Low-energy Electron Attachment Research Articles

Dissociation dynamics in low energy electron attachment to ammonia using velocity slice imaging.

The complete dissociation dynamics of low energy electron attachment to the ammonia molecule has been studied using velocity slice imaging (VSI) spectrometry. One low energy resonant peak around 5.5 eV and a broad resonance around 10.5 eV incident electron energies have been observed. The resonant states mainly dissociate via H- and NH2- fragments, though for the upper resonant state, the signature of NH- fragments is also predicted due to a three-body dissociation process. Kinetic energy and angular distributions of the NH2- fragment anions are measured simultaneously around the two resonances. Based on our experimental observations, we conclude that a temporary negative ion (TNI) state with A1 symmetry is responsible for the lower resonance. Whereas, we find strong evidence for the existence of a TNI state having A1 symmetry at the 10.5 eV resonance for the first time.

Low-energy electron-induced decomposition of 5-trifluoromethanesulfonyl-uracil: A potential radiosensitizer.

5-trifluoromethanesulfonyl-uracil (OTfU), a recently proposed radiosensitizer, is decomposed in the gas-phase by attachment of low-energy electrons. OTfU is a derivative of uracil with a triflate (OTf) group at the C5-position, which substantially increases its ability to undergo effective electron-induced dissociation. We report a rich assortment of fragments formed upon dissociative electron attachment (DEA), mostly by simple bond cleavages (e.g., dehydrogenation or formation of OTf-). The most favorable DEA channel corresponds to the formation of the triflate anion alongside with the reactive uracil-5-yl radical through the cleavage of the O-C5 bond, particularly at about 0 eV. Unlike for halouracils, the parent anion was not detected in our experiments. The experimental findings are accounted by a comprehensive theoretical study carried out at the M06-2X/aug-cc-pVTZ level. The latter comprises the thermodynamic thresholds for the formation of the observed anions calculated under the experimental conditions (383.15 K and 3 × 10-11 atm). The energy-resolved ion yield of the dehydrogenated parent anion, (OTfU-H)-, is discussed in terms of vibrational Feshbach resonances arising from the coupling between the dipole bound state and vibrational levels of the transient negative ion. We also report the mass spectrum of the cations obtained through ionization of OTfU by electrons with a kinetic energy of 70 eV. The current study endorses OTfU as a potential radiosensitizer agent with possible applications in radio-chemotherapy.

Dissociation of methyl formate (HCOOCH3) molecules upon low-energy electron attachment

Context. The methyl formate molecule (HCOOCH3) is considered to be a key molecule in astrochemistry. The abundance of this molecule in space depends on the stability upon irradiation with particles like low-energy electrons. Aims. We have investigated the decomposition of the molecule upon electron capture in the electron energy range from about 0 eV up to 15 eV. All experimentally obtained fragmentation channels of the molecular anion were investigated by quantum chemical calculations. Methods. A high resolution electron monochromator coupled with quadrupole mass spectrometer was used for the present laboratory experiment. Quantum chemical calculations of the electron affinities of the generated fragments, the thermodynamic thresholds and the activation barriers for the associated reaction channels were carried out to complement the experimental studies. Results. Electron attachment is shown to be a purely dissociative process for this molecule and proceeds within two electron energy regions of about 1 eV to 4 eV and from 5 eV to 14 eV. In our experiment five anionic fragments with m/z (and possible stoichiometric structure) 59 (C2H3O2−), 58 (C2H2O2−), 45 (CHO2−) 31 (CH3O−), and 29 (CHO−) were detected. The most abundant anion fragments that are formed through dissociative electron attachment to methyl formate are the complementary anions CH3O− and CHO−, associated with the same single bond cleavage and different survival probability. Conclusions. The low-energy electron induced dissociation of methyl formate differs from its isomers acetic acid and glycolaldehyde, which leads to possible chemical selectivity in the chemical evolution.

Dynamics of electron attachment and photodissociation in iodide-uracil-water clusters via time-resolved photoelectron imaging.

The dynamics of low energy electron attachment to monohydrated uracil are investigated using time-resolved photoelectron imaging to excite and probe iodide-uracil-water (I-·U·H2O) clusters. Upon photoexcitation of I-·U·H2O at 4.38 eV, near the measured cluster vertical detachment energy of 4.40 eV ± 0.05 eV, formation of both the dipole bound (DB) anion and valence bound (VB) anion of I-·U·H2O is observed and characterized using a probe photon energy of 1.58 eV. The measured binding energies for both anions are larger than those of the non-hydrated iodide-uracil (I-·U) counterparts, indicating that the presence of water stabilizes the transient negative ions. The VB anion exhibits a somewhat delayed 400 fs rise when compared to I-·U, suggesting that partial conversion of the DB anion to form the VB anion at early times is promoted by the water molecule. At a higher probe photon energy, 3.14 eV, I- re-formation is measured to be the major photodissociation channel. This product exhibits a bi-exponential rise; it is likely that the fast component arises from DB anion decay by internal conversion to the anion ground state followed by dissociation to I-, and the slow component arises from internal conversion of the VB anion.

Effects of the plasma-facing materials on the negative ion H− density in an ECR (2.45 GHz) plasma

Within the framework of fundamental research, the present work focuses on the role of surface material in the production of H− negative ion, with a potential application of designing cesium-free H− negative ion sources oriented to fusion application. It is widely accepted that the main reaction leading to H− production, in the plasma volume, is the dissociative attachment of low-energy electrons (Te ≤ 1 eV) on highly ro-vibrationally excited hydrogen molecules. In parallel with other mechanisms, the density of these excited molecules may be enhanced by means of the recombinative desorption, i.e. the interaction between surface absorbed atoms with other atoms (surface adsorbed or not) through the path . Accordingly, a systematic study on the role played by the surface in this reaction, with respect to the production of H− ion in the plasma volume, is here performed. Thus, tantalum and tungsten (already known as H− enhancers) and quartz (inert surface) materials are employed as inner surfaces of a test bench chamber. The plasma inside the chamber is produced by electron cyclotron resonance (ECR) driving and it is characterized with conventional electrostatic probes, laser photodetachment, and emission and absorption spectroscopy. Two different positions (close to and away from the ECR driving zone) are investigated under various conditions of pressure and power. The experimental results are supported by numerical data generated by a 1D model. The latter couples continuity and electron energy balance equations in the presence of magnetic field, and incorporates vibrational kinetics, H2 molecular reactions, H electronically excited states and ground-state species kinetics. In the light of this study, recombinative desorption has been evidenced as the most probable mechanism, among others, responsible for an enhancement by a factor of about 3.4, at 1.6 Pa and 175 W of microwave power, in the case of tantalum.

Can the Electron-Accepting Properties of Odorants Be Involved in Their Recognition by the Olfactory System?

The present study examines the possible importance of the electron-accepting properties of odorant molecules and, in particular, the formation and decay of temporary negative ions via low-energy electron attachment as a possible contribution toward understanding odorant recognition by olfactory receptors (ORs). Fragments formed by dissociative electron attachment (DEA) of mustard oil odorants represented by a series of isothiocyanates are studied experimentally using DEA spectroscopy and DFT calculations. Relative intensities for the most abundant fragment species, S- and SCN-, are found to be characteristic of structurally similar odorants under investigation. This novel approach for the investigation of odorants may contribute to understanding the initial stages of the olfactory process and may provide a means to distinguish between odorants and their interactions with the olfactory receptor system.

Ultrafast dynamics of low-energy electron attachment via a non-valence correlation-bound state.

The primary electron-attachment process in electron-driven chemistry represents one of the most fundamental chemical transformations with wide-ranging importance in science and technology. However, the mechanistic detail of the seemingly simple reaction of an electron and a neutral molecule to form an anion remains poorly understood, particularly at very low electron energies. Here, time-resolved photoelectron imaging was used to probe the electron-attachment process to a non-polar molecule using time-resolved methods. An initially populated diffuse non-valence state of the anion that is bound by correlation forces evolves coherently in ∼30 fs into a valence state of the anion. The extreme efficiency with which the correlation-bound state serves as a doorway state for low-energy electron attachment explains a number of electron-driven processes, such as anion formation in the interstellar medium and electron attachment to fullerenes.

Inducing chemical reactivity on specific sites of a molecule using the Coulomb interaction exerted by a low energy electron.

Remarkable site-specificity in the resonant attachment of low energy electrons (LEEs) to molecular targets is proposed as an efficient method for inducing chemical reactivity on specific sites of molecules. The Coulomb interaction between the attached electron and the most polarizable molecular electrons localized on the attached site is the reason for site-specific chemical reactivity. The Coulombically induced site-specific chemical reactivity is best illustrated by the LEE induced chemical transformation of a weakly bound molecular complex into a strong covalent adduct. The chemical transformation occurs due to the Coulombically induced nucleophilicity on a specific molecular moiety in the complex. A simple strategy for inducing site-specific chemical reactivity using a LEE may find new avenues in chemical synthesis.

Hydroperoxyl radical and formic acid formation from common DNA stabilizers upon low energy electron attachment.

2-Amino-2-(hydroxymethyl)-1,3-propanediol (TRIS) and ethylenediaminetetraacetic acid (EDTA) are key components of biological buffers and are frequently used as DNA stabilizers in irradiation studies. Such surface or liquid phase studies are done with the aim to understand the fundamental mechanisms of DNA radiation damage and to improve cancer radiotherapy. When ionizing radiation is used, abundant secondary electrons are formed during the irradiation process, which are able to attach to the molecular compounds present on the surface. In the present study we experimentally investigate low energy electron attachment to TRIS and methyliminodiacetic acid (MIDA), an analogue of EDTA, supported by quantum chemical calculations. The most prominent dissociation channel for TRIS is through hydroperoxyl radical formation, whereas the dissociation of MIDA results in the formation of formic and acetic acid. These compounds are well-known to cause DNA modifications, like strand breaks. The present results indicate that buffer compounds may not have an exclusive protecting effect on DNA as suggested previously.

Resonant electron attachment to mixed hydrogen/oxygen and deuterium/oxygen clusters.

Low energy electron attachment to mixed (H2)x/(O2)y clusters and their deuterated analogs has been investigated for the first time. These experiments were carried out using liquid helium nanodroplets to form the clusters, and the effect of the added electron was then monitored via mass spectrometry. There are some important differences between electron attachment to the pure clusters and to the mixed clusters. A particularly notable feature is the formation of HO2- and H2O- ions from an electron-induced chemical reaction between the two dopants. The chemistry leading to these anions appears to be driven by electron resonances associated with H2 rather than O2. The electron resonances for H2 can lead to dissociative electron attachment (DEA), just as for the free H2 molecule. However, there is evidence that the resonance in H2 can also lead to rapid electron transfer to O2, which then induces DEA of the O2. This kind of excitation transfer has not, as far as we are aware, been reported previously.

Suppression of low-energy dissociative electron attachment in Fe(CO)5 upon clustering.

In this work, we probe anion production upon electron interaction with Fe(CO)5 clusters using two complementary cluster-beam setups. We have identified two mechanisms that lead to synthesis of complex anions with mixed Fe/CO composition. These two mechanisms are operative in distinct electron energy ranges. It is shown that the elementary decomposition mechanism that has received perhaps the most attention in recent years (i.e., dissociative electron attachment at energies close to 0 eV) becomes suppressed upon increasing aggregation of iron pentacarbonyl. We attribute this suppression to the electrostatic shielding of a long-range interaction that strongly enhances the dissociative electron attachment in isolated Fe(CO)5.

Investigation of the Radio-Frequency Discharge in a High Current Negative Hydrogen Ion Source With a Global Enhanced Vibrational Kinetic Model

A numerical investigation of the radio-frequency hydrogen discharge in the high current negative hydrogen ion source (HCNHIS) is presented using a global enhanced vibrational kinetic model (GEVKM). The HCNHIS consists of a high-pressure (2–65 torr) radio-frequency discharge chamber where the main production of high-lying vibrational states of the hydrogen molecules occurs. The hydrogen plasma flow in the discharge chamber is reduced by a series of bypass tubes and enters through a nozzle into a low-pressure (1–15 mtorr) negative hydrogen ion production chamber where H− are generated mainly by the dissociative attachment of low-energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is applied to the HCNHIS discharge and involves volume-averaged equations for 21 hydrogen species (atoms, ions, and molecules in excited states) and electrons. The GEVKM is supplemented with outlet boundary conditions for the nozzle and bypass tubes of the HCNHIS and accounts for compressibility, viscous, and rarefaction effects. GEVKM simulations of the RF discharge are performed with inlet flow rates of 5–1000 sccm and absorbed powers of 200–1000 W using the HCNHIS-2 design which is configured with an extractor grid attached to a short negative ion production region. These simulations investigate the effects of the absorbed power and the inlet flow rate on the chemical composition, electron and heavy particles temperature, wall temperature, the maximum extractable H− current in the discharge chamber, as well as optimum operational parameters of HCNHIS-2. GEVKM simulations of the HCNHIS-2 discharge are used to obtain estimates of the H− current and compared with Faraday cup measurements taken at the extraction grid.

Proton Shuttling and Reaction Paths in Dissociative Electron Attachment to o- and p-Tetrafluorohydroquinone, an Experimental and Theoretical Study.

Here we present a combined experimental and theoretical study on the fragmentation of o- and p-tetrafluorohydroquinone upon low energy electron attachment. Despite an identical ring-skeleton and identical functional groups in these constitutional isomers, they show distinctly different fragmentation patterns, a phenomenon that cannot be explained by distinct resonances or different thermochemistry. Using high-level quantum chemical calculations with the computationally affordable domain localized pair natural orbital approach, DLPNO-CCSD(T), we are able to provide a complete and accurate description of the respective reaction dynamics, revealing proton shuttling and transition states for competing channels as the explanation for the different behavior of these isomers. The results represent a "schoolbook example" of how the combination of experiment and modern high-level theory may today provide a thorough understanding of complex reaction dynamics by computationally affordable means.

Analysis of mixed nitric oxide–water clusters by complementary ionization methods

Mixed clusters of nitric oxide and water are investigated by mass spectrometry using different ionization methods, namely (1) the electron ionization at 70eV (EI); (2) the doping with sodium atoms and subsequent photoionization (NaPI); and (3) the low energy electron attachment (EA). All these methods applied to the same molecular beam generated by mixed NO–water expansion yield distinctly different mass spectra. The EI spectra are dominated by the protonated water (H2O)nH+ ions, nevertheless, they show some evidence for the mixed NO–water clusters. The NaPI spectra exhibit only (H2O)n·Na+ ions which originate from the pure water clusters (H2O)N. The abundance of the pure water clusters generated in the mixed expansions is small compared to the abundance of the mixed species. The negative EA spectra clearly demonstrate the dominance of the mixed clusters, namely (H2O)n·NO2− and (H2O)n·(NO)m−, m=2, 3. None of the above mentioned methods alone can reveal the neutral cluster composition and size, however, based on their unique combination in our experiment, we can discuss the neutral clusters and ionization processes. In addition, we have investigated the negative ion intracluster chemistry by the theoretical calculations. Energetically favorable pathways from the (NO)3− ionic core to the NO2− in the presence of water are identified, justifying the generation of the (H2O)n·NO2− series. Further series in the negative ion spectra are discussed.

Stability of the Parent Anion of the Potential Radiosensitizer 8-Bromoadenine Formed by Low-Energy (

8-Bromoadenine (8BrA) is a potential DNA radiosensitizer for cancer radiation therapy due to its efficient interaction with low-energy electrons (LEEs). LEEs are a short-living species generated during the radiation damage of DNA by high-energy radiation as it is applied in cancer radiation therapy. Electron attachment to 8BrA in the gas phase results in a stable parent anion below 3 eV electron energy in addition to fragmentation products formed by resonant exocyclic bond cleavages. Density functional theory (DFT) calculations of the 8BrA- anion reveal an exotic bond between the bromine and the C8 atom with a bond length of 2.6 Å, where the majority of the charge is located on bromine and the spin is mainly located on the C8 atom. The detailed understanding of such long-lived anionic states of nucleobase analogues supports the rational development of new therapeutic agents, in which the enhancement of dissociative electron transfer to the DNA backbone is critical to induce DNA strand breaks in cancerous tissue.

Low-Energy Electron Attachment to Serine Conformers: Shape Resonances and Dissociation Dynamics

Shape resonances of electron-molecule system formed in the low-energy electron attachment to four low-lying conformers of serine (serine 1, serine 2, serine 3, and serine 4) in gas phase are investigated using the quantum scattering method with the non-empirical model potentials in single-center expansion. In the attachment energy range of 0–10 eV, three shape resonances for serine 1, serine 2, and serine 4 and four shape resonances for serine 3 are predicted. The one-dimensional potential energy curves of the temporary negative ions of electron-serine are calculated to explore the correlations between the shape resonance and the bond cleavage. The bond-cleavage selectivity of the different resonant states for a certain conformer is demonstrated, and the recent experimental results about the dissociative electron attachment to serine are interpreted on the basis of present calculations.

Why Can Unnatural Electron Acceptors Protect Photosynthesizing Organisms but Kill the Others?

The polychlorinated compounds captafol (CPL) and 2,6-dichloroisonicotinic acid (INA) are able to protect plants acting as a fungicide or an inductor of plant resistance, respectively. At the same time, CPL and INA are dangerous for the respiratory organisms, i.e. mammalians, bacteria, and fungi. The high electron-withdrawing ability of these compounds enables them to serve as unnatural electron acceptors in the cellular ambient near to electron transport pathways located in the thylakoid membrane of chloroplasts or in the mitochondrial respiratory chain. Low-energy electron attachment to CPL and INA in vacuo leads to formation of many fragment species mainly at thermal electron energy as it is shown using dissociative electron attachment spectroscopy. On the basis of the experimental findings, assigned with the support of density functional theory calculations it is suggested that the different bioactivity of CPL and INA in respiratory and photosynthetic organisms is due to the interplay between the dissociative electron attachment process and the energies of electrons leaked from the electron transport pathways.

Dissociative electron attachment to some spinochromes: Fragment anion formation

Resonance attachment of low-energy (0–14eV) electrons to a series of natural polyphenolic naphthoquinones (so-called spinochromes) derived from sea urchins is studied by means of dissociative electron attachment (DEA) spectroscopy. The experimental findings are interpreted using quantum-chemical calculations of the empty level structure to predict the energies of negative ion resonant states as well as the thermodynamic energy thresholds for formation of fragments by DEA. A variety of fragment species is found to be formed at very low energies of incident electrons, thus modeling reductive conditions in living cells near the pathways of cellular electron transfer. In particular, the temporary molecular anions of the compounds under investigation dissociate efficiently through elimination of neutral H atoms or diatomic hydrogen molecules (in analogy with previous findings in flavonoids), this process being energetically possible owing to the presence of multiple hydroxyl groups usually associated with radical scavenging activity. A likely correlation between dehydrogenation of spinochromes stimulated by electron attachment and their well-documented antioxidant protective properties is briefly discussed.

Dissociative electron attachment to the radiosensitizing chemotherapeutic agent hydroxyurea.

Dissociative electron attachment to hydroxyurea was studied in the gas phase for electron energies ranging from zero to 9 eV in order to probe its radiosensitizing capabilities. The experiments were carried out using a hemispherical electron monochromator coupled with a quadrupole mass spectrometer. Diversified fragmentation of hydroxyurea was observed upon low energy electron attachment and here we highlight the major dissociation channels. Moreover, thermodynamic thresholds for various fragmentation reactions are reported to support the discussion of the experimental findings. The dominant dissociation channel, which was observed over a broad range of energies, is associated with formation of NCO(-), water, and the amidogen (NH2) radical. The second and third most dominant dissociation channels are associated with formation of NCNH(-) and NHCONH2 (-), respectively, which are both directly related to formation of the highly reactive hydroxyl radical. Other ions observed with significant abundance in the mass spectra were NH2 (-)/O(-), OH(-), CN(-), HNOH(-), NCONH2 (-), and ONHCONH2 (-).

The Equally Important Role of Adenine Derivatives to That of Pyrimidine Derivatives in Near-0 eV Electron-Induced DNA Lesions.

The role of adenine (A) derivatives in DNA damage is scarcely studied due to the low electron affinity of base A. Experimental studies demonstrate that low-energy electron (LEE) attachment to adenine derivatives complexed with amino acids induces barrier-free proton transfer producing the neutral N7 -hydrogenated adenine radicals rather than conventional anionic species. To explore possible DNA lesions at the A sites under physiological conditions, probable bond ruptures in two models-N7 -hydrogenated 2'-deoxyadenosine-3'-monophosphate (3'-dA(N7H)MPH) and 2'-deoxyadenosine-5'-monophosphate (5'-dA(N7H)MPH), without and with LEE attachment-are studied by DFT. In the neutral cases, DNA backbone breakage and base release resulting from C3' -O3' and N9 -C1' bond ruptures, respectively, by an intramolecular hydrogen-transfer mechanism are impossible due to the ultrahigh activation energies. On LEE attachment, the respective C3' -O3' and N9 -C1' bond ruptures in [3'-dA(N7H)MPH](-) and [5'-dA(N7H)MPH](-) anions via a pathway of intramolecular proton transfer (PT) from the C2' site of 2'-deoxyribose to the C8 atom of the base moiety become effective, and this indicates that substantial DNA backbone breaks and base release can occur at non-3'-end A sites and the 3'-end A site of a single-stranded DNA in the physiological environment, respectively. In particular, compared to the results of previous theoretical studies, not only are the electron affinities of 3'-dA(N7H)MPH and 5'-dA(N7H)MPH comparable to those of hydrogenated pyrimidine derivatives, but also the lowest energy requirements for the C3' -O3' and N9 -glycosidic bond ruptures in [3'-dA(N7H)MPH](-) and [5'-dA(N7H)MPH](-) anions, respectively, are comparable to those for the C3' -O3' and N1 -glycosidic bond cleavages in corresponding anionic hydrogenated pyrimidine derivatives. Thus, it can be concluded that the role of adenine derivatives in single-stranded DNA damage is equally important to that of pyrimidine derivatives in an irradiated cellular environment.