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

Effect of morphology of thin DNA films on the electron stimulated desorption of anions

We present a comparison between the electron stimulated desorption (ESD) of anions from DNA samples prepared by lyophilization (an example of poorly organized or nonuniform films) and molecular self-assembly (well-ordered films). The lyophilization (or freeze- drying) method is perhaps the most frequently employed technique for forming DNA films for studies of low-energy electron (LEE) interactions leading to DNA damage; however, this technique usually produces nonuniform films with considerable clustering which may affect DNA configuration and enhance sample charging when the film is irradiated. Our results confirm the general validity of ESD measurements obtained with lyophilized samples, but also reveal limitations of lyophilization for LEE studies on DNA films. Specifically we observe some modulation of structures, associated with dissociative electron attachment, in the anion yield functions from different types of DNA film, confirming that conformational factors play a role in the LEE induced damage to DNA.

On the mechanism of anion desorption from DNA induced by low energy electrons

Our knowledge of the mechanisms of radiation damage to DNA induced by secondary electrons is still very limited, mainly due to the large sizes of the system involved and the complexity of the interactions. To reduce the problem to its simplest form, we investigated specific electron interactions with one of the most simple model system of DNA, an oligonucleotide tetrameter compound of the four bases. We report anion desorption yields from a thin solid film of the oligonucleotide GCAT induced by the impact of 3-15 eV electrons. All observed anions (H-, O-, OH-, CN-, and OCN-) are produced by dissociative electron attachment to the molecule, which results in desorption peaks between 6 and 12 eV. Above 14 eV nonresonant dipolar dissociation dominates the desorption yields. By comparing the shapes and relative intensities of the anion yield functions from GCAT physisorbed on a tantalum substrate with those obtained from isolated DNA basic subunits (i.e., bases, deoxyribose, and phosphate groups) from either the gas phase or condensed phase experiments, it is possible to obtain more details on the mechanisms involved in low energy electron damage to DNA, particularly on those producing single strand breaks.

Intrinsic and extrinsic factors in anion electron-stimulated desorption: D− from deuterated hydrocarbons condensed on Kr and water ice films

The results of D(-) ion desorption induced by 3-20 eV electrons incident on condensed CD(4), C(2)D(6), C(3)D(8), C(2)D(4), and C(2)D(2) are presented. These compounds were deposited in submonolayer amounts on the surfaces of multilayer solid films of Kr and nonporous and porous amorphous ice. While desorption of the D(-) anions proceeds via well-known processes, i.e., dissociative electron attachment (DEA) and dipolar dissociation, significant perturbations of these processes due to presence of the different film substrates are observed. We have shown that it is possible to distinguish between the character and nature of these perturbations. The presence of the nonporous ice perturbs the D(-) desorption intensity by affecting the intrinsic properties of the intermediate anion states through which dissociation proceeds. On the other hand, the presence of the porous ice introduces extrinsic effects, which can affect electron energy losses prior to their interaction with the hydrocarbon molecule and/or the energies and intensities of the fragment species after dissociation. Simple mechanisms responsible for the observed variations in the intensities of desorbed anionic signals are proposed and discussed. Electron transfer from transient anion states to electron states of the substrate film or nearby hydrocarbon molecules appear as the most efficient mechanism to reduce the magnitude of the DEA process.

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.