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 pi* 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.

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 68.5 • ± 0.3 • , 41.30 ± 0.02 eV and at 143.5 • ± 0.3 • , 37.30 ± 0.02 eV. 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 20 • -150 • . Results are compared with the available experimental and theoretical data. In addition, integral, momentumtransfer, and viscosity cross sections were determined by numerical integration of the measured DCS extrapolated to 0 • and to 180 • .

Relative differential electron impact cross sections sigma ( theta ) for elastic and inelastic scattering in mercury have been measured. The sigma ( theta ) are normalized to the optical oscillator strength for the 61S0-61P1 transition and put on an absolute scale. Results are presented at 15, 25, 35, 40, 50, 60 and 100 eV for elastic scattering and the 61P1 excitation. In addition the 63P1, 71S0 and 71P1 excitation sigma ( theta ) are presented at 60 eV. The angular ranges are from 2' for inelastic and from 10' for elastic scattering, up to 150'. Results are compared with the other experimental and theoretical data. Comparison with the relativistic calculations is given by Srivastava et al. in the following paper (ibid., vol.26, no.5, p.1025-30 (1993)).

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.

Absolute measurements of elastic scattering and vibrational excitation of the NO molecule by low energy electron impact (0.4-2.5 eV) are presented. They show that previous estimates of these cross sections may be in error by as much as a factor of 3 and provide compelling evidence for a reassessment of the balance between elastic scattering and vibrational excitation at incident energies below 2 eV. They also confirm the critical contribution that intermediate negative ion resonances (NO-) make to the various scattering processes for this molecule at low incident energies.