Dissociative Electron Attachment Mechanism Research Articles

Monte Carlo study of the relative role of energy absorption mechanisms in water under irradiation by photons in the energy range of 3–1000 Ry

Mechanisms of energy absorption in liquid water under 3–1000 Ry photon irradiation are studied by Monte Carlo simulation with accounting for the cascade decays of vacancies in inner electron subshells created by primary and secondary ionization processes. At incident photon energies above O1s ionization threshold, most of the energy is transferred to the medium by secondary electrons and photons produced in the processes of photoionization, electron impact ionization, and cascade decay of inner-shell vacancies. On the average, on the whole considered incident photon energy interval, 49.8% of the absorbed energy is transferred to the medium by high-energy secondary electrons and photons in inelastic processes of interaction with the atoms of the medium. Low-energy electrons and photons incapable of ionizing/exciting the atoms of the medium are responsible for 48.8% of energy absorption. Energy absorption in the processes of primary ionization by incident photons are significant only at small incident photon energies of a few Rydbergs, their average contribution to energy absorption on the whole considered energy interval makes only 1.4%. At incident photon energies above O1s-ionization threshold low-energy electrons are produced in large quantities; this can be important in radiotherapy since in the tissues of organisms, low-energy electrons may cause the DNA strand breaks via the mechanism of dissociative electron attachment.

Same-Energy UV Photons and Low-Energy Electrons Activating Methane and Ammonia Frozen in Amorphous Solid Water.

UV photons and low-energy electrons play an important role in the evolution of various molecules in the interstellar medium (ISM). Here, we examined the product molecule formation as a result of irradiation of 193 nm photons and 6.4 eV electrons (same energy under identical laboratory conditions) on D2O|CH4 + ND3|D2O sandwiched films deposited on Ru(0001) substrate at 25 K in ultrahigh vacuum as a model for processes in the ISM. Temperature-programmed desorption spectra performed following the irradiation revealed the signature of hydrazine and formamide product molecules. These molecules were, however, formed exclusively following the photons' irradiation. These results were compared with the products obtained from a D2O|CH4|D2O sample without ammonia, where deuterated formaldehyde was the dominant product, formed also by photons only. Our results indicate that the photon-induced activation of the cofrozen molecules within D2O occurs via direct (partial) dissociation of the host and embedded molecules, followed by sample annealing. The electron-induced activation occurs through a direct dissociative electron attachment mechanism. The results presented here suggest possible pathways to generate various C-N, C-O, C-C, N-O, and N-H bonds containing molecules in the ISM.

Nonadiabatic dynamics in multidimensional complex potential energy surfaces.

Despite the continuous development of theoretical methodologies for describing nonadiabatic dynamics of molecular systems, there is a lack of approaches for processes where the norm of the wave function is not conserved, i.e., when an imaginary potential accounts for some irreversible decaying mechanism. Current approaches rely on building potential energy surfaces of reduced dimensionality, which is not optimal for more involving and realistic multidimensional problems. Here, we present a novel methodology for describing the dynamics of complex-valued molecular Hamiltonians, which is a generalisation of the trajectory surface hopping method. As a first application, the complex surface fewest switches surface hopping (CS-FSSH) method was employed to survey the relaxation mechanisms of the shape resonant anions of iodoethene. We have provided the first detailed and dynamical picture of the π*/σ* mechanism of dissociative electron attachment in halogenated unsaturated compounds, which is believed to underlie electron-induced reactions of several molecules of interest. Electron capture into the π* orbital promotes C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C stretching and out-of-plane vibrations, followed by charge transfer from the double bond into the σ* orbital at the C–I bond, and, finally, release of the iodine ion, all within only 15 fs. On-the-fly dynamics simulations of a vast class of processes can be envisioned with the CS-FSSH methodology, including autoionisation from transient anions, core-ionised and superexcited states, Auger and interatomic coulombic decay, and time-dependent luminescence.

Effect of Coadsorbed Oxygen on the Photochemistry of Methane Embedded in Amorphous Solid Water

The photochemistry of methane caged within amorphous solid water (ASW) is interesting as a model for studying interstellar and high-altitude atmospheric pathways for the formation of more complex hydrocarbons. Here, we report on the photoreactivity of clean methane and in the presence of oxygen molecules, known as electron capture species, within two 50 monolayer-thick D2O-ASW films adsorbed on Ru(0001) substrate under ultrahigh vacuum conditions. Irradiation by 248 nm UV photons (5.0 eV), where none of the involved molecules absorb these photons in the gas phase, leads to the formation of diverse hydrocarbons. In all cases, the presence of oxygen results in significantly enhanced reactivity. The dissociative electron attachment mechanism with electrons generated within the metal substrate is thought to largely govern the photoreactivity in this system. Methyl radicals as the primary photoproducts subsequently react with the surrounding water and neighboring methane as well as with the stable O2– anion. P...

Experimental Evidence on the Mechanism of Dissociative Electron Attachment to Formic Acid

Two mechanisms for dissociative electron attachment in HCOOH, the formation of HCOO(-)+H, were proposed in the literature: (i) via a direct electron attachment into a σ* resonance, augmented by dipole binding of the incident electron [G. A. Gallup et al., Phys. Rev. A 79, 042701 (2009)], and (ii) with the 1.8 eV π* resonance as a doorway state, linked to the products by symmetry lowering-distortion of the temporary anion, primarily the C-H bond, from the planar symmetry [T. N. Rescigno et al., Phys. Rev. Lett. 96, 213201 (2006)]. The later mechanism implies a reduction of the cross section upon deuteration of the hydrogen bonded to the C atom, whereas the former mechanism would leave the cross section unaffected. Our experimental absolute cross sections for the four isotopomers of formic acid show that deuteration on the C atom reduces the cross section value only marginally (by 12%) compared to deuteration on the O atom (reduction by a factor of 16), and thus favor mechanism (i).

Role of excited states in low-energy electron (LEE) induced strand breaks in DNA model systems: influence of aqueous environment.

Low-energy electrons (LEEs) interact with a DNA model system to create a variety of excited states. In the gas phase, dissociative (sigma*) states are accessible by LEEs with energy <4 eV (see picture) and cause facile strand breaks through a dissociative electron attachment mechanism. However, under solvation these dissociative (sigma*) states are blue-shifted and are less accessible to LEEs.

Photochemistry of molecules at confined environment: CD3Br/O/Ru(001) and CO2@Ice

AbstractThe photochemistry of molecules constrained within a confining environment on surfaces has been studied. Orientation of methyl bromide could be controlled methyl down or up by varying the pre‐adsorbed oxygen coverage due to electrostatic interactions on Ru(001) under UHV conditions. Irradiation of the coadsorption system at 193 nm has shown that the resulting photochemical activity is sensitive to the molecular orientation. Photodesorption and dissociation cross sections were 1.0·10−19 cm−2 for methyl‐down and 3.0·10−19 cm−2 for the methyl‐up configurations. This observation represents the first report of the steric effect in electron‐molecule interaction due to the dissociative electron attachment mechanism of photochemical processes on surfaces.A second system of CO2 molecules caged within ice has also been studied. Here the trapped carbon dioxide molecules cannot leave the surface at their normal desorption temperature near 100 K, but are explosively desorbing at the onset of ice evaporation near 165 K. Upon UV irradiation, enhanced dissociation to adsorbed CO and oxygen is recorded. In addition, a new reactivity channel is observed to form H2CO, tentatively identified as formaldehyde. The relevance of photochemistry of caged molecules within ice to interstellar hydrocarbon formation as a possible route for the origin of life is discussed.

Electron stimulated decomposition of adsorbed hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane, Cu(I)(hfac)(vtms)

The effect of a low energy electron beam on the decomposition of hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane (Cu(I) precursor) on the Si(111)-(7×7) surface at room temperature has been studied using x-ray photoelectron spectroscopy and Auger electron spectroscopy. It was found that electron bombardment causes the decomposition of the Cu(I) precursor and an electron energy threshold for decomposition was found to be 4±0.5 eV. This result demonstrates that the decomposition of the Cu(I) precursor in the threshold region occurs through a dissociative electron attachment mechanism. Elemental composition studies of the grown films show that they are contaminated with carbon, and that the copper fraction is increased in the film compared to the precursor composition.

State resolved probe of an energetic surface reaction: Phosgene on silver

State resolved characterization of nascent gas phase products is used as probe for the dynamics of an energetic surface reaction. This is achieved in the photodissociation of monolayer phosgene adsorbed on Ag(110). Irradiation of adsorbed Cl2CO in a broad photon energy range (hν=1.9–6.4 eV) leads to dissociation, with Cl retained on the surface and CO desorbing into the gas phase. The translational energy of product CO (g), 〈Etrans〉=0.26 eV, is independent of hν, even at the threshold photon energy (1.9 eV). This result establishes a dissociative electron attachment mechanism involving a reactive intermediate, ClCO, whose prompt dissociation serves as a probe to surface dissociation dynamics. Consistent with translation, internal state distribution of product CO (g) also shows an energetic origin: The rotational distribution, with an overall flux-weighted mean rotational energy of 〈Erot〉=0.17 eV, can be approximated by a bimodal Boltzmann distribution with rotational temperatures of 700 K at low J(s) and 7000 K at high J(s); the relative vibrational population is Nν=1/Nν=0=0.30. Contrary to common expectation based on quenching rates, both translational and rotational energies of CO (g) from monolayer photodissociation are much higher than those from the direct photodissociation in multilayers. This is taken as evidence for concerted reaction dynamics on the surface: The high exothermicity in the Cl–Ag bond formation on the surface exerts part of the energy to the Cl–CO coordinate, leading to higher energies in CO (g).

A freely localized microwave discharge for removal of chlorofluorocarbon contamination from the atmosphere

Intense microwave beams are proposed to be used for cleaning atmospheric chlorofluorocarbon contamination which is destroying the ozone layer of the Earth. It is shown that it may be possible to excite microwave discharges freely localized in the troposphere. The relation between the quantity of the destroyed chlorofluorocarbons and discharge parameters is established, the energy required for destruction is evaluated and possible unfavourable consequences of the accompanying effects (such as nitrogen oxides production) are analysed. The mechanism of dissociative electron attachment which is manifested in a cold decaying plasma of a pulsed microwave discharge is considered to be a principle mechanism causing dissociation of chlorofluorocarbons (CF2Cl2, CFCl3, etc.). The results are presented of a model laboratory experiment in which, under conditions close to those of free space, a study is made of the efficiency of chlorofluorocarbon dissociation under the action of the discharge produced in air by intense microwave radiation. The experimental results do not contradict the conclusions of the analysis of elementary processes causing the destruction of the chlorofluorocarbon component.

Electric field effects on decomposition of dilute concentrations of CHCl 3 and CCl 4 in electron beam generated air plasma

Effects of an electric field on plasma-chemical decomposition of dilute concentrations (of order 300 ppm) of CHCl 3 and CCl 4 in dry and wet (≈10% relative humidity) air streams were studied. The atmospheric pressure, non-equilibrium plasma was produced by an electron beam. The function of the electric field is to control the average electron energy. For CHCl 3, application of an electric field tended to increase decomposition markedly, with larger increases in wet air relative to dry air. For CCl 4, it tended to slightly increase decomposition in dry air while decreasing decomposition in wet air. The results are qualitatively consistent with the primary decomposition mechanism of dissociative electron attachment to CHCl 3 and CCl 4 in competition with non-dissociative electron attachment to O 2.

Photofragmentation dynamics of submonolayers of CH3Br adsorbed on Pt(111)

Angle-resolved, photofragment translational energy distributions, Pϑ(ET)s, arising from 308 nm laser irradiation of CH3Br adsorbed on Pt(111) are presented. Methyl photofragments were observed with translational energies extending to 2 eV and with Pϑ(ET)s which varied sharply with angle of exit from the surface. The fragmentation dynamics were consistent with a mechanism of dissociative electron attachment of subvacuum level, photoexcited substrate electrons to adsorbed CH3Br. The CH3 Pϑ(ET)s and the angular variation of the CH3 yield gave evidence that submonolayers of CH3Br form islands on Pt(111).

Electron attachment by saturated nitriles, acrylonitrile (C 2H 3CN), and benzonitrile (C 6H 5CN)

The formation of negative ions by attachment of low-energy electrons (0–17 eV) to a number of saturated nitriles (CH 3CN, CH 3NC, C 2H 5CN, n-C 3H 7CN, i-C 3H 7CN) and to C 2H 3CN and C 6H 5CN has been studied including an analysis of the kinetic excess energy release. In all compounds, the energetically most favourable dissociation channel, R · + CN, is characterized by an unusually low cross-section and an unbalanced excess energy distribution with virtually no kinetic energy release. This observation is explained by the mechanism of dissociative electron attachment, which in nitriles always proceeds via electron capture into a MO with π * character and subsequent cleavage of a σ bond. Apart from CN −, various other negatively charged fragments including (M H) − and (M H 2) −· are observed. Results obtained in the present study are compared with dissociative electron attachment by the corresponding halogen compounds.