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 Kr layer and metal substrate. An increase in DEA cross section with a reduction in the distance of transient anions from the metal surface, is explained by the effect of image charges on the energy at which anion and neutral ground state potential energy curves cross. Below thicknesses of 5 ML of Kr, a decrease in DEA cross section is observed and attributed to a reduction in the electron capture probability of the halide due to competition with transfer to the metal substrate.

Cross sections (CSs) for the interaction of low-energy electrons (LEE) with condensed macromolecules are essential parameters for accurate modeling of radiation-induced molecular decomposition and chemical synthesis. Electron irradiation of dry nanometer-scale macromolecular solid films has often been employed to measure CSs and other quantitative parameters for LEE interactions. Since such films have thicknesses comparable with electron thermalization distances, energy deposition varies throughout the film. Moreover, charge accumulation occurring inside the films shields a proportion of the macromolecules from electron irradiation. Such effects complicate the quantitative comparison of the CSs obtained in films of different thicknesses and limit the applicability of such measurements. Here, we develop a simple mathematical model, termed the molecular survival model, that employs a CS for a particular damage process together with an attenuation length related to the total CS, to investigate how a measured CS might be expected to vary with experimental conditions. As a case study, we measure the absolute CS for the formation of DNA strand breaks (SBs) by electron irradiation at 10 and 100 eV of lyophilized plasmid DNA films with thicknesses between 10 and 30 nm. The measurements are shown to depend strongly on the thickness and charging condition of the nanometer-scale films. Such behaviors are in accord with the model and support its validity. Via this analysis, the CS obtained for SB damage is nearly independent of film thickness and charging effects. In principle, this model can be adapted to provide absolute CSs for electron-induced damage or reactions occurring in other molecular solids across a wider range of experimental conditions.

The absolute dissociative-electron-attachment cross section for CH3C1 condensed onto multilayer Kr films was measured and found to be as high as (2.3~0.9) && 10 ' cm (i.e., approximately 106 times larger than that calculated for gaseous CH&C1). An R-matrix model of electron attachment to CHsC1 reproduces the enhancement and its variation with Kr film thickness.Dissociative electron attachment (DEA) is a two-step process in which an electron attaches to a molecular target to form a temporary negative ion that subsequently dissociates into a neutral and a stable anionic fragment. The process is well known from gas phase experiments[1] and has more recently been observed in a number of condensed phase systems [2]. It is of both fundamental and practical importance in many areas of physics and chemistry. Although description of the condensed-phase phenomenon and its applications often requires knowledge of absolute DEA cross sections, these have so far only been measured for condensed Oq [3] and CF4 [4].In this Letter, we report the absolute DEA cross section for CH3C1 condensed onto a multilayer Kr substrate as a function of the thickness of that substrate. We find this cross section to be enhanced by four to six orders of magnitude with respect to the gas phase va-lues [5,6].To our knowledge this is the largest increase of any electron cross section resulting from a phase change. Additionally, we present a model, based on an existing 8-matrix representation of isolated e-CH3C1 scattering, that successfully reproduces the observed enhancement by including, in an approximate manner, the polarization effect of the substrate. Previous theories on the behavior of transient anions on surfaces have been applied to the calculation of the magnitude of the vibrational excitation cross section. They described changes in the adsorbate anion's lifetime and energy due to the presence of a metal substrate and the anion's distance from that substrate [7][8][9][10]. Coupling of surface transient anions to surface state resonances has been described recently by Rous [11]. The absolute cross sections for DEA to CH3Cl molecule s condensed onto a Kr film were measured with the method of Marsolais, Deschenes, and Sanche[12]. The technique is derived from low energy electron transmission (LEET) spectroscopy [13], which measures a monoenergetic electron current arriving at a metal substrate after passing through a dielectric film, as a function of the potential applied between the substrate and the electron source. When the dielectric film is uncharged and the electrons have just enough energy to enter the film, a sharp rise termed the "injection curve" (IC) is seen in the spectrum. If electrons become trapped on the film surface, the IC shifts to a higher accelerating voltage because the negative charge retards the incoming electrons. The shift in IC "AU" is related to the total surface charge Q and is a function of time t, the length of exposure of the film to the electron beam. When a dielectric film is partially covered by molecules ...

We have investigated the dynamics of low-energy (1−20 eV) electron-induced reactions in condensed thin films of methanol (CH 3 OH) through both electron-stimulated desorption (ESD) and postirradiation temperature-programmed desorption (TPD) experiments conducted under ultrahigh vacuum conditions. Results of ESD experiments, involving a high-sensitivity time-of-flight mass spectrometer, indicate that anion (H − , CH − , CH 2 − , CH 3 − , O − , OH − , and CH 3 O − ) desorption from the methanol thin film at incident electron energies below about 15 eV is dominated by processes initiated by the dissociation of temporary negative ions of methanol formed via electron capture, a resonant process known as dissociative electron attachment (DEA). However, postirradiation TPD investigation of radicals, especially •CH 2 OH and CH 3 O• remaining in the methanol thin film, demonstrates that electron impact excitation, not DEA, is the primary mechanism by which the radical−radical reaction products methoxymethanol (CH 3 OCH 2 OH) and ethylene glycol (HOCH 2 CH 2 OH) are formed. This apparent dichotomy between the results of ESD and postirradiation experiments is attributed to the low DEA cross section for methanol compared to that of species such as halomethanes. Our results suggest that for molecules such as methanol, low-energy electron-induced electronic excitation, rather than DEA, plays a dominant role in ionizing radiation-induced chemical synthesis in environments such as the interstellar medium. ■ INTRODUCTIONBecause of its simple chemical structure, methanol is a prototypical candidate for radiolysis studies of oxygencontaining biomolecules such as DNA. The radiation chemistry of methanol is of particular interest because methanol is exposed to different types of ionizing radiation in varying environments and phases. For example, liquid methanol is used as a solvent in radiation-induced grafting of copolymer composites. 1 The radiation chemistry of condensed methanol is also of astrochemical interest because methanol is found in relatively high abundance in protostar environments. Methanol is thought to be an important precursor in cosmic ices not only to species such as methyl formate (HCOOCH 3 ), ethylene glycol (HOCH 2 CH 2 OH), and dimethyl ether (CH 3 OCH 3 ) but also to many prebiotic species such as simple sugars and amino acids. 2−4 Because of such applications, the high-energy radiolysis of methanol has been extensively studied over a period spanning seven decades. 5−9 More recent advances, however, have demonstrated that studying the interactions of low-energy electrons with condensed matter is essential to obtaining a fundamental understanding of radiation chemistry because the interactions of high-energy radiation, such as cosmic rays (E max ∼ 10 20 eV), with matter produce large numbers of low-energy (<15 eV) secondary electrons, which are thought to initiate radiolysis reactions in the condensed phase. 10,11 In this publication, we investigate the chemistry induced in condensed methanol by such low-energ...

We review recent research on reactions (including dissociation) initiated by low-energy electron bombardment of monolayer and multilayer molecular solids at cryogenic temperatures. With incident electrons of energies below 20 eV, dissociation is observed by the electron stimulated desorption (ESD) of anions from target films and is attributed to the processes of dissociative electron attachment (DEA) and to dipolar dissociation. It is shown that DEA to condensed molecules is sensitive to environmental factors such as the identity of co-adsorbed species and film morphology. The effects of image-charge induced polarization on cross sections for DEA to CH3Cl are also discussed. Taking as example, the electron-induced production of CO within multilayer films of methanol and acetone, it is shown that the detection of electronic excited states by high-resolution electron energy loss spectroscopy can be used to monitor electron beam damage. In particular, the incident energy dependence of the CO indicates that below 19 eV, dissociation proceeds via the decay of transient negative ions (TNI) into electronically excited dissociative states. The electron-induced dissociation of biomolecular targets is also considered, taking as examples the ribose analog tetrahydrofuran and DNA bases adenine and thymine, cytosine and guanine. The ESD of anions from such films also show dissociation via the formation of TNI. In multilayer molecular solids, fragment species resulting from dissociation, may react with neighboring molecules, as is demonstrated in anion ESD measurements from films containing O2 and various hydrocarbon molecules. X-ray photoelectron spectroscopy measurements reported for electron-irradiated monolayers of H2O and CF4 on a Si–H passivated surface further show that DEA is an important initial step in the electron-induced chemisorption of fragment species.