Transverse single-spin asymmetry and cross section for pi(0) and eta mesons at large Feynman x in p(up arrow) + p collisions at root s=200 GeV

Measurements of the differential cross section and the transverse single-spin asymmetry, A(N), vs x(F) for pi(0) and eta mesons are reported for 0.4 < x(F) < 0.75 at an average pseudorapidity of 3.68. A data sample of approximately 6.3 pb(-1) was analyzed, which was recorded during p(up arrow) + p collisions at root s = 200 GeV by the STAR experiment at RHIC. The average transverse beam polarization was 56%. The cross section for pi(0), including the previously unmeasured region of x(F) > 0.55, is consistent with a perturbative QCD prediction, and the eta/pi(0) cross-section ratio agrees with existing midrapidity measurements. For 0.55 < x(F) < 0.75, the average A(N) for eta is 0.210 +/- 0.056, and that for pi(0) is 0.081 +/- 0.016. The probability that these two asymmetries are equal is similar to 3%.

Electron collisions with the HCOOH...(H2O)n (n=1, 2 and 3) complexes in liquid phase: the influence on the π* shape resonance of formic acid

In this conference we report cross sections for elastic collisions of low-energy electrons with the HCOOH…(H2O)n complexes, with n = 1, 2 and 3. The scattering cross sections were computed with the Schwinger multichannel method [K. Takatsuka and V. McKoy, Phys. Rev. A 24 , 2473 (1981); Phys. Rev. A 30 , 1734 (1984)] with pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)] in the static-exchange and static-exchange plus polarization approximations, for energies from 0.5 eV to 6 eV. We considered some diÆerent hydrogen-bonded structures for the complexes that were generated with classical Monte Carlo simulations [K. Coutinho and S. Canuto, J. Chem. Phys. 113, 9132, (2000)]. The aim of this work is to investigate the effect of the surrounding water molecules on the π* shape resonance of the solute. Previous theoretical and experimental studies carried out in the gas phase reported a π* state for HCOOH at around 1.9 eV. For the n = 1 case and for all complexes, the stabilization of the resonance was observed (it appears at lower energy compared to the value obtained in the gas phase), as reported previously for the CH2O…H2O complexes [T. C. Freitas, M. A. P. Lima, S. Canuto, and M. H. F. Bettega, Phys. Rev. A 80, 062710 (2009)]. This result indicates that the presence of the solvent may affect the processes related to the π* state, such as the molecular dissociation by electron impact. For the n = 2 case we have observed both stabilization and destabilization of the π* resonance, that is associated with the hydrogen bond donor or acceptor role of the water molecules in the complexes. For the n = 3 case, preliminary static-exchange results show the stabilization of the π* state. We propose an explanation of the stabilization/destabilization of the π* state in terms of the polarization of the solute due to the surrounding water molecules and the net charge in the solute.

Dissociative electron attachment to chloromethane.

A low energy electron may attach to a molecule, forming a metastable resonance, which may dissociate into a stable anion and a neutral radical. Chloromethane has been a good target for dissociative electron attachment studies, since it is a small molecule with a clear dissociative ‘sigma*’ shape resonance. We present potential energy curves for CH3Cl and its anion, as a function of the C-Cl distance. Due to the resonant nature of the anion, a correct description requires a treatment based on scattering calculations. In order to compute elastic cross sections and phase shifts we employed the Schwinger multichannel method, implemented with pseudopotentials of Bachelet, Hamann and Schlüter, at the static-exchange plus polarization approximation. At the equilibrium geometry, the resonance was found arround 3.3 eV, in accordance to experience. The incoming electron is captured by a ‘sigma*’ orbital located at the C-Cl bond, which will relax in the presence of this extra electron. We took this bond as the reaction coordinate, and performed several scattering calculations for a series of nuclear conformations. The phase shift obtained in each calculation was fitted by a two component function, consisting in the usual Breit-Wigner profile, which captures the resonant character, and a second order polynomial in the wave number, which accounts for the background contribution. That way, we obtained position and width of the resonance, which allowed us to build the potential energy curve. For larger distances, the anion becomes stable and usual electronic structure calculations suffice. Furthermore, the existence of a dipole-bound anion state is revealed when we employed a set of very diffuse functions. The knowledge on the behaviour of the neutral and anionic electronic states helps us in elucidating how the dissociation takes place.

Low energy elastic and electronically inelastic electron scattering from biomolecules.

Reactions initiated by collisions with low-energy secondary electrons has been found to be the prominent mechanism toward the radiation damage on living tissues through DNA strand breaks. Now it is widely accepted that during the interaction with these secondary species the selective breaking of chemical bonds is triggered by dissociative electron attachment (DEA), that is, the capture of the incident electron and the formation of temporary negative ion states [1,2,3]. One of the approaches largely used toward a deeper understanding of the radiation damage to DNA is through modeling of DEA with its basic constituents (nucleotide bases, sugar and other subunits). We have tried to simplify this approach and attempt to make it comprehensible at a more fundamental level by looking at even simple molecules. Studies involving organic systems such as carboxylic acids, alcohols and simple ¯ve-membered heterocyclic compounds are taken as starting points for these understanding. In the present study we investigate the role played by elastic scattering and electronic excitation of molecules on electron-driven chemical processes. Special attention is focused on the analysis of the in°uence of polarization and multichannel coupling e®ects on the magnitude of elastic and electronically inelastic cross-sections. Our aim is also to investigate the existence of resonances in the elastic and electronically inelastic channels as well as to characterize them with respect to its type (shape, core-excited or Feshbach), symmetry and position. The relevance of these issues is evaluated within the context of possible applications for the modeling of discharge environments and implications in the understanding of mutagenic rupture of DNA chains. The scattering calculations were carried out with the Schwinger multichannel method (SMC) [4] and its implementation with pseudopotentials (SMCPP) [5] at di®erent levels of approximation for impact energies ranging from 0.5 eV to 30 eV. References [1] B. Boudai®a, P. Cloutier, D. Hunting, M. A. Huels and L. Sanche, Science 287, 1658 (2000). [2] X. Pan, P. Cloutier, D. Hunting and L. Sanche, Phys. Rev. Lett. 90, 208102 (2003). [3] F. Martin, P. D. Burrow, Z. Cai, P. Cloutier, D. Hunting and L. Sanche, Phys. Rev. Lett. 93, 068101 (2004). [4] K. Takatsuka and V. McKoy, Phys. Rev. A 24, 2437 (1981); ibid. Phys. Rev. A 30, 1734 (1984). [5] M. H. F. Bettega, L. G. Ferreira and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993).

Spin Orbit Effects in the Electronic Transport Properties of Adsorbed Graphene Nanoribbons

Graphene has received great attention due to its exceptional properties, which include corners with zero effective mass, extremely large mobilities, this could render it the new template for the next generation of electronic devices. Furthermore it has weak spin orbit interaction because of the low atomic number of carbon atom in turn results in long spin coherence lengths. Therefore, graphene is also a promising material for future applications in spintronic devices - the use of electronic spin degrees of freedom instead of the electron charge. Graphene can be engineered to form a number of different structures. In particular, by appropriately cutting it one can obtain 1-D system -with only a few nanometers in width - known as graphene nanoribbon, which strongly owe their properties to the width of the ribbons and to the atomic structure along the edges. Those GNR-based systems have been shown to have great potential applications specially as connectors for integrated circuits. Impurities and defects might play an important role to the coherence of these systems. In particular, the presence of transition metal atoms can lead to significant spin-flip processes of conduction electrons. Understanding this effect is of utmost importance for spintronics applied design. In this work, we focus on electronic transport properties of armchair graphene nanoribbons with adsorbed transition metal atoms as impurities and taking into account the spin-orbit effect. Our calculations were performed using a combination of density functional theory and non-equilibrium Greens functions. Also, employing a recursive method we consider a large number of impurities randomly distributed along the nanoribbon in order to infer, for different concentrations of defects, the spin-coherence length.