Study Of Dissociative Electron Attachment Research Articles

<i>Ab initio</i> molecular dynamics study on dissociation process of 2-thiouracil and its tautomers under low-energy electron interactions

When biomolecules interact with high-energy particles and rays, they are directly ionized or dissociated, then a large number of low-energy electrons are formed as secondary particles. These low-energy electrons will attach to biomolecules, and trigger off the secondary dissociation, forming free radicals and ions with high reactivity, which can damage the structure and function of the biomolecule and cause irreversible radiation damage to the biomolecule. It is important to study the low-energy dissociative electron attachment (DEA) process of biomolecules for understanding radiation damage to biological organisms. Currently, the theoretical studies of DEA have mainly focused on the bound states of negative ions and the types of resonances in the dissociation process. The dissociation process is well described by quantum computational method, but the diversity and complexity of dissociation channels present in the dissociation process of 2-thiouracil molecule also pose a great computational challenge to these methods. In addition, the quantum computational methods are not ideal for dealing with the discrete states of chemical bonds and the problem of continuity coupling of electrons. The dissociation dynamics of biomolecules mainly results from ionization and electron attachment. <i>Ab initio</i> molecular dynamics simulation can reasonably describe these processes. In light of these considerations, <i>ab initio</i> molecular dynamics simulation is used in this work to study dynamic variation process in DEA. The low-energy electron dissociative attachment to 2-thiouracil in the gas phase is studied by using the Born-Oppenheimer molecular dynamics model combined with density functional theory. It is found that an important dehydrogenation phenomenon of 2-thiouracil and its tautomers occurs in the DEA process, and that the N—H and C—H bond are broken at specific locations. Due to the loss of hydrogen atoms at the N and C sites, the closed-shell dehydrogenated negative ion (TU-H)<sup>–</sup> forms, which is the most important negative ion fragments in the dissociation process. The potential energy curves, the bond dissociation energy and the electron affinity energy of the broken bond show that the N—H bond is the most likely to break, indicating the formation of the negative ion (TU-H)<sup>–</sup> mainly comes from the breaking of N—H bond. The theoretical calculations in this work are in good agreement with the available experimental results, indicating that the chosen calculation method is fully reliable. ​The BOMD simulations can not only dynamically recover the process of dissociative attachment of low-energy electrons to 2-thiouracil, but also more importantly provide an insight into the mechanisms of dehydrogenation and dissociation channels of 2-thiouracil molecules in DEA process.

Dissociative electron attachment studies of gas-phase acetic acid using a velocity map imaging technique

Advancing instrumentation to explore dissociative electron attachment (DEA) studies allows previously unattainable information to be acquired. Using a newly constructed velocity map imaging spectrometer, we revisited a study on DEA to gas-phase acetic acid. We discuss possible fragmentation channels and compared the corresponding ion yields with previous high electron-energy resolution results. We focus on the channels occurring at higher energies, particularly near 10 eV, and calculate their thermodynamic thresholds. Moreover, we expand previous studies and perform time-sliced imaging near the 10 eV resonance to obtain the kinetic energy distribution of the fragment ions.

Experimental and Theoretical Studies of Dissociative Electron Attachment to Metabolites Oxaloacetic and Citric Acids.

In this contribution the dissociative electron attachment to metabolites found in aerobic organisms, namely oxaloacetic and citric acids, was studied both experimentally by means of a crossed-beam setup and theoretically through density functional theory calculations. Prominent negative ion resonances from both compounds are observed peaking below 0.5 eV resulting in intense formation of fragment anions associated with a decomposition of the carboxyl groups. In addition, resonances at higher energies (3–9 eV) are observed exclusively from the decomposition of the oxaloacetic acid. These fragments are generated with considerably smaller intensities. The striking findings of our calculations indicate the different mechanism by which the near 0 eV electron is trapped by the precursor molecule to form the transitory negative ion prior to dissociation. For the oxaloacetic acid, the transitory anion arises from the capture of the electron directly into some valence states, while, for the citric acid, dipole- or multipole-bound states mediate the transition into the valence states. What is also of high importance is that both compounds while undergoing DEA reactions generate highly reactive neutral species that can lead to severe cell damage in a biological environment.

Carboxylation Enhances Fragmentation of Furan upon Resonant Electron Attachment.

We report a dissociative electron attachment study to 2-furoic acid (C5H4O3) isolated in a gas phase, which is a model molecule consisting of a carboxylic group and a furan ring. Dissociation of furan by low energy electrons is accessible only via electronic excited Feshbach resonances at energies of incident electrons above 5 eV. On the other hand, carboxylic acids are well-known to dissociate via attachment of electrons at subexcitation energies. Here we elucidate how the electron and proton transfer reactions induced by carboxylation influence stability of the furan ring. Overlap of the furan and carboxyl π orbitals results in transformation of the nondissociative π2 resonance of the furan ring to a dissociative resonance. The interpretation of hydrogen transfer reactions is supported by experimental studies of 3-methyl-2-furoic and 5-methyl-2-furoic acids (C6H6O3) and density functional theory (DFT) calculations.

Dissociative electron attachment to carbon dioxide

Our experimental progresses on the reaction dynamics of dissociative electron attachment (DEA) to carbon dioxide (CO2) are summarized in this review. First, we introduce some fundamentals about the DEA dynamics and provide an epitome about the DEAs to CO2. Second, the experimental technique developments are described, in particular, on the high-resolution velocity map imaging apparatus in which we put a lot of efforts during the past two years. Third, our findings about the DEA dynamics of CO2 are surveyed and briefly compared with the others’ work. At last, we give a perspective about the applications of the DEA studies and highlight the inspirations in the production of molecular oxygen on Mars and the catalytic transformations of CO2.

Dissociative electron attachment to polyatomic molecules

Dissociative electron attachment (DEA) to molecule plays a key role in atmosphere, interstellar space and ionization damages of biological tissue. Experimental DEA studies of polyatomic molecules in gas phase provide the dynamics details that are the fundamentals to establish the physicochemical models of the electron-induced reactions in complicated environments. Since 2012, we successively set up two ion-velocity-map-imaging apparatuses, and accomplished a series of experimental studies of the DEA dynamics. Here is a brief review about our progresses on polyatomic molecules.

Dipole-Supported Electronic Resonances Mediate Electron-Induced Amide Bond Cleavage.

Dissociative electron attachment (DEA) plays a key role in radiation damage of biomolecules under high-energy radiation conditions. The initial step in DEA is often rationalized in terms of resonant electron capture into one of the metastable valence states of a molecule followed by its fragmentation. Our combined theoretical and experimental investigations indicate that the manifold of states responsible for electron capture in the DEA process can be dominated by core-excited (shake-up) dipole-supported resonances. Specifically, we present the results of experimental and computational studies of the gas-phase DEA to three prototypical peptide molecules, formamide, N-methylformamide (NMF), and N,N-dimethyl-formamide (DMF). In contrast to the case of electron capture by positively charged peptides in which amide bond rupture is rare compared to N─C_{α} bond cleavage, fragmentation of the amide bond was observed in each of these three molecules. The ion yield curves for ions resulting from this amide bond cleavage, such as NH_{2}^{-} for formamide, NHCH_{3}^{-} for NMF, and N(CH_{3})_{2}^{-} for DMF, showed a double-peak structure in the region between 5 and 8eV. The peaks are assigned to Feshbach resonances including core-excited dipole-supported resonances populated upon electron attachment based on high-level electronic structure calculations. Moreover, the lower energy peak is attributed to formation of the core-excited resonance that correlates with the triplet state of the neutral molecule. The latter process highlights the role of optically spin-forbidden transitions promoted by electron impact in the DEA process.

Dissociative electron attachment studies with hyperthermal Rydberg atoms.

Earlier studies of the velocity distributions of heavy-Rydberg ion-pair states formed in collisions between potassium Rydberg atoms with low-to-intermediate values of n, 10 ≲ n ≲ 15, and targets that attach free low-energy electrons have shown that such measurements can provide a window into the dynamics of dissociative electron capture. Here we propose that the reaction dynamics can be explored in much greater detail through studies using hyperthermal Rydberg atoms. This is demonstrated using, as an example, helium Rydberg atoms and a semi-classical Monte Carlo collision code developed specifically to model the dynamics of Rydberg electron transfer in collisions between Rydberg atoms and attaching targets. The simulations show that the outcome of collisions is sensitive not only to the lifetime and decay energetics of the excited intermediate negative ion formed upon initial Rydberg electron capture but also to the radial electron probability density distribution in the Rydberg atom itself, i.e., to its ℓ value.

Pt(CO)2Cl2 fragmentation upon low energy electron interactions

SynopsisIn the present communication we present low energy electron interactions with Pt(CO)2Cl2, a FEBID precursor. Electron impact ionization as well as dissociative electron attachment studies are discussed.

Computational study of dissociative electron attachment to π-allyl ruthenium (II) tricarbonyl bromide

Motivated by the current interest in low energy electron induced fragmentation of organometallic complexes in focused electron beam induced deposition (FEBID) we have evaluated different theoretical protocols for the calculation of thermochemical threshold energies for DEA to the organometallic complex π-allyl ruthenium (II) tricarbonyl bromide. Several different computational methods including density functional theory (DFT), hybrid-DFT and coupled cluster were evaluated for their ability to predict these threshold energies and compared with the respective experimental values. Density functional theory and hybrid DFT methods were surprisingly found to have poor reliability in the modelling of several DEA reactions; however, the coupled cluster method LPNO-pCCSD/2a was found to produce much more accurate results. Using the local correlation pair natural orbital (LPNO) methodology, high level coupled cluster calculations for open-shell systems of this size are now affordable, paving the way for reliable theoretical DEA predictions of such compounds.

Electron interactions with the focused electron beam induced processing (FEBID) precursor tungsten hexachloride.

Secondary electrons with an energy distribution below 100 eV are formed when high-energy particles interact with matter. In the focused electron beam induced deposition, high-energy beams are used to decompose organometallic compounds on surfaces. We investigated the electron ionisation of WCl6 and dissociative electron attachment to WCl6 in the gas phase in order to better understand the decomposition mechanism driven by secondary electrons. A double-focusing mass spectrometer coupled with a Nier-type ion source was used to perform the present studies. The electron ionisation studies were performed with an electron energy of 70 eV and dissociative electron attachment studies in the energy range of ~0-14 eV. Tungsten hexachloride rapidly oxidises, leading to the formation of a mixture of pure WCl6 and WCl4 O together with WCl2 O2 species. The fragmentation of the three chlorinated compounds is effective, although electron ionisation to WCl6 leads to W(+) in contrast with WCl2 O2 and WCl4 O leading to WO2 (+) and WO(+) , respectively, as lighter fragments. With regard to electron attachment, decomposition of the precursor molecules is observed; however, W(-) was not detected within the detection limit of the instrument. Electron ionisation and dissociative electron attachment (DEA) to WCl6 , WCl4 O and WCl2 O2 lead to strong fragmentation. In electron ionisation, the fragmentation by loss of chlorine atoms was observed for both WCl6 and the oxidised species. Additionally, the loss of all chlorine ligands is observable for WCl6 as well as the oxidised species. The DEA results have shown dissociation by the scission of chlorine atoms as well as by the scission of an oxygen atom. The formation of chlorine and oxygen anions was observed, indicating the formation of a neutral counterpart containing the metal atom, free to be attacked by the next electron.

Quantum chemical study of dissociative electron attachment to d-ribose and d-fructose

We performed quantum chemistry calculations using long-range corrected density-functional theory to understand the interaction of an excess electron with d-ribose/d-fructose. Previous experimental studies have shown that these sugar molecules undergo dissociative electron attachment to form various fragments upon capture of a nearly zero-energy electron. First, we studied doorway states, including dipole-bound and valence-bound anions in pyranose, furanose, and open-chain structures. Subsequently, we explored the reaction pathways on the anionic potential energy surfaces for selected structures to understand the detailed reaction mechanisms of dissociative electron attachment to these sugar molecules.

Low-energy dissociative electron attachment toCF2

We present the results of a theoretical study of dissociative electron attachment (DEA) of low-energy electrons to ${\mathrm{CF}}_{2}$. We carried out electron scattering calculations using the complex Kohn variational method at the static-exchange and relaxed self-consistent field (SCF) level at the equilibrium geometry and compare our differential cross sections to other results. We then repeated these calculations as a function of the three internal degrees of freedom to obtain the resonance energy surfaces and autoionization widths. We use this data as input to form the Hamiltonian relevant to the nuclear dynamics. The multidimensional wave equation is solved using the multiconfiguration time-dependent Hartree (MCTDH) approach within the local approximation.

Dissociative electron attachment to chlorine dioxide probed by velocity slice im-aging

SynopsisWe report the study of dissociative electron attachment (DEA) to chlorine dioxide (OClO) in the energy range of 1 to 10 eV using velocity slice imaging technique. DEA to OClO shows Cl−, O−, O2−, and OCl− as fragment channels at various electron energies. Based on the momentum images of these ions we unravel the molecular dynamics leading to DEA.

Dissociative electron attachment dynamics of ozone using velocity slice imaging

SynopsisWe report the study of dissociative electron attachment to ozone (O3) in the energy range of 1 to 10 eV using velocity slice imaging technique. Based on the momentum images that we obtained for O− and O2− channels at various electron energies we unravel the molecular dynamics leading to DEA.

Dissociative electron attachment to molecular chlorine

SynopsisWe report the study of dissociative electron attachment to molecular chlorine in the energy range of 1 to 20 eV using velocity slice imaging technique. Based on the angular distributions at various electron energies, we identify the partial waves involved in the resonance formation that lead to DEA.

Complete data acquisition and analysis system for low-energy electron–molecule collision studies

A complete data acquisition system has been developed that can work with any personal computer irrespective of the operating system installed on it. The software can be used in low and intermediate electron-energy collision studies with ground-state molecules in gas phase using a combination of RS-232, GPIB, and USB-interfaced devices. Various tabletop instruments and nuclear instrumentation module (NIM) -based electronics have been interfaced and have communicated with the software, which is based on LabVIEW. This is tested with dissociative electron attachment (DEA) and polar dissociation studies to oxygen molecule and successfully used in a DEA study of carbon monoxide and carbon dioxide.

Electron-molecule chemistry and charging processes on organic ices and Titan’s icy aerosol surrogates

Electron-induced polymerization processes and charging events that can occur within Titan’s atmosphere or on its surface were simulated using electron irradiation and dissociative electron attachment (DEA) studies of nitrogen-containing organic condensates. The DEA studies probe the desorption of H− from hydrogen cyanide (HCN), acetonitrile (CH3CN), and aminoacetonitrile (NH2CH2CN) ices, as well as from synthesized tholin materials condensed or deposited onto a graphite substrate maintained at low temperature (90–130K). The peak cross sections for H− desorption during low-energy (3–15eV) electron irradiation were measured and range from 3×10−21 to 2×10−18cm2. Chemical and structural transformations of HCN ice upon 2keV electron irradiation were investigated using X-ray photoelectron and Fourier-transform infrared spectroscopy techniques. The electron-beam processed materials displayed optical properties very similar to tholins produced by conventional discharge methods. Electron and negative ion trapping lead to 1011chargescm−2 on a flat surface which, assuming a radius of 0.05μm for Titan aerosols, is ∼628charges/radius (in μm). The facile charge trapping indicates that electron interactions with nitriles and complex tholin-like molecules could affect the conductivity of Titan’s atmosphere due to the formation of large negative ion complexes. These negatively charged complexes can also precipitate onto Titan’s surface and possibly contribute to surface reactions and the formation of dunes.

Dissociative electron attachment studies on acetone.

Dissociative electron attachment (DEA) to acetone is studied in terms of the absolute cross section for various fragment channels in the electron energy range of 0-20 eV. H(-) is found to be the most dominant fragment followed by O(-) and OH(-) with only one resonance peak between 8 and 9 eV. The DEA dynamics is studied by measuring the angular distribution and kinetic energy distribution of fragment anions using Velocity Slice Imaging technique. The kinetic energy and angular distribution of H(-) and O(-) fragments suggest a many body break-up for the lone resonance observed. The ab initio calculations show that electron is captured in the multi-centered anti-bonding molecular orbital which would lead to a many body break-up of the resonance.

Dissociative electron attachment to titatinum tetrachloride and titanium tetraisopropoxide

Here we present a dissociative electron attachment study of titanium tetrachloride and titanium tetraisopropoxide in the incident electron energy range from about 0–18 eV. The results are compared to electron impact ionization and fragmentation of these compounds and discussed in relation to the role of secondary electrons in focused electron beam induced deposition. We also use the opportunity and describe in detail a recently constructed crossed beam apparatus for the study of the energy dependency of ion formation in low energy electron interaction with gas phase molecules.