the collision is initially elastic. As a result, magnitude correlation in the dissociation rate well corresponds with that in the impulsive stress. This result is important in that it suggests a new method to evaluate mechanical property of materials such as Youngs modulus by analyzing the collision with nanocluster ions.

physics.atm-clus↗