Andrew D. Bass

Absolute cross sections for dissociative electron attachment to condensed CH3Cl and CH3Br: Effects of potential energy curve crossing and capture probability

We report cross sections for the trapping of 0–10 eV electrons by CH3Cl and CH3Br physisorbed onto a Kr covered Pt substrate, measured as a function of Kr film thickness and methyl halide concentration. The molecules stabilize electrons incident at the surface by the dissociation of transient CH3Cl− and CH3Br− ions into an atomic anion and a neutral fragment [dissociative electron attachment DEA]. For CH3Cl, the condensed phase absolute DEA cross section at ≈0.5 eV, reaches 13×10−18 cm2±50%, which is 104–106 times larger than the gas phase cross section. At higher energies (5–10 eV) for CH3Cl, our measurements provide a lower limit for the DEA cross section. For CH3Br, the maximum DEA cross section occurs below the vacuum level; we measure an absolute magnitude of 3.0×10−16 cm2±50% near 0 eV, which is 100 times larger than the corresponding gas phase value. These enhancements in cross section arise from the lowering of the potential energy surfaces of intermediate anions due to polarization induced in the...

Reactive scattering of O− in organic films at subionization collision energies

Anion desorption stimulated by the impact of 0–20 eV electrons on O2/hydrocarbon mixed films is reported. It is shown that part of the H−, OH−, CH−, and CH2− desorption yields from O2/hydrocarbon films is the result of reactive scattering of O− fragments produced via dissociative electron attachment (DEA) in the alkane and alkene thin films. These results support the interpretation that the DEA O− react with the hydrocarbon molecules to form a transient molecular anion complex which, in addition to autodetachment, may decay by dissociation into various anion and neutral fragments, and thus cause chemical modification of the solid.

Measurements of charge accumulation induced by monochromatic low-energy electrons at the surface of insulating samples

We investigate charging of insulators with an apparatus that allows measurements of trapped charges resulting from the impact of monoenergetic electrons of 0.1–28 eV. Details are given on the construction and operation of this instrument. A high-resolution electron monochromator provides a pulsed electron beam of variable energy and current. Accumulated surface charge is monitored using a Kelvin probe and a high-sensitivity electrometer. An ultraviolet source of adjustable maximum frequency allows the sample to be discharged for multiple measurements on the same sample. We illustrate the use of the instrument with preliminary measurements for ∼100 μm thick samples cut from an industrial polyethylene cable. The incident electron-energy dependence of the trapping probability exhibits large variation and indicates that electrons with energies <5 eV are the most efficiently trapped; charging near 10 eV is attributed to dissociative electron attachment to polyethylene molecules.

An improved electron transmission method for measuring electron trapping cross sections at the surface of dielectric films

We investigate several problems inherent in the low energy electron transmission (LEET) technique for measuring cross sections σCT for charge trapping, by submonolayer (ML) quantities of a target molecule deposited onto the surface of a dielectric film. In particular we see that the energy of the incident electron beam while charging the film was poorly defined in the original method. Furthermore, we demonstrate that interactions between trapped charges and the metallic substrate set a limit of about 100 mV for the maximum surface potential that should be allowed in absolute measurements of CT cross sections. To surmount these problems, we show how the surface potential ΔV generated by electrons of specific incident energy can be obtained rapidly, by monitoring the relative transmitted current Ir at a single reference energy EREF, rather than by recording multiple LEET spectra (or injection curves). The approach allows several measurements at different incident electron energies to be made on a single fil...

Absolute cross section for loss of supercoiled topology induced by 10 eV electrons in highly uniform /DNA/1,3-diaminopropane films deposited on highly ordered pyrolitic graphite.

It was recently shown that the affinity of doubly charged, 1-3 diaminopropane (Dap(2+)) for DNA permits the growth on highly ordered pyrolitic graphite (HOPG) substrates, of plasmid DNA films, of known uniform thickness [O. Boulanouar, A. Khatyr, G. Herlem, F. Palmino, L. Sanche, and M. Fromm, J. Phys. Chem. C 115, 21291-21298 (2011)]. Post-irradiation analysis by electrophoresis of such targets confirms that electron impact at 10 eV produces a maximum in the yield of single strand breaks that can be associated with the formation of a DNA(-) transient anion. Using a well-adapted deterministic survival model for the variation of electron damage with fluence and film thickness, we have determined an absolute cross section for strand-break damage by 10 eV electrons and inelastic scattering attenuation length in DNA-Dap complex films.

Absolute cross sections for anion production by low energy electron impact on physisorbed CO2

Abstract Total charge trapping cross sections are reported for CO 2 physisorbed onto multilayer Kr substrates. For incident electron energies between 1 and 14 eV, charge trapping proceeds via a dissociative electron attachment (DEA) mechanism, related to that observed in the gas phase. However, near 9.7 eV, coupling between CO 2 ∗− and Kr ∗− states results in a significant increase in DEA, and hence charge trapping. Between 1 and 6 eV, we measure absolute total cross sections for anion production with peak values approximately 43 times larger than those for gas phase CO 2 ; this is attributed to effects of the solids polarization energy.

CHARGE STABILIZATION BY CHLOROMETHANE MOLECULES ON MULTILAYER KR FILMS

We have measured the charge trapping cross section for the chloromethane molecules CCl4, CHCl3, and CH2Cl2 physisorbed onto Kr films. Within the 0–10 eV range investigated, charge is stabilized at the surface as atomic anions which are formed by dissociative electron attachment (DEA) to the molecules. Intermediate anion states previously observed in gas phase experiments are apparent. Below ≊4 eV electron impact energy, the charge trapping cross section becomes an absolute cross section for DEA because of the absence of a desorbed ion signal. At higher energies, our results provide a lower limit for this same quantity. Comparisons with gas phase cross sections, show substantial and systematic differences that can be understood in terms of the interaction between the intermediate anion states and electronic polarization field they induce on the Kr surface.

Cross sections for electron trapping by DNA and its component subunits I: Condensed tetrahydrofuran deposited on Kr

We report cross sections for electron capture processes occurring in condensed tetrahydrofuran (THF) for incident electron energies in the range of 0-9 eV. The charge trapping cross section for 6-9 eV electrons is very small, and an upper limit of 4 x 10(-19) cm2 is estimated from our results. This latter is thus also an upper bound for the cross section for dissociative electron attachment process that is known to occur at these energies for condensed THF. At energies close to zero eV electron trapping proceeds via intermolecular stabilization. The cross section for this process is strongly dependent on the quantity of deposited THF. Since THF may model the furyl ring found in deoxyribose, these measurements indicate that this ring likely plays little role in either initiating or enhancing strand break damage via the attachment of the low energy secondary electrons produced when DNA is exposed to ionizing radiation.

Sample morphology and porosity in electron stimulated desorption: N2* from N2 adsorbed onto nanoscale ice films

We have measured the electron-stimulated desorption of metastable molecular nitrogen (N2*) from N2 condensed onto solid thin films of water of varying preparation. Structures seen in the N2* yield function permit the quantity of N2 at the film–vacuum interface to be monitored and hence allow one to study changes in the effective surface area of the water film. This latter quantity is itself related to the porosity of the film. When deposited onto porous water, N2 molecules penetrate into the film and throughout its pores covering their large surface area. Under these conditions, the signal of desorbing N2* particles is inversely proportional to the surface area of the pores. We present a simple model for N2 film growth on a porous film. Its mathematical formulation allows the pore area and pore volume of amorphous water films to be measured. The surface region of crystalline water films is also found to be rough. Moreover, the porosity of solid water films is found to be dependent on the substrate on whic...

Dissociative electron attachment and charge transfer in condensed matter

Abstract Experiments using energy-selected beams of electrons incident from vacuum upon thin vapour deposited solids show that, as in the gas-phase, scattering cross sections at low energies are dominated by the formation of temporary negative ions (or resonances) and that molecular damage may be effected via dissociative electron attachment (DEA). Recent results also show that charge transfer between anionic states of target molecules and their environment is often crucial in determining cross sections for electron driven processes. Here, we review recent work from our laboratory, in which charge transfer is observed. For rare gas solids, electron exchange between the electron–exciton complex and either a metal substrate or co-adsorbed molecule enhances the desorption of metastable atoms and/or molecular dissociation. We discuss how transient electron capture by surface electron states of a substrate and subsequent electron transfer to a molecular adsorbate enhances the effective cross sections for DEA. We also consider the case of DEA to CF 2 Cl 2 condensed on water and ammonia ices, where electron exchange between pre-solvated electron states of ice and transient molecular anions can also increase DEA cross sections. Electron transfer from molecular resonances into pre-solvated electron states of ice is also discussed.