The effects of temperature and morphology on electron transmission and stimulated desorption of H− from thin hydrocarbon films

Chemical transformation of molecular ices containing N2O and C2D2 by low energy electrons: New chemical species of astronomical interest

We have employed electron stimulated desorption (ESD) and x-ray photoelectron spectroscopy (XPS) to study the chemical species generated from multilayer films of N₂O, C₂D₂, and mixtures thereof (i.e., N₂O/C₂D₂) by the impact of low energy electrons with energies between 30 and 70 eV. Our ESD results for pure films of N₂O show the production of numerous fragment cations and anions, and of larger molecular ions, of sufficient kinetic energy to escape into vacuum, which are likely formed by ion-molecule scattering in the film. Ion-molecule scattering is also responsible for the production of cations from C₂D₂ films that contain as many as six or seven carbon atoms. Many of the same anions and cations desorb from N₂O/C₂D₂ mixtures, as well as new species, which is the result of ion-molecule scattering in the film. Anion desorption signals further indicate the formation of C-N containing species within the irradiated films. XPS spectra of N1s, C1s, and O1s lines reveal the fragmentation of N-O bonds and gradual formation of molecules containing species containing O-C=O, C=O, and C-O functional groups. A comparison between ESD and XPS findings suggests that species observed in the ESD channel are primarily products of reactions taking place at the film-vacuum interface, while those observed in the XPS derive from reactions occurring within the solid.

Glycine formation in CO2:CH4:NH3 ices induced by 0-70 eV electrons

Glycine (Gly), the simplest amino-acid building-block of proteins, has been identified on icy dust grains in the interstellar medium, icy comets, and ice covered meteorites. These astrophysical ices contain simple molecules (e.g., CO₂, H₂O, CH₄, HCN, and NH₃) and are exposed to complex radiation fields, e.g., UV, γ, or X-rays, stellar/solar wind particles, or cosmic rays. While much current effort is focused on understanding the radiochemistry induced in these ices by high energy radiation, the effects of the abundant secondary low energy electrons (LEEs) it produces have been mostly assumed rather than studied. Here we present the results for the exposure of multilayer CO₂:CH₄:NH₃ ice mixtures to 0-70 eV electrons under simulated astrophysical conditions. Mass selected temperature programmed desorption (TPD) of our electron irradiated films reveals multiple products, most notably intact glycine, which is supported by control measurements of both irradiated or un-irradiated binary mixture films, and un-irradiated CO₂:CH₄:NH₃ ices spiked with Gly. The threshold of Gly formation by LEEs is near 9 eV, while the TPD analysis of Gly film growth allows us to determine the "quantum" yield for 70 eV electrons to be about 0.004 Gly per incident electron. Our results show that simple amino acids can be formed directly from simple molecular ingredients, none of which possess preformed C-C or C-N bonds, by the copious secondary LEEs that are generated by ionizing radiation in astrophysical ices.

Polarizability-Induced Vibrational Shifts for Infrared Spectroscopy of Molecular Surfaces: Xe as a Surface Probe of Hexadecanethiol Self-Assembled Monolayers and Hexane Films on Au(111)

Adsorption of atomic and molecular species on several model film surfaces is shown to lead to systematic and reproducible spectral shifts for infrared absorption peaks associated with vibrations at the film/adsorbate interface. These shifts to lower frequencies are demonstrated to be versatile spectral signatures of this exposed interface. While this is found to occur for a wide variety of adsorbates, this work focuses on the adsorption of Xe onto molecular surfaces. By using the pre-Xe film reflectivity as a reference in calculating the spectra, the changes induced by Xe adsorption due to van der Waals forces are isolated from the conventional film spectrum which contains both surface and subsurface spectral contributions. A model-free algorithm containing two user-defined fitting parameters is described to reconstruct the spectrum of the surface from which these shifted features originate. The high surface sensitivity of the technique is exhibited for the case of a hexadecanethiol self-assembled monolayer (SAM), where the reconstruction algorithm successfully reproduces all peaks attributed to the methyl tail groups with minimal spectral contributions from the methylene groups composing the subsurface chain backbone; such methylene spectral features dominate the conventional surface infrared spectra. The algorithm is applied to films of hexane to demonstrate its application to disordered multilayer films as well. The technique and reconstruction algorithm are shown to extract spectroscopic information from the uppermost ∼0.3 nm of the film, representing the fundamental limit of vibrational surface sensitivity. Rudimentary modeling of a CO-Xe₃ system using Morse/Lennard-Jones interactions demonstrates the expected distance dependence of the van der Waals interaction which gives rise to the observed vibrational spectral shifts.

Low energy electron and O– reactions in films of O2 coadsorbed with benzene or toluene

A detailed understanding of nascent reactive events leading to DNA damage is required to describe ionizing radiation effects on living cells. These early, sub-picosecond events involve mainly low energy (E < 20 eV) secondary electrons (SE), and low energy (E < 5 eV) secondary ion (and neutral) fragments; the latter are created either by the primary radiation, or by SE via dissociative electron attachment (DEA). While recent work has shown that SE initiate DNA strand break formation via DEA, the subsequent damage induced by the DEA ion fragments in DNA, or its basic components is unknown. Here, we report 0-20 eV electron impact measurements of anion desorption from condensed films containing O2 and either benzene (C6H6), or toluene (C6H5CH3); these molecules represent the most fundamental structural analogs of pyrimidine bases. Our experiments show that all of the observed OH- yields are the result of reactive scattering of 1-5 eV O- fragments produced initially by DEA to O2. These O- reactions involve hydrogen abstraction from benzene or toluene, and result in the formation of benzyl radicals, or toluene radicals centered on either the ring or exocyclic methyl group. O- scatters over nm distances comparable to DNA dimensions, and reactions involve a transient anion collision complex. Anion desorption is found to depend on both, the temperature of hydrocarbon film formation (morphology), and the order of overlayer adsorption, e.g. O2 on benzene, or benzene on O2. Our measurements support the notion that in irradiated DNA similar secondary-ion reactions can be initiated by the abundant secondary electrons, and may lead to clustered damage.