Léon Sanche

Electron stimulated desorption of H− from thin films of thymine and uracil

We present measurements of low energy (0–38 eV) electron stimulated desorption of H− from room temperature thin films of pure thymine (T) and uracil (U), condensed on polycrystalline Pt, and describe in detail the experimental methods required for such studies. The nominal film thicknesses are estimated to range from 0.08 to 2.7 monolayers; sublimation of the films at 69 °C (T) and 82 °C (U) onto the room temperature Pt substrate leads to nonuniform film growth, i.e., volumetric clustering, particularly in the submonolayer regime. H− formation by electron impact occurs via dissociative electron attachment (DEA) to the molecules, and results in strong desorption peaks near 8.6 eV for either molecule, whereas above 12–13 eV nonresonant dipolar dissociation dominates the desorption yields. Comparison of the present condensed phase results with gas phase measurements suggests that the desorbing H− produced at the DEA peak are mainly the result of CH bond cleavage, while near the desorption threshold of about ...

Enhancements in dissociative electron attachment to CF4, chlorofluorocarbons and hydrochlorofluorocarbons adsorbed on H2O ice

We report that the absolute cross sections for dissociative attachment of approximately 0 eV electrons to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are strongly enhanced by the presence of H2O ice. The absolute cross sections for CFCl3, CHF2Cl, and CH3CF2Cl on water ice are measured to be approximately 8.9 x 10(-14), approximately 5.1 x 10(-15), and approximately 4.9 x 10(-15) cm2 at approximately 0 eV, respectively. The former value is about 1 order of magnitude higher than that in the gas phase, while the latter two are 3-4 orders higher. In contrast, the resonances at electron energies > or = 2.0 eV are strongly suppressed either for CFCs and HCFCs or for CF4 adsorbed on H2O ice. The cross-section enhancement is interpreted to be due to electron transfer from precursor states of the solvated electron in ice to an unfilled molecular orbital of CFCs or HCFCs followed by its dissociation. This study indicates that electron-induced dissociation is a significant process leading to CFC and HCFC fragmentation on ice surfaces.

Degradation of functionalized alkanethiolate monolayers by 0–18 eV electrons

Electron stimulated desorption of neutral molecular fragments is used to study degradation of ordered organic thin films under low-energy (0–18 eV) electron impact, and total electron doses ranging between 180–550 μC/cm2. Different saturated linear thiols HS(CH2)nX (n=2 or 15, and X=CH3 or COOH) are adsorbed from solution onto a gold surface to produce a self-assembled monolayer (SAM). Here, we present yield function measurements for electron stimulated desorption of moities such as H2, CH3, CH3CH2, CH3CH2CH2, CO, and CO2 from such thin chemisorbed films. For CH3-terminated SAMs, neutral fragment desorption thresholds lie between 5–7 eV, whereas for COOH-terminated SAMs, desorption thresholds as low as 0.2 and 3–5 eV are observed. The results suggest that the incident electrons interact with functional groups localized at the film–vacuum interface, which then leads to predominantly methyl group C–H, and C–COOH bond cleavage. In addition to nonresonant degradation mechanisms, which vary monotonically from ...

Large enhancement in dissociative electron attachment to HCl adsorbed on H2O ice via transfer of presolvated electrons

We report that dissociative electron attachment (DEA) to HCl is strongly enhanced by adsorption on the surface of H2O ice. The absolute DEA cross section at ∼0 eV for HCl adsorbed on ice is measured to be ∼4.0×10−15 cm2, which is two orders of magnitude higher than in the gas phase. This enhancement is essentially due to electron transfer from precursor states of the solvated electron in ice to an unfilled molecular orbital of HCl followed by its dissociation. This study indicates that electron-induced dissociation may be a significant process leading to HCl dissociation on ice surfaces in polar stratospheric clouds due to ionization by cosmic rays.

Dosimetry of ultrasoft x-rays (1.5 keV AlKα) using radiochromatic films and colour scanners

This work explores the possibility of measuring the absorbed dose of ultrasoft x-rays (USX, 1.5 keV Al(Kalpha)) with GAFCHROMIC HD-810 radiochromatic dosimetry films (HD-810 films) and colour scanners. HD-810 films were exposed to USX, soft x-rays (14.8 keV) and gamma-rays (60Co) for various times. The response of HD-810 films to absorbed doses of gamma-rays in water was calibrated with Fricke dosimetry and used for the calibration of USX. The optical density of the HD-810 films was quantified with an HP ScanJet 6100C scanner and Corel Picture Paint 7. The choice of the reading channel and colour adjustment settings were optimized to either improve sensitivity or expand the measurable dose range. The response of the HD-810 films to the absorbed dose in water decreased by 50% when the effective photon energy decreased from 1.25 MeV to 14.8 keV. The ratio of the mass energy absorption coefficient of the active layer of HD-810 films to that of water was found to play a major role in this decrease. The mean absorbed doses of the active layer of the HD-810 films exposed to USX were derived. The calculation of the initial photon fluence rate and the mean absorbed doses of USX to biological samples such as plasmid DNA is discussed. This study suggests that radiochromatic dosimetry films are promising secondary dosimeters for measuring the absorbed dose of USX.

Condensed-phase effects on absolute cross sections for dissociative electron attachment to CFCs and HCFCs adsorbed on Kr

We present measurements of absolute dissociative electron attachment (DEA) cross sections to CFCl3, CHF2Cl, and CH3CF2Cl adsorbed on the surface of Kr as a function of electron energy (0–10 eV). The DEA cross sections are measured to be ∼7.2×10−15 cm2 at ∼0 eV, ∼4.2×10−16 cm2 at 0.65 eV, and ∼7.8×10−16 cm2 at 0.89 eV for CFCl3, CHF2Cl, and CH3CF2Cl, respectively. This cross section is similar to the gas-phase value for CFCl3, while for the latter two molecules, it is orders of magnitude higher than the gaseous values. These results can be explained by considering the changes in the survival probability of the anion resonance and in the electron capture probability due to the decrease of the nuclear wave function overlap in the Franck–Condon region.