Formation of interstellar SH+^+ from vibrationally excited H2_2: Quantum study of S+^+ + H2_2 ⇄\rightleftarrows SH+^+ + H reactions and inelastic collisions

The rate constants for the formation, destruction, and collisional excitation of SH$^+$ are calculated from quantum mechanical approaches using two new SH$_2^+$ potential energy surfaces (PESs) of $^4A''$ and $^2A''$ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH$^+$ ($^3\Sigma^-$) + H($^2S$) channel. The formation of SH$^+$ through the S$^+$ + H$_2$ reaction is endothermic by $\approx$ 9860 K, and requires at least two vibrational quanta on the H$_2$ molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H$_2$($v=2$) are in very good agreement with the quantum results above 100K. Further quasi-classical calculations are then performed for $v=3$, 4, and 5 to cover all vibrationally excited H$_2$ levels significantly populated in dense photodissociation regions (PDR). The new calculated formation and destruction rate constants are two to six times larger than the previous ones and have been introduced in the Meudon PDR code to simulate the physical and illuminating conditions in the Orion bar prototypical PDR. New astrochemical models based on the new molecular data produce four times larger SH$^+$ column densities, in agreement with those inferred from recent ALMA observations of the Orion bar.Comment: 8 pages, 7 figure

Hoje tem consulta de quê?: Projeto Espaço Liso

Trabalho apresentado no 31º SEURS - Seminário de Extensão Universitária da Região Sul, realizado em Florianópolis, SC, no período de 04 a 07 de agosto de 2013 - Universidade Federal de Santa Catarina.O presente texto apresenta e discute alguns resultados do projeto de extensão “Espaço Liso: arte, cuidado e saúde”, desenvolvido na Faculdade de Educação da Universidade Federal do Rio Grande do Sul. Iniciado em 2012, o projeto é conduzido por um grupo multidisciplinar, envolvendo docentes, profissionais e estudantes das mais diferentes áreas e cursos. O espaço liso – conceito filosófico retirado da filosofia de Gilles Deleuze e Félix Guattari – diz respeito a um território marcado por fluxos e desejos, sujeito ao acaso, ao inesperado e às forças de criação coletiva. Uma das ações desenvolvidas pelo grupo é o “Receituário Mais do que especial”, atividade realizada em diversos espaços públicos e privados de Porto Alegre, uma alusão aos receituários de “controle especial” utilizados para a dispensa de medicamentos psiquiátricos controlados. Num mundo marcado por um crescente aumento da medicalização, o que se quer com esta atividade é a criação de um espaço de conversa onde os aspectos de saúde são colocados em primeiro plano. Ao modo lúdico, monta-se um estande onde as “consultas” são realizadas. Ao invés de medicamentos, balas coloridas de chocolate são oferecidas. As prescrições feitas são lúdicas e as mais inusitadas, direcionadas a crianças, adolescentes, adultos e idosos. O objetivo é que, através da delicadeza, da ocupação do espaço público e do cuidado, os sujeitos experimentem o lugar da fala e da escuta, e que, diante da velocidade e atropelamento da vida cotidiana, consigam olhar para seus próprios movimentos de vida e para aquilo que lhes incita prazer

Quantum and quasi-classical calculations for the S+ + H2(v,j) → SH+(v′,j′) + H reactive collisions

10 págs; 14 figs.; 2 tabs.State-to-state cross-sections for the S + H(v,j) → SH(v′,j′) + H endothermic reaction are obtained using quantum wave packet (WP) and quasi-classical (QCT) methods for different initial ro-vibrational H(v,j) over a wide range of translation energies. The final state distribution as a function of the initial quantum number is obtained and discussed. Additionally, the effect of the internal excitation of H on the reactivity is carefully studied. It appears that energy transfer among modes is very inefficient that vibrational energy is the most favorable for the reaction, and rotational excitation significantly enhances the reactivity when vibrational energy is sufficient to reach the product. Special attention is also paid to an unusual discrepancy between classical and quantum dynamics for low rotational levels while agreement improves with rotational excitation of H. An interesting resonant behaviour found in WP calculations is also discussed and associated with the existence of roaming classical trajectories that enhance the reactivity of the title reaction. Finally, a comparison with the experimental results of Stowe et al. for S + HD and S + D reactions exhibits a reasonably good agreement with those results.O. R. and N. B. acknowledge CSIC for the travelling grant I-LINK0775. Financial support from the Scientific and Technological Research Council of TURKEY (TUBITAK) (Project No. TBAG- 112T827) and the Ministerio de Economía e Innovación (Spain), for grants CSD2009-00038 and FIS2014-52172-C2, is gratefully acknowledged. The computations have been performed on the High Performance and Grid Computing Center (TR-Grid) at ULAKBIM/TURKEY and CESGA computer center. We also thank the support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007- 2013)/ERC Grant Agreement no. 610256 (NANOCOSMOS). We also acknowledge the COST action CM1401 ‘Our Astrochemical History’.Peer Reviewe

Realidade virtual e a medicina

Universidade Federal de São Paulo (UNIFESP) Escola Paulista de Medicina Departamento de CirurgiaUniversidade Federal de São Paulo (UNIFESP) Escola Paulista de MedicinaUNIFESP, EPM, Depto. de CirurgiaUNIFESP, EPMSciEL

Long-range multipolar potentials of the 18 spin-orbit states arising from the C(3^3P) + OH(X2Π^2\Pi) interaction

31 pages (texte+figures)We present the multipolar potentials at large intermolecular distances for the 18 doubly-degenerate spin-orbit states arising from the interaction between the two open-shell systems, C($^3$P) and OH(X$^2\Pi$). With OH fixed at its ground vibrational state averaged distance $r_0$, the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance $R$ and the angle $\gamma$ between $R$ and $r$. The 18 $\times$ 18 diabatic potential matrix elements are built up from the perturbation theory up to second order and from a two-center expansion of the coulombic interaction potential, resulting in a multipolar expansion of the potential matrix expressed as a series of terms varying in $R^{-n}$. The expressions for the long-range coefficients of the expansion are explicitly given in terms of monomer properties such as permanent multipole moments, static and dynamic polarizabilities. Accurate values for the monomer properties are used to properly determine the long-range interaction coefficients. The diagonalization of the full 18 $\times$ 18 potential matrix generates adiabatic long-range PESs in good agreement with their {\it ab initio} counterparts

Communication: theoretical exploration of Au+H2, D2, and HD reactive collisions

The following article appeared in Journal of Chemical Physic 135.9 (2011): 091102 and may be found at http://scitation.aip.org/content/aip/journal/jcp/135/9/10.1063/1.3635772A quasi-classical study of the endoergic Au(1S)+ H2(X1Σg+) → AuHAuH+(2Σ+)+H(2S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au+H+H, is the dominant reaction channel for energies above the H2 dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertionA.D.-U. acknowledges a JAE fellowship supported by CSIC. This work is supported by Comunidad Autónoma de Madrid, Grant No. S2009/MAT/1467, and by Ministerio de Ciencia e Innovación, Grant Nos. CSD2009-00038 and FIS2010-18132. The calculations have been performed at CESGA and IFF computing centers. P.B.A. thanks the National Science Foundation for suppor

Dynamically biased statistical model for the ortho/para conversion in the H2 + H3+ → H3+ + H2 reaction

In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)10.1063/1.2430711]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H 5+ complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasi-classical trajectory calculations and an approximation is done in which the initial ZPE of the reactants is reduced in QCT calculations to obtain a new ZPE-biased scrambling matrix. This reduction of the ZPE is explained by the need of correcting the pure classical level number of the H 5+ complex, as done in classical simulations of unimolecular processes and to get equivalent quantum and classical rate constants using Rice-Ramsperger-Kassel-Marcus theory. This matrix allows to obtain a ratio of hop/exchange mechanisms, α(T), in rather good agreement with recent experimental results by Crabtree [J. Chem. Phys. 134, 194311 (2011)10.1063/1.3587246] at room temperature. At lower temperatures, however, the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough. This demonstrates the importance of applying quantum methods to simulate this reaction at the low temperatures of astrophysical interestThis work has been supported by the program CONSOLIDER-INGENIO 2010 of Ministerio de Ciencia e Innovación under Grant No. CSD2009-00038, entitled “Molecular strophysics: the Herschel and Alma era,” and by Grant Nos. FIS2011-29596-C02 and QCQ2008-02578, and by Comunidad Autónoma de Madrid (CAM) under Grant No. S-2009/MAT/146

Chirality in Bare and Passivated Gold Nanoclusters

Chiral structures have been found as the lowest-energy isomers of bare (Au$_{28}$ and Au$_{55}) and thiol-passivated (Au$_{28}(SCH$_{3})$_{16}$and Au$_{38}(SCH_{3})_{24}) gold nanoclusters. The degree of chirality existing in the chiral clusters was calculated using the Hausdorff chirality measure. We found that the index of chirality is higher in the passivated clusters and decreases with the cluster size. These results are consistent with the observed chiroptical activity recently reported for glutahione-passivated gold nanoclusters, and provide theoretical support for the existence of chirality in these novel compounds.Comment: 5 pages, 1 figure. Submitted to PR

Molecular dynamics simulations of lead clusters

Molecular dynamics simulations of nanometer-sized lead clusters have been performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf 269/270}, 1109 (1992)). The binding energies of clusters forming crystalline (fcc), decahedron and icosahedron structures are compared, showing that fcc cuboctahedra are the most energetically favoured of these polyhedral model structures. However, simulations of the freezing of liquid droplets produced a characteristic form of ``shaved'' icosahedron, in which atoms are absent at the edges and apexes of the polyhedron. This arrangement is energetically favoured for 600-4000 atom clusters. Larger clusters favour crystalline structures. Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect fcc Wulff particle, containing a number of parallel stacking faults. The effects of temperature on the preferred structure of crystalline clusters below the melting point have been considered. The implications of these results for the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other minor changes for publicatio

Electron transport signature of H2 dissociation on atomic gold wires

Nonequilibrium Green’s functions calculations based on density functional theory show a direct link between the initial stages of H2 dissociation on a gold atomic wire and the electronic current supported by the gold wire. The simulations reveal that for biases below the stability threshold of the wire, the minimum-energy path for H2 dissociation is not affected. However, the electronic current presents a dramatic drop when the molecule initiates its dissociation. This current drop is traced back to quantum interference between electron paths when the molecule starts interacting with the gold wireThis work has been supported by Comunidad Autónoma de Madrid (CAM) under Grant No. S-2009/MAT/1467, by the Ministerio de Ciencia e Innovación under Grant No. FIS2011-29596-C02, and by the European-Union Integrated Project AtMol (http://www.atmol.eu). We would like to thank as well the CESGA computing center for the computing time under the ICTS grant