### Session P5: Electron, Positron, and Photon Collision

Chair: Stephen Buckman, Australian National University
Room: A705

 Thursday, June 16, 2011 2:00PM - 2:12PM P5.00001: Electron Collision Cross Sections for Iodine K. Bartschat , O. Zatsarinny , G. Garcia , F. Blanco , L.R. Hargreaves , D.B. Jones , R. Murrie , J.R. Brunton , M.J. Brunger , M. Hoshino , S.J. Buckman We performed a joint experimental and theoretical study of elastic electron scattering from atomic and molecular iodine. The experimental results for atomic iodine were obtained by subtracting known cross sections from the measured data obtained with a pyrolyzed mixed beam containing a variety of atomic and molecular species. The calculations were performed using both a fully relativistic Dirac \hbox{$B$-spline} \hbox{$R$-matrix} (close-coupling) method [1] and an optical model potential approach~[2]. The agreement between the two sets of theoretical predictions and the experimental data for the angle-differential and the angle-integrated elastic cross sections at 40~eV and 50~eV is very encouraging. It suggests that the present results are suitable for use in modeling plasma kinetic behavior when iodine is an important constituent.\\[4pt] [1] O. Zatsarinny and K. Bartschat, Phys. Rev. A {\bf 77} (2008) 062701.\\[0pt] [2] F. Blanco and G. Garcia, Phys. Lett. A {\bf 317} (2003) 458 (2003). Thursday, June 16, 2011 2:12PM - 2:24PM P5.00002: Narrow electron beam production and transmission properties with glass capillaries Buddhika Dassanayake , John Tanis , Asma Ayyad Transmission of 300-1000 eV electrons through a single cylindrically-shaped glass capillary of Borosilicate glass was studied [1,2]. The capillary had a diameter of $\sim$0.20 mm and a length of $\sim$15 mm. Transmitted electron intensities revealed three distinct regions with different characteristics: (1) for sample tilt angles (with respect to incident beam) less than 1\r{ } the transmission was dominated by the direct beam (no interactions with inner capillary wall), (2) for tilt angles between 1\r{ } and $\sim$3.5\r{ } the majority of transmission was due to Coulombic repulsion by charge deposition at the capillary entrance, and (3) for tilt angles larger than 3.5\r{ } transmission was governed by inelastic scattering and as a result lost energy. Energy dependence and time evolution studies were carried out to seek more insight into the transmission process. \\[4pt] [1] B.S. Dassanayake \textit{et}.\textit{ al}., \textit{Phys. Rev. A }\textbf{81}, 020701(R) (2010). \\[0pt] [2] B.S. Dassanayake \textit{et}.\textit{ al}., \textit{Phys. Rev. A }\textbf{83}, 012707 (2011). Thursday, June 16, 2011 2:24PM - 2:36PM P5.00003: Low energy (e,2e) studies from CH4: Results from symmetric coplanar experiments and M3DW theory Haari Ch , Kate Nixon , Andrew Murray , Chuangang Ning , Don Madison Low energy experimental and theoretical triply differential cross sections (TDCS) will be presented for electron impact ionization of Methane (CH4) for both the highest occupied molecular orbital (HOMO) and next highest occupied molecular orbital (NHOMO). The HOMO is a predominantly p-type orbital which is labeled 1t2 and the NHOMO is a predominantly s-type orbital labeled 2a1. Coplanar symmetric (symmetric both in final state electron energies and observation angles) will be presented for final state electron energies ranging from (1.4 eV, 1.5 eV) to (20 eV, 20 eV). The experimental results are in reasonably good qualitative agreement with theoretical M3DW (molecular 3-body distorted wave) calculations. The molecular results will be compared with the ionization of the 2s and 2p shells of neon which is the isoelectronic atom. Thursday, June 16, 2011 2:36PM - 2:48PM P5.00004: Remarkable Rise in Electron-Ion Recombination of O~II for Low Temperature Nebular Plasmas Sultana Nahar , Anil Pradhan Recombination is dominant in cooler nebular plasmas and recombination lines (RCL) of O II are commonly detected. Collisionally excited lines (CEL) are also common. But a longstanding discrepancy of lower and higher oxygen abundance exists predicted from the RCL and CEL respectively. This is a puzzle since existent atomic parameters for O~II are known to be accurate. We have studied the low energy photoionization and low temperature recombination of O II using the unified method based on relativistic Breit-Pauli R-matrix method and close coupling approximation. We find that the fine structure effects in the low temperature region, studied for the first time, cause considerable enhancement in electron-ion recombination rates. The enhancement comes from the intense narrow resonances, allowed in fine structure but not in LS coupling approximation, in a small energy region right at the ionization threshold. Due to the small energy range that contains these resonances, experiment was unable to detect them. We will present detailed features and recombination rates at low temperature which are expected to narrow the gap of discrepancy in oxygem abundance in nebular plasmas. Thursday, June 16, 2011 2:48PM - 3:00PM P5.00005: Imiging molecular dynamics of dissociative electron attachment to polyatomic molecules: ammonia, methane, and methanol Hidehito Adaniya , Daniel Slaughter , Thorsten Weber , Ali Belkacem Momentum imaging experiment is performed to study molecular dynamics of dissociative electron attachment(DEA) to the ammonia, methane, and methanol molecules for their well known resonances. The momentum spheres of the negative ion fragments, H$^{-}$,NH$^{-}$,NH2$^{-}$ from ammonia, H$^{-}$,CH$^{-}$,CH2$^{-}$ from methane, H$^{-}$,O$^{-}$,CH3O$^{-}$ from methanol, show unique angular distribution in their respective resonances, indicating each resonance involves different molecular dynamics. Thursday, June 16, 2011 3:00PM - 3:12PM P5.00006: Electron and Positron Scattering from Pyrimidine P. Palihawadana , J. Machacek , C. Makochekanwa , J. Sullivan , S. Buckman , M. Brunger , C. Winstead , V. McKoy , G. Garcia , F. Blanco Pyrimidine (C$_{4}$H$_{4}$N$_{2})$ is a model molecule for studying both electron and positron interactions with DNA/RNA bases, as three of the five nucleobases (cytosine, thymine, and uracil) are pyrimidine derivatives. In this work we present absolute elastic differential cross sections for scattering of low-energy electrons (3-50 eV) by pyrimidine measured using a crossed electron-target beam apparatus at the Australian National University. Cross sections calculated using the Schwinger variational technique and a screening-corrected form of the independent-atom method are provided for comparison with the measured results. We also present absolute total and differential cross sections, recently measured at the positron beamline facility at the Australian National University, for positron scattering from pyrimidine. The measurements are taken using a magnetically confined positron beam (1-200 eV) with typical energy resolution of 60 meV. These measured cross sections are used to discuss key features between electron and positron scattering processes. Thursday, June 16, 2011 3:12PM - 3:24PM P5.00007: Trends in Positron Interactions with Bio-Molecules J.R. Machacek , P. Palihawadana , C. Makochekanwa , J.P. Sullivan , M.J. Brunger , S.J. Buckman We present new measurements of positron scattering from a series of biologically relevant molecules; Uracil (C$_{4}$H$_{4}$N$_{2}$O$_{2})$, Tetrahydrofuran or THF (C$_{4}$H$_{8}$O) and 3-hydroxy-THF (C$_{4}$H$_{8}$O$_{2})$. These measurements were taken using the high resolution, low energy positron beam at the Australian National University. The energy of the magnetically confined positron beam can be tuned between 1 and 200 eV, and the energy resolution of the beam is typically 60 to 100 meV. The measurements include absolute total scattering and positronium formation cross sections and have enabled us to assemble a set of cross sections which can be used to investigate positron transport in systems which contain these molecules, or for which these molecules serve as useful analogues, such as DNA and RNA. One of the ultimate aims of this work is to combine state-of-the-art measurements of positron interactions with bio-molecules, with models of positron transport in these systems. Thursday, June 16, 2011 3:24PM - 3:36PM P5.00008: Trends in Positron Scattering from the Noble Gases J. Machacek , C. Makochekanwa , A. Jones , P. Caradonna , R. McEachran , J. Sullivan , S. Buckman We have a program of low energy (0.5 - 60 eV), high resolution (60 meV), positron scattering from atoms and molecules which is facilitated by a high-flux, trap-based positron beamline facility at the Australian National University [1]. The positron beam is pulsed, operating at about 100 Hz and typically containing about 1000 positrons. For noble gases, our goals range from establishing benchmarks' for positron scattering cross sections, to the investigation of threshold effects in processes such as positronium formation and ionization. This paper will present examples of trends observed in a number of scattering processes in He, Ne, Ar, Kr and Xe, and include investigations of elastic differential cross sections and the observation of Wigner cusps in the elastic channel at the Ps threshold. Where possible, the current experimental results will be compared with the best available theoretical calculations, and other experimental data from the literature. \\[4pt] [1] J. P. Sullivan, A. Jones, P. Caradonna, C. Makochekanwa, S. J. Buckman, Rev. Sci. Intrum. \textbf{79}, 113105 (2008) Thursday, June 16, 2011 3:36PM - 3:48PM P5.00009: A space-time resolved approach for interacting quantum field theories Q. Su , R. Wagner , R. Grobe We present an alternative approach to the usual perturbative S-matrix evaluation of quantum field theories which is non-perturbative and provides full space-time resolution [1]. This method is used to study the dynamical development of the force between two fermion wave packets for a one-dimensional Yukawa system. The spatial distribution of the virtual bosons that act as mediators of the force can be analyzed along with the fermionic densities. The fermion-fermion interaction can be approximated by a potential function, which is used to develop classical and quantum models. It is shown that these models are good approximations to the exact field theoretical calculations when the Fock space is restricted to only one boson, but in the full quantum field theory the fermion-fermion force is enhanced by higher-order multi-boson processes. Furthermore, the normally attractive fermion-fermion Yukawa force can in principle be manipulated to even be repulsive if the momentum modes available to the virtual bosons are restricted. \\[4pt] [1] R.E. Wagner, M.R. Ware, B.T. Shields, Q. Su and R. Grobe, Phys. Rev. Lett. 106, 023601 (2011). Thursday, June 16, 2011 3:48PM - 4:00PM P5.00010: Collision-Induced Infrared Absorption by Collisional Complexes in dense Hydrogen-Helium gas mixtures at Thousands of Kelvin Martin Abel , Lothar Frommhold , Xiaoping Li , Katharine L.C. Hunt The interaction-induced absorption by collisional pairs of H$_{2}$ molecules is an important opacity source in the atmospheres of the outer planets and cool stars. The emission spectra of cool white dwarf stars differ significantly in the infrared from the expected blackbody spectra of their cores, which is largely due to absorption by collisional H$_{2}$--H$_{2}$, H$_{2}$--He, and H$_{2}$--H complexes in the stellar atmospheres. Using quantum-chemical methods we compute the atmospheric absorption from hundreds to thousands of kelvin [1]. Laboratory measurements of interaction-induced absorption spectra by H$_{2}$ pairs exist only at room temperature and below. We show that our results reproduce these measurements closely [1], so that our computational data permit reliable modeling of stellar atmosphere opacities even for the higher temperatures [1]. \\[4pt] [1] Xiaoping Li, Katharine L. C. Hunt, Fei Wang, Martin Abel, and Lothar Frommhold, `Collision-Induced Infrared Absorption by Molecular Hydrogen Pairs at Thousands of Kelvin'', International Journal of Spectroscopy, vol. 2010, Article ID 371201, 11 pages, 2010. doi: 10.1155/2010/371201