Radmila Panajotovic

Effective Cross Sections for Production of Single-Strand Breaks in Plasmid DNA by 0.1 to 4.7 eV Electrons

Abstract Panajotovic, R., Martin, F., Cloutier, P., Hunting, D. and Sanche, L. Effective Cross Sections for Production of Single-Strand Breaks in Plasmid DNA by 0.1 to 4.7 eV Electrons. Radiat. Res. 165, 452–459 (2006). We determined effective cross sections for production of single-strand breaks (SSBs) in plasmid DNA [pGEM 3Zf(-)] by electrons of 10 eV and energies between 0.1 and 4.7 eV. After purification and lyophilization on a chemically clean tantalum foil, dry plasmid DNA samples were transferred into a high-vacuum chamber and bombarded by a monoenergetic electron beam. The amount of the circular relaxed DNA in the samples was separated from undamaged molecules and quantified using agarose gel electrophoresis. The effective cross sections were derived from the slope of the yield as a function of exposure and had values in the range of 10−15– 10−14 cm2, giving an effective cross section of the order of 10−18 cm2 per nucleotide. Their strong variation with incident electron energy and the resonant enhancement at 1 eV suggest that considerable damage is inflicted by very low-energy electrons to DNA, and it indicates the important role of π* shape resonances in the bond-breaking process. Furthermore, the fact that the energy threshold for SSB production is practically zero implies that the sensitivity of DNA to electron impact is universal and is not limited to any particular energy range.

Critical minima in elastic electron scattering by argon

We determined experimentally two critical points in elastic electron scattering by argon where the differential cross section (DCS) attains its smallest values. The points were found to be at and at . Special attention was given to improve the angular resolution in order to determine the exact positions of the minima. These minima are important because they are a sensitive test of the validity of experimental procedures, and are used to verify theoretical predictions of DCS shapes and magnitudes, and of the polarization of scattered electrons. Normalized DCS were determined by measuring the angular distributions of elastically scattered electrons at incident energies of 10, 15, 20, 25, 30, 40, 50, 60, 75, 80, 90 and 100 eV in the angular range . Results are compared with the available experimental and theoretical data. In addition, integral, momentum-transfer, and viscosity cross sections were determined by numerical integration of the measured DCS extrapolated to and to .

Electron collisions with ethylene

We have measured absolute elastic scattering and vibrational excitation cross sections for electron impact on ethylene. The experimental data have been obtained on two different crossed-beam electron spectrometers and they cover the energy range from 1 to 100 eV and scattering angles between 10° and 130°. Both differential (in angle) and energy-dependent cross sections have been measured. The differential cross sections have also been analysed using a molecular phase shift analysis technique in order to derive the integral elastic and elastic momentum transfer cross sections. Comparison is made with earlier data, where available, and also with a number of recent theoretical calculations.

Invited Article: An improved double-toroidal spectrometer for gas phase (e,2e) studies

A new spectrometer is described for measuring the momentum distributions of scattered electrons arising from electron-atom and electron-molecule ionization experiments. It incorporates and builds on elements from a number of previous designs, namely, a source of polarized electrons and two high-efficiency electrostatic electron energy analyzers. The analyzers each comprise a seven-element retarding-electrostatic lens system, four toroidal-sector electrodes, and a fast position-and-time-sensitive two-dimensional delay-line detector. Results are presented for the electron-impact-induced ionization of helium and the elastic scattering of electrons from argon and helium which demonstrate that high levels of momentum resolution and data-collection efficiency are achieved. Problematic aspects regarding variations in collection efficiency over the accepted momentum phase space are addressed and a methodology for their correction presented. Principles behind the present design and previous designs for electrostatic analyzers based around electrodes of toroidal-sector geometry are discussed and a framework is provided for optimizing future devices.

Electron scattering from tetrafluoroethylene.

We report experimental results for electron scattering from tetrafluoroethylene, C2F4, obtained from measurements in two laboratories. An extensive set of differential, integral, and momentum transfer cross sections is provided for elastic scattering for incident electron energies from 1 to 100 eV and inelastic (vibrational excitation) scattering for incident electron energies at 3, 6, 7.5, 8, and 15 eV, and for scattering angles ranging from 10 degrees to 130 degrees. To highlight the role of intermediate negative ions (resonances) in the scattering process we have also measured excitation functions for elastic scattering and vibrational excitation of the ground electronic state of C2F4 for incident energies between 1.5 and 20 eV. Our results are compared with recent theoretical calculations and a limited number of other experimental results.

Cross sections for low-energy electron scattering from adenine in the condensed phase.

Measurements of the vibrational and electronic excitation of a sub-monolayer up to a monolayer film of adenine were performed with a high resolution electron energy-loss (HREEL) spectrometer. The integral cross sections (over the half-space angle) for excitation of the normal vibrational modes of the ground electronic state and electronically excited states are calculated from the measured reflectivity EEL spectra. Most cross sections for vibrational excitation are of the order of 10(-17) cm(2), the largest being the out-of-plane wagging of the amino-group and the six-member ring deformations. A wide resonance feature appears in the incident energy dependence of the vibrational cross sections at 3-5 eV, while a weak shoulder is present in this dependence for combined ring deformations and bending of hydrogen atoms. For the five excited electronic states, at 4.7, 5.0, 5.5, 6.1 and 6.6 eV, the cross sections are of the order of 10(-18) cm(2), except in the case of the state at the energy of 6.1 eV, for which it is two to three times higher.

An experimental and theoretical study of transient negative ions in Mg, Zn, Cd and Hg

A range of experimental and theoretical techniques have been applied to the study of transient negative ions (resonances) formed in electron scattering from the Group II metals Mg, Zn, Cd, and Hg at incident electron energies below the first ionization potential. A wealth of resonance structures have been observed and from the experimental observations and theoretical information, classifications are proposed for some of these negative ion states.

Electron scattering from perfluorocyclobutane (c-C4F8)

We report experimental results for electron scattering from perfluorocyclobutane, c-C(4)F(8), obtained from measurements in our two laboratories. A set of differential, integral, and momentum transfer cross sections is provided for elastic scattering for incident electron energies from 1.5 to 100 eV. Inelastic scattering (vibrational excitation) cross sections have been measured for incident electron energies of 1.5, 2, 5, 6, and 7 eV. In order to investigate the role of intermediate negative ions (resonances) in the scattering process we have also measured an excitation function for elastic scattering and vibrational excitation of the ground electronic state of C(4)F(8) for incident energies between 0.6 and 20 eV. These results are compared with the limited amount of data available in the literature for scattering from this molecule.

Angle-resolving time-of-flight electron spectrometer for near-threshold precision measurements of differential cross sections of electron-impact excitation of atoms and molecules

This article presents a new type of low-energy crossed-beam electron spectrometer for measuring angular differential cross sections of electron-impact excitation of atomic and molecular targets. Designed for investigations at energies close to excitation thresholds, the spectrometer combines a pulsed electron beam with the time-of-flight technique to distinguish between scattering channels. A large-area, position-sensitive detector is used to offset the low average scattering rate resulting from the pulsing duty cycle, without sacrificing angular resolution. A total energy resolution better than 150 meV (full width at half maximum) at scattered energies of 0.5-3 eV is achieved by monochromating the electron beam prior to pulsing it. The results of a precision measurement of the differential cross section for electron-impact excitation of helium, at an energy of 22 eV, are used to assess the sensitivity and resolution of the spectrometer.