L. Parenteau

DISSOCIATIVE ELECTRON ATTACHMENT IN NANOSCALE ICE FILMS : THICKNESS AND CHARGE TRAPPING EFFECTS

The yield and kinetic energy (KE) distributions of D− ions produced via dissociative electron attachment (DEA) resonances in nanoscale D2O ice films are collected as a function of film thickness. The 2B1, 2A1, and 2B2 DEA resonances shift to higher energies and their D− ion yields first increase and then decrease as the D2O films thicken. The D− KE distributions also shift to higher energy with increasing film thickness. We interpret the changes in the DEA yield and the D− KE distributions in terms of modifications in the electronic and geometric structure of the surface of the film as it thickens. A small amount of charge build-up occurs following prolonged electron beam exposure at certain energies, which primarily affects the D− KE distributions. Charge trapping measurements indicate that an enhancement in the trapping cross section occurs at energies near zero and between 6 and 10 eV.

Dissociative electron attachment in nanoscale ice films: Temperature and morphology effects

The electron-stimulated desorption (ESD) of D− ions from condensed D2O films is investigated. Three low-energy peaks are observed which are identified as arising from excitation of 2B1, 2A1, and 2B2 dissociative electron attachment (DEA) resonances. A fourth, higher energy feature is also seen in the D− yield which is likely due to the formation of a transient anion state that dissociates and/or decays into a dissociative excited state. The energies and ion yields of the resonances vary with the temperature and morphology of the D2O film. Below 60 K, the work function of the ice films changes with temperature and the DEA resonances shift in energy. The D− ESD yield generally increases with temperature, but it deviates from this trend at temperatures corresponding to structural phase transitions in ice. The (2B1) D− temperature dependence is remarkably similar to that observed for the ESD of low-energy D+ ions from D2O ice, even though the two originate from different electronic excitations. These results ...

Surface reactions between O2 and hydrocarbons induced by dissociative electron attachment

Desorption of O− and OH− ions induced by low‐energy (4–20 eV) electron impact on O2 and hydrocarbon molecules (CnH2n+2, n=5 and 8; CnH2n, n=2, 3, and 4) coadsorbed on Pt is reported. The magnitude of the O− and OH− signals is investigated as a function of incident electron energy and substrate coverage. Beyond monolayer coverage, results are provided for two types of coadsorption: a single hydrocarbon layer physisorbed on a multilayer O2 film and a multilayer film containing 25% volume O2 mixed with hydrocarbon molecules. For all experiments, the OH− yield function can be correlated with that of the O− signal from pure O2 and hydrocarbon–O2 mixture films. This result indicates that the OH− ions are produced by the abstraction reactions O−+CnH2n+2→OH−+CnH2n+1 and O−+CnH2n→OH−+CnH2n−1 where O− ions are generated by the dissociative attachment reaction e+O2 (3∑−g)→O−2 (2∏u,2∑+g,2∑+u)→O− (2P)+O(3P,1D). The observed reaction efficiency for OH− formation, defined as the ratio of the OH− intensity to that of O−,...

Dissociative attachment reactions in electron stimulated desorption from condensed O2 and O2‐doped rare‐gas matrices

Desorption of the ions O−, O−2,O−3 (and/or O2⋅O−) induced by electron impact on pure O2 multilayer films and Ar, Kr, and Xe matrix films containing O2 is reported. In addition to these anions, the ionic complexes M⋅O− (M=Ar and Kr) are also observed to desorb from Ar and Kr matrices, respectively. In the range 4–16 eV, the incident electron energy (Ei) dependence of the yields (i.e., the yield functions) of all the diatomic and triatomic anions exhibit features which can be correlated with the O− yield function; indicating that, these anions are produced by dissociative attachment reactions whose first step involves the formation of O−2 quasibound states. From analysis of all yield functions and variations of the anion yields as a function of O2 concentration in the matrices, we find that the simplest dissociative transient state, which can propel in vacuum an M⋅O− or O2⋅O− ion, must have the configuration M⋅O2⋅O−*2. To explain the formation of O−2 and O−3 ions below Ei≂6 eV, the existence of an electroni...

Dynamics of dissociative attachment reactions in electron stimulated desorption: Cl− from condensed Cl2

Energy analysis of Cl− ions produced by dissociative attachment in electron stimulated desorption from Cl2 condensed on a platinum substrate is reported. The electron energy dependence of the Cl− signal exhibits two peaks around 2 and 5 eV which arise, respectively, from the 2Πg and 2Πu core‐excited Cl−*2 resonant states. At higher Cl2 coverages, a third peak is observed around 11.5 eV. From kinetic energy distributions, it is possible to ascribe this latter peak to Cl− ions formed via the 2Πu resonance by electrons which have suffered energy losses through the excitation of low‐lying electronic states of molecular chlorine. In the energy range of the 2Πu Cl−*2 resonance, we observe that multiple scattering processes are also important and that the curve representing the kinetic energy of Cl− ions formed via a single scattering process as a function of incident electron energy is a straight line with a slope 1/2. This indicates that the chlorine lattice is not involved in the dissociation dynamics.

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.

Electron stimulated desorption of O− and metastable CO* from physisorbed CO2

We report electron stimulated desorption (ESD) measurements of O− yields produced by dissociative electron attachment (DEA) to physisorbed CO2. The molecules are condensed at about 17–20 K on polycrystalline Pt, either as pure multilayer films, or in submonolayer (ML) quantities onto thick rare gas substrates. For the pure disordered multilayer solids, we observe four peaks in the O− yield function at incident electron energies, E(e), of about 4.1, 8.5, 11.2, and 15 eV. The lowest two are assigned, respectively, to the 2Πu and 2Πg resonance states of CO−2, which dissociate into O−(2P)+CO(X 1Σ+), and are known to dominate the gas phase DEA O− production cross section for E(e)≤20 eV. Measurements of ESD CO* metastable yields from similar CO2 multilayer solids on Pt(111), also presented here, suggest that the 11.2 and 15 eV O− peaks are associated with the manifold of close‐lying CO2*− states which dissociate into O−(2P)+CO* (a 3Π, a′ 3Σ+, or d 3Δ). For 0.15 ML of CO2 physisorbed on 20 ML thick rare gas subs...

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.

Charge trapping and the desorption of anionic and metastable fragments by dissociative electron attachment to condensed

We have studied dissociative electron attachment (DEA) to condensed by measuring the charge trapping (CT) cross section for submonolayer quantities of on Kr, and by monitoring the electron-stimulated desorption (ESD) of and metastable from and doped Kr and films. Several new channels for DEA are revealed. The largest CT cross section, of approximately , is observed over the incident energy range 0.15 - 1.5 eV and is attributed to DEA, although the possibility that is also stabilized at the lowest energies cannot be excluded. The magnitude of the CT cross section represents an enhancement of more than a factor of 35 relative to the peak gas phase DEA cross section at 2.3 eV. This increase can be attributed to the interaction of the anion state with its polarizable environment. Additional anion states, at , , and possibly , contribute to the DEA yield of . Of these, the 9 and 15.9 eV resonances have no experimentally observed analogues in the gas phase. The charge and energy transfer between the 9.0 eV feature and the state of multilayer Kr films indicates a Rydberg character for this state.

The effects of different substrates on the electron stimulated desorption dynamics of O− from physisorbed O2

We report condensed phase measurements of kinetic energy (Ek) distributions of O−, produced by dissociative electron attachment (DEA) at 6 eV incident electron energy; they are obtained under identical experimental conditions from submonolayer quantities of 16O2 deposited on disordered multilayer substrates of 18O2, Ar, Kr, Xe, CH4, and C2H6, all condensed at 20 K on polycrystalline platinum (Pt). The results suggest that the desorption dynamics of O− DEA fragments is, in part, determined by large angle elastic scattering of O− prior to desorption, as well as the net image charge potential (Ep) induced in the condensed dielectric solid and the Pt metal. The measurements also indicate that, particularly at small Kr substrate thicknesses, the Ep may not necessarily be uniform across the surface, but may fluctuate due to surface roughness. Thus, in addition to energy losses in the substrate prior to, and during, DEA, these effects may influence the dissociation dynamics of the O2− resonance itself, as well a...