99 research outputs found
Chemical Accelerator Studies of Isotope Effects on Collision Dynamics of IonâMolecule Reactions: Elaboration of a Model for Direct Reactions
This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/53/2/10.1063/1.1674042.Crossedâbeam studies on isotopic variants of the reaction Ar+â+âH2âArH+ are reported. Both velocity and angular distributions of the ionic product as a function of initial translational energy, down to 0.1 eV (center of mass), have been measured. At lowest energies there is a gain in the translational energy of the products over that of the reactants, but at higher energies there is increasing conversion of kinetic into internal energy. While this represents the most probable course of the reaction there is a fairly wide distribution about the median values. Results confirm that this reaction is predominantly direct at all energies and provide no evidence for intermediate persistent complex formation. They are also consistent with a model for direct reactions previously proposed. The data on reaction with HD permit further development of this mechanism. The reactants are mutually accelerated by their longârange attractive potential until hydrogen atom transfer occurs. The liberated H (or D) atom is reflected from the ArD+(ArH+ and the products separate, being decelerated in the process by the attractive potential acting between them. This âpolarizationâreflectionâ model yields a reasonable value for the radius at which transfer occurs, and it accounts quantitatively for the magnitudes of, and isotopic effects on, the median product velocities. It also predicts the significant back scattering observed at very low as well as very high energies. With appropriate modification for the attractive potentials involved the model can provide a simple representation of direct reactions in general
Translational energy dependence of reaction mechanism: Xe++CH4âXeH++CH3
This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/74/9/10.1063/1.441715.The dynamics of the exoergic ionâmolecule reaction Xe+(CH4,CH3)XeH+ were studied by chemical accelerator techniques over the relative translational energy range 0.2 to 8 eV. Results of the kinematicmeasurements are reported as scattering intensity contour maps in Cartesian velocity space. Centerâofâmass angular and energy distributions, derived from these maps, provide information on the reaction mechanism and on the partitioning of available energy between internal and translational modes in the products. The results suggest that reaction proceeds via the formation of a longâlived complex at low collision energies (below 0.5 eV) and by a direct mechanism approaching spectator stripping at higher energies
Role of impact parameter in branching reactions: Chemical accelerator studies of the reaction Xe++CH4âXeCH3 ++H
This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/74/9/10.1063/1.441716.Integral reaction cross sections and product velocity distributions have been measured for the ionâmolecule reaction Xe+(CH4,H)XeCH3 + over the relative reactant translational energy range of 0.7â5.5 eV by chemical accelerator techniques. The kinematic results indicate that reaction proceeds in a direct manner by a rebound mechanism over the energy range studied, suggesting that this substitution reaction occurs predominantly in small impact parameter collisions. This finding contrasts with the results obtained for the competing reaction, Xe+(CH4,CH3)XeH+, where the strong forward scattering of the XeH+ product indicates that Hâatom abstraction occurs primarily in large impact parameter collisions
Translational energy dependence of cross sections for reactions of OHâ (H2O) n with CO2 and SO2
This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/80/10/10.1063/1.446510.A tandem mass spectrometer has been used to measure cross sections for reactions of the solvated negative ions OHâ(H2O) n , where 0â€nâ€3, with the neutral molecules CO2 and SO2 over the range of reactant translational energy 0.15â25.0 eV (LAB). The reactions observed include solvent switching, collisional dissociation, and charge transfer. The exoergic solvent switching reactions are very rapid, having cross sections which exceed a hundred square Angstroms at low energies. These cross sections decrease approximately as (energy)â0.5 up to 1 eV and then decrease much more rapidly at higher collision energies. Estimates of bond dissociation energies for the cluster ions are derived from the measured translational energy thresholds for the endothermic collisional dissociationreactions
Search for the Theta^+(1540) in lattice QCD
We report on a study of the pentaquark Theta^+(1540), using a variety of
different interpolating fields. We use Chirally Improved fermions in
combination with Jacobi smeared quark sources to improve the signal and get
reliable results even for small quark masses. The results of our quenched
calculations, which have been done on a 12^3 x 24 lattice with a lattice
spacing of a = 0.148 fm, do not provide any evidence for the existence of a
\Theta^+ with positive parity. We do observe, however, a signal compatible with
nucleon-kaon scattering state. For the negative parity the results are
inconclusive, due to the potential mixture with nucleon-kaon and N^*-kaon
scattering states.Comment: 5 pages, 2 figure
Does facial soft tissue protect against zygomatic fractures?: results of a finite element analysis
Introduction: Zygomatic fractures form a major entity in craniomaxillofacial traumatology. Few studies have dealt with biomechanical basics and none with the role of the facial soft tissues. Therefore this study should investigate, whether facial soft tissue plays a protecting role in lateral midfacial trauma
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