Absolute and effective cross-sections for low-energy electron-scattering processes within condensed matter

Abstract

Within the last two decades, a number of experimental techniques have been developed to measure mean free paths and absolute and effective cross-sections for various processes related to the interaction of low-energy electrons with condensed matter. In all of the experiments, a monochromatic electron beam impinges on a thin multilayer film composed of atoms and/or molecules condensed on a metal or semiconductor substrate held at cryogenic temperatures in an ultra-high-vacuum system. Depending on the apparatus, cross-sections are obtained from low-energy electron transmission (LEET), high-resolution electron energy loss (HREEL), x-ray photoelectron (XPS) spectroscopy, electron-stimulated desorption (ESD) of neutral and ions, or a combination of these techniques. Quasi-elastic and inelastic mean free paths have been extracted from LEET data. This method has also served to generate absolute cross-sections for electron trapping and fragment production from the dissociation of transient molecular anions. In amorphous ice, a complete set of absolute cross-sections for all inelastic losses by 1-20 eV electrons has been obtained from HREEL data. Effective cross-sections for neutral and ionic radical formation were generated by desorption and XPS experiments. These various methods are briefly described in this article, and the corresponding cross-sections in the range 0-20 eV summarized.

Induction of strand breaks in DNA films by low energy electrons and soft X-ray under nitrous oxide atmosphere

“…More importantly, for N 2 O molecules condensed within the DNA films, studies on DEA to condensed N 2 O indicates the presence of a peak in the yield of O − production near 1 eV. Its cross section at 1 eV is 350 × 10 −18 cm 2 that represents an enhancement in DEA of more than 40 over the gasphase value (Bass et al, 1997), while condensed-phase cross section for DEA to O 2 is 27 × 10 −18 cm 2 at 6 eV (Bass and Sanche, 1998). Thus, in the DNA films, the formation of O − from condensed N 2 O is possible with much lower energy than the formation of O − from O 2 .…”

Protection by organic ions against DNA damage induced by low energy electrons

“…(3). 46 At shorter distances from the metal surface (≤1 nm), the electric field is sufficiently high to decrease the capture cross section and reduce the average lifetime τā by increasing electron transfer to the metal. 46,48 This latter mechanism has been found to decrease σ DEA to condensed O 2 by two orders of magnitude when the target molecule goes from lying on a thick dielectric layer to the bare substrate.…”

“…46 At shorter distances from the metal surface (≤1 nm), the electric field is sufficiently high to decrease the capture cross section and reduce the average lifetime τā by increasing electron transfer to the metal. 46,48 This latter mechanism has been found to decrease σ DEA to condensed O 2 by two orders of magnitude when the target molecule goes from lying on a thick dielectric layer to the bare substrate. 48 Considering the sensitivity of DEA to the modification of a near electric field, it is very likely that the modification of electric fields within DNA caused by the addition of cations at specific sites would modify σ DEA leading to base damage and rupture of the C-O bond of the backbone.…”

“…It is known that DEA to molecular solids and biomolecules including DNA is highly dependent on environment. 46,47 In particular, modification of an electric field close to the target molecule can considerably alter the DEA cross section. 46 This has been verified by changing the distance between a molecule undergoing DEA and its metal substrate by intercalating an inert dielectric layer between them.…”

“…46,47 In particular, modification of an electric field close to the target molecule can considerably alter the DEA cross section. 46 This has been verified by changing the distance between a molecule undergoing DEA and its metal substrate by intercalating an inert dielectric layer between them. At large distances (≥1 nm), the electric field created by the polarization field in the dielectric and in the metal (induced by the transient anion) increases the DEA cross section principally by reducing R c in Eq.…”