Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session RR2: Charged Particle Molecule Collisions |
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Chair: Tom Kirchner, York University Room: Saratoga Hilton Ballroom 2 |
Thursday, October 22, 2009 8:00AM - 8:30AM |
RR2.00001: Ion induced ionization and fragmentation of biomolecules and biomolecular clusters Invited Speaker: The interaction of keV to MeV protons and heavy ions with DNA is the basis of ion therapy and risk assessment for manned space flight. The reason is, that in tissue the originally fast ions are decelerated to MeV energies and below (the Bragg peak region) where DNA damage is highest. In living cells, DNA damage always occurs in the presence of a chemical environment. The great complexity of such condensed systems rules out the investigation of fragmentation dynamics on an event by event level. For such studies, we need finite target systems still sufficiently complex to reflect relevant chemical aspects of the cell nucleus. We investigate ion collisions with isolated DNA building blocks and their neutral clusters to bridge the gap between isolated molecules and the condensed phase. In these studies, we could for instance show that intermolecular hydrogen bonds strongly affect the nucleobase fragmentation dynamics. Because of limitations of the cluster approach, we recently commissioned a novel setup interfacing an electrospray ion source with a keV ion beamline. This allows for generation of a protonated/deprotonated beam of virtually any biomolecule. The charged biomolecules are then collected and cooled in an RF trap and subsequently collided with keV ions. Time-of-flight mass spectrometry is used to study ionization and fragmentation dynamics. First results on small protonated peptides and DNA building blocks prove the great potential of the technique. [Preview Abstract] |
Thursday, October 22, 2009 8:30AM - 9:00AM |
RR2.00002: Double ionization dynamics of Helium dimers investigated during charged particle impact Invited Speaker: Helium dimers (He$_{2})$ are a Van-der-Waals bound system and the most extreme quantum matter in AMO physics with a binding energy below 0.1 $\mu $eV (1.1 mK). Its internuclear distance varies from 2 to several hundreds Angstrom (even larger than the DNA diameter). Their existence was predicted theoretically by Slater in 1928 and the experimental prove followed 1994 by Sch\"{o}llkopf in a diffraction experiment. Singly and doubly charged projectiles with energies of 25 - 150 keV/u were used to fragment the Helium dimer into two He$^{+}$ ions. Using the COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) imaging technique we measured the three dimensional momentum vector of all fragments (He$^{+ }$ions and emitted electrons). A detailed analysis shows different breakup channels, as a two-step-two mechanism, charge transfer and the so far only in single photon absorption observed interatomic coulombic decay (ICD), induced by the transfer of a virtual photon. [Preview Abstract] |
Thursday, October 22, 2009 9:00AM - 9:15AM |
RR2.00003: Positron Interactions with Biologically Important Molecules C. Makochekanwa, W. Tattersall, J. Sullivan, A. Jones, P. Caradonna, D. Slaughter, S. Buckman, A. Bankovic, Z. Lj. Petrovic, K. Nixon, M. Brunger Positron interactions with molecules in the body are likely to play a key role in diagnostic techniques such as Positron Emission Tomography (PET). PET scans involve the injection of a high-energy positron into the body, and the detection of the resultant gamma rays that arise on annihilation with an electron. The processes between the emission of the high-energy particle and the gamma ray production involve positron-molecule scattering, and yet there is essentially no fundamental, quantitative knowledge of these interactions. We will discuss a program of measurements that is underway to quantify such scattering processes and illustrate our first results with the water molecule and formic acid. The ultimate aim of this program is to underpin the development of positron transport studies in soft matter and a new dosimetry for PET. [Preview Abstract] |
Thursday, October 22, 2009 9:15AM - 9:30AM |
RR2.00004: Dependence of positron-molecule binding energies on molecular parameters J.R. Danielson, J.A. Young, C.M. Surko Studies of positron annihilation on many molecules show evidence of vibrational Feshbach resonances and thus provide evidence that positrons bind to these species. The downshifts in the resonances from the mode energies provide a measure of the binding energy.\footnote{J. A. Young and C. M. Surko, Phys. Rev. A {\bf 77}, 052704 (2008); and Phys. Rev. A {\bf 78}, 032702 (2008).} Regression analysis on the existing data set of 30 such molecules indicates that the binding energy can be expressed as a linear combination of the molecular dipole polarizability and the permanent dipole moment with an additional term for aromatics that is proportional to the number of $\pi$ bonds. This result is compared with binding energy calculations and tested with other annihilation data. Predictions of the model are discussed, including positron binding to atomic clusters, polycyclic aromatic molecules (PAHs), and other chemical species. The relationship of these positron-molecule bound states to analogous electron-molecule states is also discussed.\footnote{H. Abdoul-Carmine and C. Desfrancios, European Phys. J. {\bf 2}, 149 (1998).} [Preview Abstract] |
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