L. Sanche

RESONANT DISSOCIATION OF DNA BASES BY SUBIONIZATION ELECTRONS

We have measured the electron energy dependence for production of a great variety of anion fragments, induced by resonant attachment of subionization electrons to thymine (T) and cytosine (C) within femto-second time scales. At the lowest electron energies we also observe stable molecular anions of these bases, viz., T− and C−. Our measurements suggest that this resonant mechanism may relate to critical damage of irradiated cellular DNA by subionization electrons prior to thermalization.

Transmission of 0–15 eV monoenergetic electrons through thin‐film molecular solids

The transmission coefficient for monoenergetic electrons (0.04 eV FWHM) passing through thin films (∼100 A) of molecular solids is measured as a function of electron energy in the range 0–15 eV. The visibility of sharp maxima in the transmission spectra is enhanced by measuring the negative value of the second energy derivative of the transmitted current with respect to electron energy. In benzene, fluorobenzene, pyridine, benzaldehyde, furan, cyclohexene and 1,3‐ and 1,4‐cyclohexadiene, the spectra exhibit from six to eight maxima, whereas in pyrrole and thiophene four structures are resolved. With the exception of the first two features in fluorobenzene and the first feature for the other solids investigated, all other maxima correlate within at least ±0.5 eV with the gas‐phase electronic transitions. This correlation is achieved by increasing the energy scale of the transmission spectra relative to the gas‐phase electronic energy levels. These findings corroborate previous experiments on solid organic ...

Electron stimulated desorption via dissociative attachment in amorphous H2O

Low energy (0–12 eV) electron impact on condensed amorphous H2O and D2O films is shown to induce electron stimulated desorption of H− and D−, respectively, via dissociative electron attachment. The onsets for H− and D− detection are at 5.5 eV, with a maximum yield for anion desorption at ∼7.4 eV. The kinetic energy distributions of the desorbing anions are peaked near 0 eV, indicating that the anions suffer post‐dissociation collisions at or near the surface, with a large probability of anion trapping on the surface. The present results provide direct information on the dissociation products, prior to the interferences of subsequent reaction processes in the condensed film.

Cross Sections for Low-Energy (10 – 50 eV) Electron Damage to DNA

Abstract Boudaïffa, B., Cloutier, P., Hunting, D., Huels, M. A. and Sanche, L. Cross Sections for Low-Energy (10 – 50 eV) Electron Damage to DNA. Radiat. Res. 157, 227 – 234 (2002). We report direct measurements of the formation of single-, double- and multiple strand breaks in pure plasmid DNA as a function of exposure to 10 – 50 eV electrons. The effective cross sections to produce these different types of DNA strand breaks were determined and were found to range from approximately 10−17 to 3  × 10−15 cm2. The total effective cross section and the effective range for destruction of supercoiled DNA extend from 3.4 to 4.4  × 10−15 cm2 and 12 to 14 nm, respectively, over the range 10 – 50 eV. The variation of the effective cross sections with electron energy is discussed in terms of the electrons inelastic mean free path, penetration depth, and dissociation mechanisms, including resonant electron capture; the latter is found to dominate the effective cross sections for single- and double-strand breaks at 10 eV. The most striking observations are that (1) supercoiled DNA is approximately one order of magnitude more sensitive to the formation of double-strand breaks by low-energy electrons than is relaxed circular DNA, and (2) the dependence of the effective cross sections on the incident electron energy is unrelated to the corresponding ionization cross sections. This finding suggests that the traditional notion that radiobiological damage is related to the number of ionization events would not apply at very low energies.

Low‐energy electron‐stimulated production of molecular hydrogen from amorphous water ice

We have observed, via quadrupole mass spectrometry (QMS), stimulated production of D2 (H2) during low‐energy (5–50 eV) electron–beam irradiation of D2O (H2O) amorphous ice. The upper limit for the D2 (H2) production threshold is 6.3±0.5 eV; well below the first excited state of condensed water at 7.3 eV. The D2 (H2) yield increases gradually until another threshold is reached at ∼17 eV and continues to increase monotonically (within experimental error) up to 50 eV. We assign the 6.3 eV threshold to D− (H−)+D2O (H2O)→D2 (H2)+OD− (OH−) condensed phase (primarily surface) reactions that are initiated by dissociative attachment. We associate the yield below ∼11 eV with the dissociation of Frenkel‐type excitons and attribute the yield above ∼11 eV mainly to the recombination of D2O+, or D3O+, with quasifree or trapped electrons. Exciton dissociation and ion–electron recombination processes can produce reactive energetic D (H) atom fragments or D2 (H2) directly via molecular elimination. The importance of D+ (H...

Dissociative electron attachment to gas-phase 5-bromouracil

We report measurements of dissociative electron attachment (DEA) to gaseous 5-bromouracil (BrU) for incident electron energies between 0 and 16 eV. Low energy electron impact on BrU leads not only to the formation of a long lived parent anion BrU−, but also various anion fragments resulting from endo- and exo-cyclic bond ruptures, such as Br−, uracil-yl anions, i.e., (U-yl)−, OCN−, and a 68 amu anion tentatively attributed to H2C3NO−. The incident electron energy dependent signatures of either the Br− and (U-yl)− yields (at 0, 1.4, and 6 eV), or the OCN− and H2C3NO− yields (at 1.6 and 5.0 eV) suggests competing DEA channels for anion fragment formation. The production cross sections, at 0 eV incident electron energy, for BrU−, Br−, and (U-yl)− are estimated to be about 6×10−15, 6×10−14, and 1.0×10−15 cm2, respectively.

Low-energy (5-40 eV) electron-stimulated desorption of anions from physisorbed DNA bases.

Abstract Abdoul-Carime, H., Cloutier P. and Sanche, L. Low-Energy (5–40 eV) Electron-Stimulated Desorption of Anions from Physisorbed DNA Bases. We present the results of experiments on anion desorption from the physisorbed DNA bases adenine, thymine, guanine and cytosine induced by the impact of low-energy (5–40 eV) electrons. Electron bombardment of DNA base films induces ring fragmentation and desorption of H–, O–, OH–, CN–, OCN– and CH2– anions through either single or complex multibond dissociation. We designate the variation of the yield of an anion with electron energy as the yield function. Below 15 eV incident electron energy, bond cleavage is controlled mainly by dissociative electron attachment. Above 15 eV, the portion of a yield function that increases linearly is attributed to nonresonant processes, such as dipolar dissociation. A resonant structure is superimposed on this signal around 20 eV in the anion yield functions. This structure implicates dissociative electron attachment and/or resonant decay of the transient anion into the dipolar dissociation channel, with a minimal contribution from multiple inelastic electron scattering. The yields of all desorbing anions clearly show that electron resonances contribute to the damage of all DNA bases bombarded with 5–40 eV electrons. Comparison of the ion yields indicates that adenine is the least sensitive base to slow electron attack. Electron-irradiated guanine films exhibit the largest yields of desorbed anions.

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.

Substrate dependence of electron‐stimulated O− yields from dissociative electron attachment to physisorbed O2

We present measurements of O− electron stimulated desorption yields obtained under identical experimental conditions from 0.15 monolayers (ML) of O2 deposited onto disordered substrates consisting of 4 ML of either Kr, Xe, C2H6, C2H4, N2O, CH3Cl, or H2O, all condensed on Pt (polycrystalline). The resulting O− yield functions, for incident electron energies below 20 eV, are compared to that obtained from the O2/Kr solid; this allows us to assess the order of magnitude effects of the local substrate environment on dissociative electron attachment (DEA) via the 2Πu and gas phase forbidden 2Σ+g,u resonances of O−2. We note that, in addition to electron energy losses in the substrate prior to DEA to O2 and post‐dissociation interactions of the O− with the substrate molecules, charge or energy transfer from the O−2 transient anion to a substrate molecule, and capture of the incident electron into a dissociative anion resonance of the substrate molecule may contribute to a reduced O− yield from the physisorbed O...

The 2Πg shape resonance of N2 near a metal surface and in rare gas solids

Abstract The measurement of the well known 2 Π g transient N 2 − ion is reported as a function of its distance from a Pt substrate, for N 2 deposited on the surface or embedded in the bulk of an Ar, Kr and Xe multilayers film. The excitation function for the v=1 vibration loss were recorded between 0 and 4 eV with an electron spectrometer having a resolution of 18 meV. By comparing the oscillatory structure observed in these experiments with the gas-phase structure, the energy shift and the broadening of the temporary anion state are deduced. In the distance to substrate experiment the resonance is shifted progressively from a value of −0.62 eV at 32 layers to a value of −1.34 eV at 1 layer. In the surface to bulk experiment the resonance is displaced from a surface value of −0.62, −0.72, and −0.89 eV to an averaged bulk value of −0.89, −1.15, and −1.35 eV for Ar, Kr, and Xe, respectively.