Absolute cross sections for anion production by low energy electron impact on physisorbed CO2

Cooperative Chemisorption-Induced Physisorption of CO2 Molecules by Metal-Organic Chains

Effective CO2 capture and reduction can be achieved through a molecular scale understanding of interaction of CO2 molecules with chemically active sites and the cooperative effects they induce in functional materials. Self-assembled arrays of parallel chains composed of Au adatoms connected by 1,4-phenylene diisocyanide (PDI) linkers decorating Au surfaces exhibit self-catalyzed CO2 capture leading to large scale surface restructuring at 77 K (ACS Nano 2014, 8, 8644-8652). We explore the cooperative interactions among CO2 molecules, Au-PDI chains and Au substrates that are responsible for the self-catalyzed capture by low temperature scanning tunneling microscopy (LT-STM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), and dispersion corrected density functional theory (DFT). Decorating Au surfaces with Au-PDI chains gives the interfacial metal-organic polymer characteristics of both a homogeneous and heterogeneous catalyst. Au-PDI chains activate the normally inert Au surfaces by promoting CO2 chemisorption at the Au adatom sites even at <20 K. The CO2(δ-) species coordinating Au adatoms in-turn seed physisorption of CO2 molecules in highly ordered two-dimensional (2D) clusters, which grow with increasing dose to a full monolayer and, surprisingly, can be imaged with molecular resolution on Au crystal terraces. The dispersion interactions with the substrate force the monolayer to assume a rhombic structure similar to a high-pressure CO2 crystalline solid rather than the cubic dry ice phase. The Au surface supported Au-PDI chains provide a platform for investigating the physical and chemical interactions involved in CO2 capture and reduction.

O(-) from amorphous and crystalline CO2 ices

O(-) desorbed from amorphous and crystalline films of CO2 at 18 K under low energy electron impact is studied using time of flight mass spectrometry. The nature of the CO2 film is characterized by Fourier transform infrared spectrometry as a function of film thickness. It is found that the desorption rate from amorphous films is considerably larger than that from crystalline films. The desorption signal from the 4 eV resonance is found to be the dominant one as compared to that from the higher energy resonances, notably the one at 8 eV observed in the gas phase. This is explained in terms of the large enhancement in the dissociative electron attachment cross section for the 4 eV resonance in the condensed phase reported earlier using the charge trapping method.

Electron photoemission from charged films: Absolute cross section for trapping 0–5eV electrons in condensed CO2

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.

Low-energy electron scattering cross section for the production of CO within solid films of carbon dioxide

We report absolute electron scattering cross sections sigma(p) for the production of CO within thin solid film of carbon dioxide (CO(2)) condensed on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the carbon dioxide film, are detected in situ by recording energy losses to their lowest triplet electronic state a (3)Pi using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron exposure, film thickness, and incident electron energy between 2 and 30 eV, a range within which most of the secondary electrons are created in systems irradiated by high-energy particles. The energy dependence is characterized by a feature around 4 eV with sigma(p)=(7.0+/-4.0)x10(-18) cm(2), a minimum around 7 eV, a strong rise up to a large and broad maximum around 15 eV with sigma(p)=(5.4+/-2.5)x10(-17) cm(2), a decrease to a minimum around 18.5 eV, and finally a monotonous increase up to 30 eV. The CO production is discussed in terms of the formation of electron resonances or transient anion states, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding excited neutral and positive ion states.

Alteration of protein constituents induced by low-energy (<35 eV) electrons: II. Dissociative electron attachment to amino acids containing cyclic groups

We report measurements of the desorption of anions from thin condensed films of tryptophan (Trp), histidine (His) and proline (Pro) stimulated by 5-35 eV electron impact. H-, O-, OH- and CN- desorb from Trp, His and Pro, whereas CH2- is observed only from Pro fragmentation. Below 12 eV, the anion yield functions exhibit resonant structures indicative of dissociative electron attachment. For all three amino acids, this process is likely to be initiated by the resonant capture of the incident electron at the NH3(+)-CH-.....-COO- and/or NH2-CH-.....-COOH group of the molecule. Temporary electron attachment to the ring leads to anion desorption only for tryptophan and proline. The energy-averaged yields measured at the detector of the mass spectrometer are (4.9, 0.3 and 54.0) x 10(-8) H-/incident electron and (3.4, 2.9, 1.8) x 10(-11) O-/incident electron, respectively, from Trp, His and Pro dissociation. Fragmentation of amino acids is found to be as intense as that of the nucleic acid bases. These results are discussed within the context of radiobiological damage induced by secondary electrons.