Ultrafast Molecular Transport on Carbon Surfaces: The Diffusion of Ammonia on Graphite

We present a combined experimental and theoretical study of the self-diffusion of ammonia on exfoliated graphite. Using neutron time-of-flight spectroscopy we are able to resolve the ultrafast diffusion process of adsorbed ammonia, NH$_3$, on graphite. Together with van der Waals corrected density functional theory calculations we show that the diffusion of NH$_3$ follows a hopping motion on a weakly corrugated potential energy surface with an activation energy of about 4 meV which is particularly low for this type of diffusive motion. The hopping motion includes further a significant number of long jumps and the diffusion constant of ammonia adsorbed on graphite is determined with D=3.9 \cdot 10^{-8}~\mbox{m}^2 /\mbox{s} at 94 K

Molecular diffusion on surfaces: The diffusive behavior of aromatic compounds absorbed on graphitic surfaces studied with Quasi-Elastic-Neutron Scattering (QENS)

Este trabajo está dedicado al estudio de la dinámica de anillos aromáticos en fase adsorbida sobre el plano basal de microcristales de grafito, (es decir la familia de planos cuya dirección normal es paralela a la dirección [0001]). Se trata de un trabajo experimental basado en técnicas de dispersión quasi-elástica de neutrones. Hemos medido con técnicas espectroscópicas de tiempo de vuelo, la difusión de benceno hidrogenado, C6H6, para cuatro recubrimientos: desde 0.1 ML (sólo un 10 % de los sitios de adsorpción están ocupados), 0.2 ML, 0.5 ML y 1.0 ML (el 100 % de los sitios de adsorbción están ocupados) y la difusión de benceno deuterado, C6D6, para recubrimientos de 0.5 ML y 0.9 ML. Además hemos explorado un amplio rango de temperaturas: desde 60 K hasta 140 K, siendo la temperatura de desorpción del benceno en grafito de 150 K [1]. Observamos que la dinámica de las moléculas es muy sensible al recubrimiento. A bajos recubrimientos (0.1ML) las moléculas cumplen un régimen balístico mientras que a medios y altos recubrimientos (a partir de 0.2 ML) la difusión es Browniana. Podemos extraer del análisis de los espectros la velocidad cuadrática promedia de las moléculas en régimen balístico y que satisfacen el principio de equipartición de la energía. En el caso de recubrimientos por encima de 0.2 ML hemos obtenido los parámetros de fricción del orden de 2.5±0.1 ps-1 (para 0.5 ML a 140K) y que es coherente con estudios recientes [2]. Por otro lado, una estimación de la fricción fonónica en el plano basal del grafito conduce a un valor de 0.02 ps-1 que es al menos dos órdenes de magnitud menor que los parámetros de fricción extraídos de los ajuste de datos. Concluimos que la interacción entre adsorbatos gobierna el comportamiento difusivo de las moléculas mientras que la interacción entre adsorbatos y substrato juega un papel secundario. Referencias: [1] R. Zacharia et al. Phys. Rev. B 69, 155406 (2004). [2] H. Hedgeland et al. Nature Phys. 5, 561 (2009)

Nanoscopic diffusion of water on a topological insulator.

The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of inter-molecular and molecule-surface interactions. Here we present an experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI), Bi[Formula: see text]Te[Formula: see text]. Understanding the chemistry and motion of molecules on TI surfaces, while considered a key to design and manufacturing for future applications, has hitherto been hardly addressed experimentally. By combining helium spin-echo spectroscopy and density functional theory calculations, we are able to obtain a general insight into the diffusion of water on Bi[Formula: see text]Te[Formula: see text]. Instead of Brownian motion, we find an activated jump diffusion mechanism. Signatures of correlated motion suggest unusual repulsive interactions between the water molecules. From the lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor.Blavatnik Foundation TU Graz Open Access Publishing Fund Aarhus University Research Foundation VILLUM FONDEN SPP1666 of the DFG (Grant No. HO 5150/1-2) FWF (Austrian Science Fund) within the projects J3479-N20 and P29641-N36 Ramón Areces foundation the Center of Materials Crystallography (CMC) Danish National Research Foundation (DNRF93

Role of age and comorbidities in mortality of patients with infective endocarditis

[Purpose]: The aim of this study was to analyse the characteristics of patients with IE in three groups of age and to assess the ability of age and the Charlson Comorbidity Index (CCI) to predict mortality. [Methods]: Prospective cohort study of all patients with IE included in the GAMES Spanish database between 2008 and 2015.Patients were stratified into three age groups:<65 years,65 to 80 years,and ≥ 80 years.The area under the receiver-operating characteristic (AUROC) curve was calculated to quantify the diagnostic accuracy of the CCI to predict mortality risk. [Results]: A total of 3120 patients with IE (1327 < 65 years;1291 65-80 years;502 ≥ 80 years) were enrolled.Fever and heart failure were the most common presentations of IE, with no differences among age groups.Patients ≥80 years who underwent surgery were significantly lower compared with other age groups (14.3%,65 years; 20.5%,65-79 years; 31.3%,≥80 years). In-hospital mortality was lower in the <65-year group (20.3%,<65 years;30.1%,65-79 years;34.7%,≥80 years;p < 0.001) as well as 1-year mortality (3.2%, <65 years; 5.5%, 65-80 years;7.6%,≥80 years; p = 0.003).Independent predictors of mortality were age ≥ 80 years (hazard ratio [HR]:2.78;95% confidence interval [CI]:2.32–3.34), CCI ≥ 3 (HR:1.62; 95% CI:1.39–1.88),and non-performed surgery (HR:1.64;95% CI:11.16–1.58).When the three age groups were compared,the AUROC curve for CCI was significantly larger for patients aged <65 years(p < 0.001) for both in-hospital and 1-year mortality. [Conclusion]: There were no differences in the clinical presentation of IE between the groups. Age ≥ 80 years, high comorbidity (measured by CCI),and non-performance of surgery were independent predictors of mortality in patients with IE.CCI could help to identify those patients with IE and surgical indication who present a lower risk of in-hospital and 1-year mortality after surgery, especially in the <65-year group

A theoretical study of rotational and translational diffusion dynamics of molecules with a six-fold point symmetry adsorbed on a hexagonal lattice by neutron scattering

A complete analytical model for the rotational and translational diffusion of molecules with a six-fold point symmetry on a hexagonal lattice is presented. It can be applied, in particular, to the diffusion of benzene molecules adsorbed flat on the basal plane of graphite in the case of incoherent scattering. Under the weak hindered approximation, the classical mechanics framework and making use of the van Hove formalism of correlation functions, the intermediate scattering function and its Fourier transform, the scattering law, are both obtained. They can be expressed as sums of exponential decays or Lorentzian functions, respectively, containing the contribution of each of the dynamical processes taking place. In the case of benzene lying flat on the substrate we expect translational diffusion, continuous rotations of isolated molecules and hindered rotations of molecules within clusters. Each particular diffusive mechanism can be recognized owing to its particular signature in the dependence of the quasi-elastic broadening on the momentum transfer. © 2012 IOP Publishing Ltd.Support from the Ministerio de Ciencia e Innovacion (Spain) under project FIS2010-18132 is acknowledgedPeer Reviewe

Ballistic diffusion in polyaromatic hydrocarbons on graphite

6 págs.; 3 figs.; 2 tabs.This work presents an experimental picture of molecular ballistic diffusion on a surface, a process that is difficult to pinpoint because it generally occurs on very short length scales. By combining neutron time-of-flight data with molecular dynamics simulations and density functional theory calculations, we provide a complete description of the ballistic translations and rotations of a polyaromatic hydrocarbon (PAH) adsorbed on the basal plane of graphite. Pyrene, CH, adsorbed on graphite is a unique system, where at relative surface coverages of about 10-20% its mean free path matches the experimentally accessible time/space scale of neutron time-of-flight spectroscopy (IN6 at the Institut Laue-Langevin). The comparison between the diffusive behavior of large and small PAHs such as pyrene and benzene adsorbed on graphite brings a strong experimental indication that the interaction between molecules is the dominating mechanism in the surface diffusion of polyaromatic hydrocarbons adsorbed on graphite. © 2016 American Chemical SocietyI.C.-A. is grateful to the Ramón Areces foundation for the funding of her postdoctoral research position. M.S. thanks the U.K.’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202); this work used the ARCHER U.K. National Supercomputing Service. E.B. acknowledges financial support by the graduate college of the Université de Grenoble (France) and A.T. thanks the FWF (Austrian Science Fund) for financial support within the project J3479-N20. S.M.-A. acknowledges the funding from the project FIS2014-52172- C2-1-P, from Ministerio de Economia y Competitividad (Spain).Peer Reviewe

Benzene diffusion on graphite described by a rough hard disk model

9 pags.; 5 figs.; 1 tab.; 2 apps.© 2014 Elsevier Ltd. All rights reserved. New insight into the nature of diffusion and the origin of friction of a prototype system for weak physisorption-benzene molecules, C6H6, adsorbed on the basal plane (0001) of graphite-has been obtained with quasi-elastic neutron scattering (QENS). Spectra were measured at relative adsorbate coverages between 0.1 and 1.0 monolayers (ML) and at sample temperatures between 60 K and 140 K. Our experimental observations require a substantial modification of the model of interaction between benzene molecules and graphite surfaces: in contrast to recent studies we find only weak kinetic surface friction, but a substantial dissipative interaction during inter-molecular collisions. At coverages of up to 0.5 ML the molecular dynamics are successfully modeled by a rough hard disk model, which we derive from a three-dimensional rough hard sphere model. At the full monolayer, three body and higher order collisions are dominant and the rough hard disk model breaks down, as expected.Peer Reviewe