Cross-over mechanism of the melting transition in monolayers of alkanes adsorbed on graphite and the universality of energy scaling

http://arxiv.org/ftp/arxiv/papers/0902/0902.4422.pdfThe interplay between the torsional potential energy and the scaling of the 1-4 van der Waals and Coulomb interactions determines the stiffness of flexible molecules. In molecular simulations often ad-hoc values for the scaling factor (SF) are adopted without adequate justification. In this letter we demonstrate for the first time that the precise value of the SF has direct consequences on the critical properties and mechanisms of systems undergoing a phase transition. By analyzing the melting of n-alkanes (hexane C6, dodecane C12, tetracosane C24) on graphite, we show that the SF is not a universal feature, that it monotonically decreases with the molecular length, and that it drives a cross-over between two distinct mechanisms for melting in such systems.Acknowledgment is made to the donors of The American Chemical Society Petroleum Research Fund (PRF43277-B5) for the support of this research. This material is based upon work supported in part by the Department of Energy under award number DE-FG02-07ER46411. Computational resources were provided by the University of Missouri Bioinformatics Consortium

Melting of hexane monolayers adsorbed on graphite: the role of domains and defect formation

http://arxiv.org/ftp/arxiv/papers/0903/0903.1065.pdfWe present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduces all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) fully atomistic representation of hexane; (ii) explicit site-by-site interaction with carbon atoms in graphite; (iii) CHARMM force field with carefully chosen adjustable parameters of non-bonded interaction; (iv) numerous $\ge$ 100 ns runs, requiring a total computation time of ca. 10 CPU-years. This has allowed us to determine correctly the mechanism of the transition: molecular reorientation within lamellae without perturbation of the overall adsorbed film structure. We observe that the melted phase has a dynamically reorienting domain-type structure whose orientations reflect that of graphite.This material is based upon work supported in part by the Department of Energy under Award Number DE-FG02-07ER46411. Acknowledgment is made to the Donors of The American Chemical Society Petroleum Research Fund (PRF43277-B5). Computational support was provided by the University of Missouri Bioinformatics Consortium

Structural and phase properties of tetracosane (C24H50) monolayers adsorbed on graphite. Explicit Hydrogen Molecular Dynamics study

http://arxiv.org/ftp/arxiv/papers/0805/0805.1435.pdfWe discuss Molecular Dynamics (MD) computer simulations of a tetracosane (C24H50) monolayer physisorbed onto the basal plane of graphite. The adlayer molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom structure having six graphene layers. The tetracosane dynamics modeled in the fully atomistic manner agree well with experiment. The low-temperature ordered solid organizes in rectangular centered structure, incommensurate with underlying graphite. Above T = 200 K, as the molecules start to lose their translational and orientational order via gauche defect formation, a weak smectic mesophase (observed experimentally but never reproduced in United Atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions are included in the model. If the electrostatic charges are set to zero (as it is in UA force field), the melting temperature increases by ~70 K with respect to the experimental value. When the non-bonded 1-4 interaction is not scaled, the melting temperature decreases by ~90 K. If the scaling factor is set to 0.5, the melting occurs at T = 350 K, in very good agreement with experimental data.Acknowledgment is made to the Donors of The American Chemical Society Petroleum Research Fund (PRF43277 - B5), and the University of Missouri Research Board, for the support of this research. This material is based upon work supported in part by the Department of Energy under Award Number DE-FG02-07ER46411

Measurement of shower development and its Moli\`ere radius with a four-plane LumiCal test set-up

A prototype of a luminometer, designed for a future e+e- collider detector, and consisting at present of a four-plane module, was tested in the CERN PS accelerator T9 beam. The objective of this beam test was to demonstrate a multi-plane tungsten/silicon operation, to study the development of the electromagnetic shower and to compare it with MC simulations. The Moli\`ere radius has been determined to be 24.0 +/- 0.6 (stat.) +/- 1.5 (syst.) mm using a parametrization of the shower shape. Very good agreement was found between data and a detailed Geant4 simulation.Comment: Paper published in Eur. Phys. J., includes 25 figures and 3 Table

Performance of fully instrumented detector planes of the forward calorimeter of a Linear Collider detector

Detector-plane prototypes of the very forward calorimetry of a future detector at an e+e- collider have been built and their performance was measured in an electron beam. The detector plane comprises silicon or GaAs pad sensors, dedicated front-end and ADC ASICs, and an FPGA for data concentration. Measurements of the signal-to-noise ratio and the response as a function of the position of the sensor are presented. A deconvolution method is successfully applied, and a comparison of the measured shower shape as a function of the absorber depth with a Monte-Carlo simulation is given.Comment: 25 pages, 32 figures, revised version following comments from referee

Observation of two new Ξb−\Xi_b^- baryon resonances

Two structures are observed close to the kinematic threshold in the $\Xi_b^0 \pi^-$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content $bds$ are expected in this mass region: the spin-parity $J^P = \frac{1}{2}^+$ and $J^P=\frac{3}{2}^+$ states, denoted $\Xi_b^{\prime -}$ and $\Xi_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m(\Xi_b^{\prime -}) - m(\Xi_b^0) - m(\pi^{-}) = 3.653 \pm 0.018 \pm 0.006$ MeV$/c^2$, $m(\Xi_b^{*-}) - m(\Xi_b^0) - m(\pi^{-}) = 23.96 \pm 0.12 \pm 0.06$ MeV$/c^2$, $\Gamma(\Xi_b^{*-}) = 1.65 \pm 0.31 \pm 0.10$ MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of $\Gamma(\Xi_b^{\prime -}) < 0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.Comment: 17 pages, 2 figure

Observation of an Excited Bc+ State

Using pp collision data corresponding to an integrated luminosity of 8.5 fb-1 recorded by the LHCb experiment at center-of-mass energies of s=7, 8, and 13 TeV, the observation of an excited Bc+ state in the Bc+π+π- invariant-mass spectrum is reported. The observed peak has a mass of 6841.2±0.6(stat)±0.1(syst)±0.8(Bc+) MeV/c2, where the last uncertainty is due to the limited knowledge of the Bc+ mass. It is consistent with expectations of the Bc∗(2S31)+ state reconstructed without the low-energy photon from the Bc∗(1S31)+→Bc+γ decay following Bc∗(2S31)+→Bc∗(1S31)+π+π-. A second state is seen with a global (local) statistical significance of 2.2σ (3.2σ) and a mass of 6872.1±1.3(stat)±0.1(syst)±0.8(Bc+) MeV/c2, and is consistent with the Bc(2S10)+ state. These mass measurements are the most precise to date

Measurement of the Bs0→J/ψηB_{s}^{0} \rightarrow J/\psi \eta lifetime

Using a data set corresponding to an integrated luminosity of $3 fb^{-1}$, collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV, the effective lifetime in the $B^0_s \rightarrow J/\psi \eta$ decay mode, $\tau_{\textrm{eff}}$, is measured to be $\tau_{\textrm{eff}} = 1.479 \pm 0.034~\textrm{(stat)} \pm 0.011 ~\textrm{(syst)}$ ps. Assuming $CP$ conservation, $\tau_{\textrm{eff}}$ corresponds to the lifetime of the light $B_s^0$ mass eigenstate. This is the first measurement of the effective lifetime in this decay mode.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2016-017.htm