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arXiv2014

Design and Development of Surface Modified p and n Type Silicon Sensor for Nitrogen Gas Flow Measurement

We report a gas flow driven voltage generation of Octyltrichlorosilane (OTS) molecules self assembled on silicon wafers (Si wafers). OTS Self assembled Monolayer (SAM) has been coated on both p-type and n-type doped silicon wafers (p-Si and n-Si wafers) using dip coating method. We have measured the flow induced voltage generation on OTS SAM coated Si wafers/ Uncoated Si wafers at modest gas flow velocities of subsonic regime (Mach number < 0.2) using national instruments NI-PXI-1044 Workstation. The gas flow driven voltage generation is mainly due to the interplay mechanisms of Bernoulli principle and Seebeck effect. The surface morphology of OTS SAM coated p-Si and n-Si wafers were characterized by SEM analysis. In this study, our results shows that OTS SAM coated p-Si and n-Si wafers shows better sensitivity towards nitrogen gas flow when compared with the uncoated Si wafers. OTS SAM also exhibits high thermal stability and hydrophobicity.

physics.ins-det↗

arXiv2014

Water hexamer: Self-consistent phonons versus reversible scaling versus replica exchange molecular dynamics

Classical free energies for the cage and prism isomers of water hexamer computed by the self- consistent phonons (SCP) method and reversible scaling (RS) method are presented for several flexible water potentials. Both methods have been augmented with a rotational correction for improved accuracy when working with clusters. Comparison of the SCP results with the RS results suggests a fairly broad temperature range over which the SCP approximation can be expected to give accurate results for systems of water clusters, and complements a previously reported assessment of SCP. Discrepancies between the SCP and RS results presented here, and recently published replica exchange molecular dynamics (REMD) results bring into question the convergence of the REMD and accompanying replica exchange path integral molecular dynamics results. In addition to the ever-present specter of unconverged results, several possible sources for the discrepancy are explored based on inherent characteristics of the methods used.

physics.atm-clus↗

arXiv2014

Humblonium: Classical Atoms and the Earnshaw Plasma

It is shown that electrostatic and diamagnetic forces can combine to give long lasting metastable bound dimers of macro and mesoscopically sized objects for a physically attainable material regime. This can be a large enough effect to support itself in a trap against Earth's gravity and they can stable at very high temperatures. For a more restricted material parameter set, we investigate the possibility of stable many particle collections that lose their identity as bound pairs and create a kind of plasma. These would constitute a kind of transitional state between fluids and granular materials but, unlike usual approaches, the fluid is a gas rather than a liquid.

physics.atm-clus↗

arXiv2014

A mini review on the chemistry and catalysis of the water gas shift reaction

Water gas shift (WGS) reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen. It is an important reaction industrially used in conjunction with steam reforming of hydrocarbons for the production of high purity hydrogen. Grenoble et al examined the roles of both active metals and metal oxide support on the kinetics of the WGS reaction. They found out that the turn over numbers of various Al2O3 supported transition metals decreased in the trend of Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh, and Ir, which corresponds nicely to the observed volcano shaped correlation between catalytic activities and respective CO adsorption heat. This is a strong indication that CO gets activated on the metal surface during the reaction and different metals have different activation energies. The authors also observed that the turn over number of Pt/Al2O3 was one order of magnitude higher than that of Pt/SiO2, indicating a strong support effect, which the authors ascribed to the activation of H2O.

physics.chem-ph↗

arXiv2014

Anomalous quantum and isotope effects in water clusters: Physical phenomenon, model artifact, or bad approximation?

Free energy differences $ΔF:=F-F_{\text{prism}}$ are computed for several isomers of water hexamer relative to the "prism" isomer using the self-consistent phonons method. %$ΔF:=F-F({prism})$ We consider the isotope effect defined by the quantity $δF_{D_2O}:=ΔF_{\rm D_2O}-ΔF_{\rm H_2O}$, and the quantum effect, $δF_{\hbar=0}:=ΔF_{\hbar=0}-ΔF_{\rm H_2O}$, and evaluate them using different flexible water models. While both $δF_{D_2O}$ and $δF_{\hbar=0}$ are found to be rather small for all of the potentials, they are especially small for two of the empirical models, q-TIP4P/F and TTM3-F, compared to q-SPC/Fw and the two {\it abinitio}-based models, WHBB and HBB2-pol. This qualitative difference in the properties of different water models cannot be explained by one being "more accurate" than the other. We speculate as to whether the observed anomalies are caused by the special properties of water systems, or are an artifact of either the potential energy surface form/parametrization or the numerical approximation used.

physics.atm-clus↗

arXiv2014

Calculation of ground state energy of harmonically confined two dipolar fermions

We calculate the ground state energies of a system of two dipolar fermions trapped in a harmonic oscillator potential. The dipoles are assumed to be aligned parallel to each other. We perform the calculations of ground state energy as a function of strength of interaction between two fermions by employing variational method with Hylleraas-like explicitly correlated wave function. Furthermore, we perform calculations of ground state energy within Hartree-Fock approximation and the magnitude of correlation energy is estimated by subtracting these results from the corresponding wave function based results. We also carry out calculations of ground state energies within the realm of density functional theory by using recently reported expressions for exchange and correlation energies under local density approximation. By comparing correlated wave function based results with those obtained using density functional theory approach we examine the role of fermion-fermion correlation and assess the accuracy of local density approximation based expression for the correlation energy functional.

physics.atm-clus↗

arXiv2014

Tunable Catalysis of Water to Peroxide with Anionic, Cationic, and Neutral Atomic Au, Ag, Pd, Rh, and Os

Fundamental anionic, cat-ionic, and neutral atomic metal predictions utilizing density functional theory calculations validate the recent discovery identifying the interplay between the resonances and the RT minimum obtained through complex angular momentum analysis as the fundamental atomic mechanism underlying nano-scale catalysis. Here we investigate the optimization of the catalytic behavior of Au, Ag, Pd, Rh, and Os atomic systems via polarization effects and conclude that anionic atomic systems are optimal and therefore ideal for catalyzing the oxidation of water to peroxide, with anionic Os being the best candidate. The discovery that cat-ionic systems increase the transition energy barrier in the synthesis of peroxide could be important as inhibitors in controlling and regulating catalysis. These findings usher in a fundamental and comprehensive atomic theoretical framework for the generation of tun-able catalytic systems.

physics.atm-clus↗

arXiv2014

Direct signature of light-induced conical intersections in diatomics

Nonadiabatic effects are ubiquitous in physics, chemistry and biology. They are strongly amplified by conical intersections (CIs) which are degeneracies between electronic states of triatomic or larger molecules. A few years ago it has been revealed that CIs in molecular systems can be formed by laser light even in diatomics. Due to the prevailing strong nonadiabatic couplings, the existence of such laser-induced conical intersections (LICIs) may considerably change the dynamical behavior of molecular systems. By analyzing the photodissociation process of the D2+ molecule carefully, we found a robust effect in the angular distribution of the photofragments which serves as a direct signature of the LICI providing undoubted evidence for its existence.

physics.atm-clus↗

arXiv2014

On the dynamics of photo-electrons from C$_{60}$

We explore photo-electron spectra (PES) and photo-electron angular distributions (PAD) of C$_{60}$ with time-dependent density functional theory (TDDFT) in real time. To simulate experiments in gas phase, we consider isotropic ensembles of cluster orientations and perform orientation averaging of the TDDFT calculations. First, we investigate ionization properties of C$_{60}$ by one-photon processes in the range of VUV energies. The PES map the energies of the occupied single-particle states, while the weights of the peaks in PES are given by the depletion of the corresponding level. The different influences can be disentangled by looking at PES from slightly different photon frequencies. PAD in the one-photon regime can be characterized by one parameter, the anisotropy. This single parameter unfolds worthwhile information when investigating the frequency and state dependences. We also discuss the case of multi-photon ionization induced by strong infrared laser pulses in C$_{60}$. In agreement with measurements, we find that the PES show a regular comb of peaks separated by the photon energy. Our calculations reveal that this happens because only very few occupied states of C$_{60}$ near the ionization threshold contribute to emission and that these few states happen to cooperate filling the same peaks. The PAD show a steady increase of anisotropy with increasing photon order.

physics.atm-clus↗

arXiv2014

Monte Carlo tests of Orbital-Free Density Functional Theory

The relationship between the exact kinetic energy density in a quantum system in the frame of Density Functional Theory and the semiclassical functional expression for the same quantity is investigated. The analysis is performed with Monte Carlo simulations of the Kohn-Sham potentials. We find that the semiclassical form represents the statistical expectation value of the quantum nature. Based on the numerical results, we propose an empirical correction to the existing functional and an associated method to improve the Orbital-Free results.

physics.atm-clus↗

arXiv2015

Non-Newtonian behavior and molecular structure of Cooee bitumen under shear flow: a non-equilibrium molecular dynamics study

The rheology and molecular structure of a model bitumen (Cooee bitumen) under shear are investigated in the non-Newtonian regime using non-equilibrium molecular dynamics simulations. The shear viscosity, normal stress differences and pressure of the bitumen mixture are computed at different shear rates and different temperatures. The model bitumen is shown to be a shear-thinning fluid at all temperatures. In addition, the Cooee model is able to reproduce experimental results showing the formation of nanoaggregates composed of stacks of flat aromatic molecules in bitumen. These nanoaggregates are immersed in a solvent of saturated hydrocarbon molecules. At a fixed temperature, the shear-shinning behavior is related to the inter- and intramolecular alignment of the solvent molecules, but also to the decrease of the average size of the nanoaggregates at high shear rates. The variation of the viscosity with temperature at different shear rates is also related to the size and relative composition of the nanoaggregates. The slight anisotropy of the whole sample due to the nanoaggregates is considered and quantified. Finally, the position of bitumen mixtures in the broad literature of complex systems such as colloidal suspensions, polymer solutions and associating polymer networks is discussed.

physics.chem-ph↗

arXiv2015

On the behavior of scattering phases in collisions of electrons with multi-atomic objects

We have studied the energy dependence of several first scattering phases with multi-atomic object. As concrete examples representing the general trends endohedrals Neon inside C60 and Argon inside C60 are considered. It appeared that the presence of an inner atom, either Ne or Ar, qualitatively affects the scattering phases, in spite of the fact that the fullerene consists of 60 carbon atoms, while the atom staffed inside is only one. Calculations are performed in the one-electron Hartree-Fock (HF) and random phase approximation with exchange (RPAE) for the inner atom while the fullerenes shell is substituted by static potential without and with the polarization potential. It appeared that the total endohedral scattering phase is simply a sum of atomic, Ne or Ar, and fullerenes C60 phases, contrary to the intuitive assumption that the total phases on C60 and Neon inside C60 or Ar inside C60 has to be the same.

physics.atm-clus↗

arXiv2015

Atom- photon entanglement beyond the multi-photon resonance condition

The density matrix equations of motion in near-degenerate three-level V-type closed-loop atomic system are calculated numerically in Floquet frame. The dynamical behavior of atom- photon entanglement between the dressed atom and its spontaneous emission is studied in semi classical approach beyond the two-photon resonance condition in such a system. The quantum entropy of these two subsystems is investigated by using the von Neumann entropy. It is shown that, the degree of entanglement measure (DEM) can be controlled via the intensity and the detuning of coupling optical field and quantum interference induced by spontaneous emission. Moreover in the absence of quantum interference the steady state behavior of DEM can be achieved even in beyond the two- photon resonance condition. Furthermore in the absence of quantum interference for special parameters of Rabi frequency and detuning of driving laser field disentanglement can be occurred. Also the electromagnetically induced transparency condition can be obtained when the system is disentangled.

physics.atm-clus↗

arXiv2015

Strengthening gold-gold bonds by complexing gold clusters with noble gases

We report an unexpectedly strong and complex chemical bonding of rare-gas atoms to neutral gold clusters. The bonding features are consistently reproduced at different levels of approximation within density-functional theory and beyond: from GGA, through hybrid and double-hybrid functionals, up to renormalized second-order perturbation theory. The main finding is that the adsorption of Ar, Kr, and Xe reduces electron-electron repulsion within gold dimer, causing strengthening of the Au-Au bond. Differently from the dimer, the rare-gas adsorption effects on the gold trimer's geometry and vibrational frequencies are mainly due to electron occupation of the trimer's lowest unoccupied molecular orbital. For the trimer, the theoretical results are also consistent with far-infrared multiple photon dissociation experiments.

cond-mat.mtrl-sci↗

arXiv2015

Size distribution of islands according to 2D growth model with 2 kinds of diffusion atoms

We simulated the growth of 2D islands with 2 kinds of diffusion atoms using the kinetic Monte- Carlo (kMC) method. As a result, we found that the slow atoms tend to create nuclei and determine the island volume distribution, along with additional properties such as island density. We also conducted a theoretical analysis using the rate equation of the point-island model to confirm these results.

physics.atm-clus↗

arXiv2018

Coulomb cluster explosion boosted by a quasi-dc pulse -- diagnostic tool and ultimate test of laser fusion efficiency in clusters

To greatly enhance output of nuclear fusion produced neutrons in a laser-initiated Coulomb explosion of Deuterium clusters, we propose to accelerate the resulting ions by a quasi-$dc$ electrical pulse to the energies where the $D^+ + D$ collision cross-section is the highest. With $D^+$ ions bombarding then a Deuterium-rich solid-state cathode, this allows one to solve a few problems simultaneously by (a) completely removing electron cloud hindering the Coulomb explosion of ionic core, (b) utilizing up to 100% of the cluster ions to collide with the high-density packed nuclei, and (c) reaching highly increased cross-section of neutron production in a single $D^+ + D$ collision, in particular by using a multi-layered target. We also consider the use of E-pulse acceleration for diagnostic purposes.

physics.atm-clus↗

arXiv2016

First-principles data set of 45,892 isolated and cation-coordinated conformers of 20 proteinogenic amino acids

We present a structural data set of the 20 proteinogenic amino acids and their amino-methylated and acetylated (capped) dipeptides. Different protonation states of the backbone (uncharged and zwitterionic) were considered for the amino acids as well as varied side chain protonation states. Furthermore, we studied amino acids and dipeptides in complex with divalent cations (Ca2+, Ba2+, Sr2+, Cd2+, Pb2+, and Hg2+). The database covers the conformational hierarchies of 280 systems in a wide relative energy range of up to 4 eV (390 kJ/mol), summing up to an overall of 45,892 stationary points on the respective potential-energy surfaces. All systems were calculated on equal first-principles footing, applying density-functional theory in the generalized gradient approximation corrected for long-range van der Waals interactions. We show good agreement to available experimental data for gas-phase ion affinities. Our curated data can be utilized, for example, for a wide comparison across chemical space of the building blocks of life, for the parametrization of protein force fields, and for the calculation of reference spectra for biophysical applications.

q-bio.BM↗

arXiv2015

Partial Ionic Bonding in Homogeneous Sodium Clusters

In this work, we report an interesting observation of partial ionic bonding due to charge transfer in homogeneous sodium clusters. The charge transfer causes the electronic charge to accumulate on the surface, and the resulting charges on atoms range between +0.4 to -1.0 |e^-|. We also demonstrate that this disparity among effective charges on atoms is geometry dependent, such that atoms experiencing similar surrounding, have equal effective charge. It is speculated that this phenomenon will occur among other homogeneous clusters as well, and its extent will be defined by the valence electron delocalization.

cond-mat.mes-hall↗