Mechanism Of Electron Attachment Research Articles

Atomistic modeling of thermal effects in focused electron beam-induced deposition of Me$$_2$$Au(tfac)

The role of thermal effects in the focused electron beam induced deposition (FEBID) process of Me$_2$Au(tfac) is studied by means of irradiation-driven molecular dynamics simulations. The FEBID of Me$_2$Au(tfac), a commonly used precursor molecule for the fabrication of gold nanostructures, is simulated at different temperatures in the range of $300-450$ K. The deposit's structure, morphology, growth rate, and elemental composition at different temperatures are analyzed. The fragmentation cross section for Me$_2$Au(tfac) is evaluated on the basis of the cross sections for structurally similar molecules. Different fragmentation channels involving the dissociative ionization (DI) and dissociative electron attachment (DEA) mechanisms are considered. The conducted simulations of FEBID confirm experimental observations that deposits consist of small gold clusters embedded into a carbon-rich organic matrix. The simulation results indicate that accounting for both DEA- and DI-induced fragmentation of all the covalent bonds in Me$_2$Au(tfac) and increasing the amount of energy transferred to the system upon fragmentation increase the concentration of gold in the deposit. The simulations predict an increase in Au:C ratio in the deposit from 0.18 to 0.25 upon the temperature increase from 300 K to 450 K, being within the range of experimentally reported values.

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.

Investigating resonant low-energy electron attachment to formamide: Dynamics of model peptide bond dissociation and other fragmentation channels

We report experimental results on three-dimensional momentum imaging measurements of anions generated via dissociative electron attachment to gaseous formamide. From the momentum images, we analyze the angular and kinetic energy distributions for NH$_2^{-}$, O$^{-}$, and H$^{-}$ fragments and discuss the possible electron attachment and dissociation mechanisms for multiple resonances for two ranges of incident electron energies, from 5.3~eV to 6.8~eV, and from 10.0~eV to 11.5~eV. {\it Ab initio} theoretical results for the angular distributions of the NH$_2^{-}$ anion for $\sim$6~eV incident electrons, when compared with the experimental results, strongly suggest that one of the two resonances producing this fragment is a $^2$A$''$ Feshbach resonance.

Electron Attachment to Isolated Molecules as a Probe to Understand Mitochondrial Reductive Processes.

This chapter describes the complementary experimental techniques Electron Transmission Spectroscopy and Dissociative Electron Attachment Spectroscopy, two of the most suitable means for investigating interactions between electrons and gas-phase molecules, resonance formation of temporary molecular negative ions, and their possible decay through the dissociative electron attachment (DEA) mechanism. The latter can be seen as the gas-phase counterpart of the transfer of a solvated electron in solution, accompanied by dissociation of the molecular anion, referred to as dissociative electron transfer (DET). DET takes place in vivo under reductive conditions, for instance, in the intermembrane space of mitochondria under interaction of xenobiotic molecules possessing high electron affinity with electrons "leaked" from the mitochondrial respiratory chain. A likely mechanism of the toxic activity of dichlorodiphenyltrichloroethane based on its DEA properties is briefly outlined, and compared with the well-established harmful effects of the model toxicant carbon tetrachloride ascribed to reductive dechlorination in a cellular ambient. A possible mechanism of the antioxidant activity of polyphenolic compounds present near the main site of superoxide anion production in mitochondria is also briefly discussed.

Ab Initio Molecular Dynamics Simulations of Solvated Electrons in Ammonia Clusters.

We investigated excess electron solvation dynamics in (NH3)n– ammonia clusters in the n = 8–32 size range by performing finite temperature molecular dynamics simulations. In particular, we focused on three possible scenarios. The first case is designed to model electron attachment to small neutral ammonia clusters (n ≤ ∼10) that form hydrogen-bonded chains. The excess electron is bound to the clusters via dipole bound states, and persists with a VDE of ∼160 meV at 100 K for the n = 8 cluster. The coupled nuclear and electronic relaxation is fast (within ∼100 fs) and takes place predominantly by intermolecular librational motions and the intramolecular umbrella mode. The second scenario illustrates the mechanism of excess electron attachment to cold compact neutral clusters of medium size (8 ≤ n ≤ 32). The neutral clusters show increasing tendency with size to bind the excess electron on the surface of the clusters in weakly bound, diffuse, and highly delocalized states. Anionic relaxation trajectories launched from these initial states provide no indication for excess electron stabilization for sizes n < 24. Excess electrons are likely to autodetach from these clusters. The two largest investigated clusters (n = 24 and 32) can accommodate the excess electron in electronic states that are mainly localized on the surface, but they may be partly embedded in the cluster. In the last 500 fs of the simulated trajectories, the VDE of the solvated electron fluctuates around ∼200 meV for n = 24 and ∼500 meV for n = 32, consistent with the values extrapolated from the experimentally observed linear VDE–n–1/3 trend. In the third case, we prepared neutral ammonia cluster configurations, including an n = 48 cluster, that contain possible electron localization sites within the interior of the cluster. Excess electrons added to these clusters localize in cavities with high VDEs up to 1.9 eV for n = 48. The computed VDE values for larger clusters are considerably higher than the experimentally observed photoelectric threshold energy for the solvated electron in bulk ammonia (∼1.4 eV). Molecular dynamics simulations launched from these initial cavity states strongly indicate, however, that these cavity structures exist only for ∼200 fs. During the relaxation the electron leaves the cavity and becomes delocalized, while the cluster loses its initial compactness.

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...

Contrast between the mechanisms for dissociative electron attachment to CH3SCN and CH3NCS.

The kinetics of thermal electron attachment to methyl thiocyanate (CH3SCN), methyl isothiocyanate (CH3NCS), and ethyl thiocyanate (C2H5SCN) were measured using flowing afterglow-Langmuir probe apparatuses at temperatures between 300 and 1000 K. CH3SCN and C2H5SCN undergo inefficient dissociative attachment to yield primarily SCN- at 300 K (k = 2 × 10-10 cm3 s-1), with increasing efficiency as temperature increases. The increase is well described by activation energies of 0.17 eV (CH3SCN) and 0.14 eV (C2H5SCN). CN- product is formed at <1% branching at 300 K, increasing to ∼30% branching at 1000 K. Attachment to CH3NCS yields exclusively SCN- ionic product but at a rate at 300 K that is below our detection threshold (k < 10-12 cm3 s-1). The rate coefficient increases rapidly with increasing temperature (k = 6 × 10-11 cm3 s-1 at 600 K), in a manner well described by an activation energy of 0.51 eV. Calculations at the B3LYP/def2-TZVPPD level suggest that attachment to CH3SCN proceeds through a dissociative state of CH3SCN-, while attachment to CH3NCS initially forms a weakly bound transient anion CH3NCS-* that isomerizes over an energetic barrier to yield SCN-. Kinetic modeling of the two systems is performed in an attempt to identify a kinetic signature differentiating the two mechanisms. The kinetic modeling reproduces the CH3NCS data only if dissociation through the transient anion is considered.

Comparative study of hydrolytic and electron-driven processes in carboplatin biotransformation

The results of computational simulation of reaction courses mimicking the transformation of carboplatin from pro-drug into its active shape, responsible for cytotoxic effect, are reported.Implementing the density functional theory (DFT) calculations and the supermolecular approach, we explored the pathways representing two disparate models of carboplatin bioactivation: (1) based on paradigm of carboplatin aquation, and (2) based on new hypothesis that transformation is controlled by electron-transfer processes. The calculated geometrical and thermodynamic parameters were used for evaluation of pathways. In contrast to carboplatin hydrolysis, representing a typical two stage SN2 mechanism, the postulated electron-driven reactions proceed under the dissociative electron attachment (DEA) mechanism. The reaction profiles predict endothermic effect in both stages of hydrolytic course and final exothermic effects for electron-driven processes. The most effective are hybrid processes including two-stages: water and subsequent electron impact on transformed carboplatin. The aqua-products, manifesting strong electron-affinity, can be the active form of drug capable to cytotoxic interaction with DNA, not only as alkylating agent but also as electron-acceptor. Concluding, the hybrid transformation of carboplatin is more favourable than hydrolytic.

Negative ion states of 5-bromouracil and 5-iodouracil.

The valence anion states of the potential radiosensitisers 5-bromouracil and 5-iodouracil were investigated through elastic scattering calculations. These compounds have rich spectra of negative ion states that trigger off different mechanisms for dissociative electron attachment. For each molecule, we obtained a bound π* anion, two π* shape resonances and a low lying σ* anion state, in addition to a dipole-bound state (the latter was obtained using bound-state techniques). The σ* anion, formed by electron attachment to an anti-bonding carbon-halogen orbital, was found to have resonant character in 5-bromouracil, and bound-state character in 5-iodouracil. The present calculations place the σCBr* resonance around 0.7 eV, considerably below the energy inferred from the electron transmission data (1.3 eV). The signature of this anion state, not evident in the measurements, would be obscured by the large background arising from the dipolar interaction, not by the strong signature of the π2*, as presumed. Our results support the π2* resonance as a precursor state to dissociative electron attachment around 1.5 eV in both 5-bromouracil and 5-iodouracil, while the interplay among π1*, σ* and dipole-bound states would be expected close to 0 eV. We also discuss the suppression of the hydrogen elimination channels in these species.

ETS and DEAS studies of the reduction of xenobiotics in mitochondrial intermembrane space.

This chapter describes the complementary experimental techniques electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS), two of the most suitable means for investigating interactions between electrons and gas-phase molecules, resonance formation of temporary molecular negative ions, and their possible decay through the dissociative electron attachment (DEA) mechanism. The latter can be seen as the gas-phase counterpart of the transfer of a solvated electron in solution, accompanied by dissociation of the molecular anion, referred to as dissociative electron transfer (DET). DET takes place in vivo under reductive conditions, for instance, in the intermembrane space of mitochondria under interaction of xenobiotic molecules with electrons "leaked" from the respiration chain. Experimental procedures supported by suitable quantum chemical calculations are described in detail and illustrated by an example of ETS/DEAS study of rhodanine which shows rich fragmentation under gas-phase resonance electron attachment.

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).

Low-energy electron scattering by cellulose and hemicellulose components

We report elastic integral, differential and momentum transfer cross sections for low-energy electron scattering by the cellulose components β-D-glucose and cellobiose (β(1 → 4) linked glucose dimer), and the hemicellulose component β-D-xylose. For comparison with the β forms, we also obtain results for the amylose subunits α-D-glucose and maltose (α(1 → 4) linked glucose dimer). The integral cross sections show double peaked broad structures between 8 eV and 20 eV similar to previously reported results for tetrahydrofuran and 2-deoxyribose, suggesting a general feature of molecules containing furanose and pyranose rings. These broad structures would reflect OH, CO and/or CC σ* resonances, where inspection of low-lying virtual orbitals suggests significant contribution from anion states. Though we do not examine dissociation pathways, these anion states could play a role in dissociative electron attachment mechanisms, in case they were coupled to the long-lived π* anions found in lignin subunits [de Oliveira et al., Phys. Rev. A, 2012, 86, 020701(R)]. Altogether, the resonance spectra of lignin, cellulose and hemicellulose components establish a physical-chemical basis for electron-induced biomass pretreatment that could be applied to biofuel production.

Selective terminal function modification of SAMs driven by low-energy electrons (0–15 eV)

Low-energy electron induced degradation of a model self-assembled monolayer (SAM) of acid terminated alkanethiol was studied under ultra-high vacuum (UHV) conditions at room and low (~40 K) temperatures. Low-energy electron induced chemical modifications of 11-mercaptoundecanoic acid (MUA, HS-(CH2)10-COOH) SAMs deposited on gold were probed in situ as a function of the irradiation energy (<11 eV) by combining two complementary techniques: High Resolution Electron Energy Loss Spectroscopy (HREELS), a surface sensitive vibrational spectroscopy technique, and Electron Stimulated Desorption (ESD) analysis of neutral fragments. The SAM's terminal functions were observed to be selectively damaged at around 1 eV by a resonant electron attachment mechanism, observed to decay by CO, CO2 and H2O formation and desorption. CO2 and H2O were also directly identified at low temperature by vibrational analysis of the irradiated SAMs. At higher irradiation energy, both terminal functions and spacer alkyl chains are damaged upon electron irradiation, by resonant and non-resonant processes.

Low-energy electron collisions with biomolecules

We report recent progress in applying the Schwinger multichannel computational method to the interactions of slow electrons with biomolecules. Calculations on constituents of DNA, including nucleobases, phosphate esters, and models of the backbone sugar, have provided insight into the nature of the low-energy shape resonances, and thereby into possible sites and mechanisms for electron attachment that may lead to strand-breaking. At the same time, more approximate calculations on larger assemblies such as nucleosides and deoxyadenosine monophosphate indicate how the resonance properties of the subunits will or will not persist in DNA itself. We are pursuing a similar strategy for another major class of biomolecules, the proteins, by beginning with fixed-nuclei studies of the constituent amino acids; here we present preliminary results for the simplest amino acid, glycine. We also describe efforts directed at an improved understanding electron collisions with alcohols, which, in addition to basic scientific interest, may prove useful in the modeling of ignition and combustion within biofuel-powered engines.

Elastic Scattering of Low-Energy Electron by Lignin Precursors

We present cross sections for electron collisions with monolignol precursors obtained with the Schwinger Multichannel method. For Cs phenol system, π* resonances are found in the A" irreducible representation. So, mechanisms for dissociative electron attachment could give rise if σ* resonances are found in the monolignols.

Low-energy electron scattering by pyrrole

We present elastic cross sections for electron-pyrrole scattering obtained with the Schwinger Multichannel Method. For this C2v system, we found three shape resonances in the A1, B2 and A2 irreducible representations that could give rise to direct and indirect mechanisms for dissociative electron attachment.

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 Vinyl Chloride Physisorbed on Ag(111) through Molecular Anion Formation Induced by Substrate Electron Attachment

Pulsed 266 and 355 nm ultraviolet laser irradiation of monolayer vinyl chloride physisorbed on Ag(111) results in molecular dissociation leading to C2H3 and Cl, much of which is adsorbed to the surface. On the basis of observations made on dissociation dependences on chlorine isotope and photon energy, it is deduced that upon excitation vinyl chloride forms a transient negative ion through a substrate mediated, vertical electron attachment mechanism. The anion either dissociates or relaxes through energy transfer to the neutral state causing the neutral molecule to desorb. The threshold for vertical attachment of substrate electron is estimated to be 0.8 eV below the vacuum level, in agreement with the experimentally observed wavelength dependence in photoinduced dissociation. Chemisorbed Cl on the Ag(111) surface inhibits the photodissociation process by increasing the substrate work function and consequently the energy threshold for electron vertical attachment. Upon heating the Ag(111) surface, adsorbed vinyl combines to produce 1,3-butadiene in a first order, diffusion limited, process with an activation energy of 10.4 kcal/mol.

Low-density plasma channels generated by femtosecond pulses

We present an investigation of the plasma channel formed along the trail of an intense femtosecond (fs) laser pulse propagating through helium, air and nitrogen at low pressures. The lifetime of the generated plasma channel of up to 60 μs was measured. We determined the electron-attachment mechanism as a dominant mechanism for the plasma relaxation. The initial electron densities, electron-attachment coefficients and plasma channel conductivity for different experimental conditions were obtained.

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.