Low-energy Electron Attachment Research Articles

Low-energy electron attachment to chloroform (CHCl3) molecules: A joint experimental and theoretical study

Results from a joint experimental and theoretical study of electron attachment to chloroform (CHCl3) molecules in the gas phase are reported. In an electron swarm study involving a pulsed Townsend technique with equal gas and electron temperatures, accurate attachment rate coefficients were determined over the temperature range 295–373K; they show an Arrhenius-type rise with increasing temperature, corresponding to an activation energy of 0.11(1)eV. In a high resolution electron beam experiment involving two versions of the laser photoelectron attachment method, the relative cross section for Cl− formation from CHCl3 over the energy range 0.001–1.25eV at the gas temperature TG=300K was measured. It exhibits clear downward cusp structure at the threshold for excitation of one quantum of the vibrational symmetric deformation mode ν3, indicating that this mode is active in the primary attachment process. With reference to our thermal attachment rate coefficient k(T=300K)=3.9(2)×10−9cm3s−1, a new highly resolved absolute attachment cross section for TG=300K was determined. This cross section is extended to higher energies by measurements, carried out with a pulsed electron beam apparatus which also provided new data for the distinctly weaker fragment anions HCl2− and CCl2−. The resulting total absolute cross section for anion formation is used to calculate the dependence of the attachment rate coefficient k(Te;TG) on electron temperature Te over the range 50–15000K at the fixed gas temperature TG=300K. In addition, we report the dependence of the relative cross section for Cl− formation on gas temperature (TG=310–435K). For comparison with the experimental data, R-matrix calculations have been carried out for the dominant anion channel Cl−. The results recover the main experimental observations and predict the dependence of the DEA cross section on the initial vibrational level ν3 and on the vibrational temperature. Our results are compared with those of previous electron beam and electron swarm experiments.

The possibility of the decomposition of 2′‐deoxyribose moiety of thymidine induced by the low energy electron attachment

To evaluate the possibility of the decomposition of 2-deoxyribose moiety of thymidine induced by low energy electrons (LEE) attachment, the transition states and the energy barriers of the bond breaking processes of the ribose of the nucleoside have been studied theoretically by applying the density functional theory with the double zeta basis sets (DZP++). The energy barriers for the breakage of the C-C bonds (C(1')-C(2'), C(2')-C(3'), C(3')-C(4'), and C(4')-C(5')) of the ribose group of the radical anion of thymidine are found to be high (ca. 42-57 kcal/mol). The total energies of the C-C bond-broken products are significantly higher than that of the radical anion dT(*-). The decomposition of dT(*-) through the C-C bond rupture is unlikely to take place. The rupture of the C(1')-O(4') bond of dT(*-) needs an activation energy as low as 10.4 kcal/mol. However, the reversed reaction (C(1')-O(4') bond formation) needs the activation energy low as 0.3 kcal/mol. Therefore, the intermediate product LM1(C1')-(O4') is unlikely to be stable and the C(1')-O(4') bond-broken is not favored. The activation energy of the C(4')-O(4') bond rupture process amounts to 20.5 kcal/mol. The total energy of the C(4')-O(4') bond broken product is about 6.5 kcal/mol lower than that of the reactant dT(*-). The subsequent N1-glycosidic bond breaking process is found to have a very low energy barrier. Therefore, the LEE-induced base release through the C(4')-O(4') bond rupture might be a possible pathway.

Low energy electron attachment to condensed formic acid

Dissociative electron attachment to formic acid in the condensed phase is studied using improved mass spectrometric detection of the negative ion fragments. The desorbed yields are measured as a function of incident electron energy in the range between 3 to 20 eV. Unlike previous work, the formation of the dehydrogenated anion HCOO− is observed and the signal to noise ratio is much higher for all other ions detected, i.e. OH−, O− and H−. Resonant structure seen in all anion yield functions, is attributed to dissociative electron attachment (DEA), whereas above 14 eV nonresonant dipolar dissociation (DD) dominates the desorption yields.

Fragmentation of metastable SF6−∗ ions with microsecond lifetimes in competition with autodetachment

Fragmentation of metastable SF(6)(-*) ions formed in low energy electron attachment to SF(6) has been investigated. The dissociation reaction SF(6)(-*)-->SF(5) (-)+F has been observed approximately 1.5-3.4 micros and approximately 17-32 micros after electron attachment in a time-of-flight and a double focusing two sector field mass spectrometer, respectively. Metastable dissociation is observed with maximum intensity at approximately 0.3 eV between the SF(6)(-*) peak at zero and the SF(5)(-) peak at approximately 0.4 eV. The kinetic energy released in dissociation is low, with a most probable value of 18 meV. The lifetime of SF(6)(-*) decreases as the electron energy increases, but it is not possible to fit this decrease with statistical Rice-Ramsperger-Kassel/quasiequilibrium theory. Metastable dissociation of SF(6)(-*) appears to compete with autodetachment of the electron at all electron energies.

The Role of πσ* Excited States in Electron-Induced DNA Strand Break Formation: A Time-Dependent Density Functional Theory Study

Mechanisms of low-energy electron attachment to the DNA and subsequent single-strand break (SSB) formation are of crucial importance to understand the DNA damage. In the present study, we used time-dependent density functional theory (TD-BH&HLYP/6-31G*) to explore the mechanism of SSB, in detail, by calculating the lowest excited states of 5‘-dTMPH radical anion considered as a model of DNA. The C5‘−O5‘ bond dissociation of 5‘-dTMPH radical anion was considered as the reaction coordinate along the potential energy surface of 5‘-dTMPH. The dissociative σ* state, mainly localized on the phosphate of the 5‘-dTMPH radical anion, is considered responsible for the SSB formation in DNA. Our work predicts that in this DNA model system there are available π* and σ* excited states, near 2 eV, which can couple vibronically and lead to a DNA single strand break via the dissociative σ* surface.

Excision of CN− and OCN− from acetamide and some amide derivatives triggered by low energy electrons

Low energy electron attachment to acetamide and some of its derivatives shows unique features in that the unimolecular reactions of the transient anions are remarkably complex, involving multiple bond cleavages and the formation of new molecules. Each of the three compounds acetamide (CH(3)C(O)NH(2)), glycolamide (CH(2)OHC(O)NH(2)) and cyanoacetamide (CH(2)CNC(O)NH(2)) shows a pronounced resonance located near 2 eV and decomposing into CN(-) along a concerted reaction forming a neutral H(2)O molecule and the corresponding radical (methyl and methoxy). From glycolamide an additional reaction pathway resulting in the loss of water is operative, in this case generating two fragments and observable via the complementary anion (M-H(2)O)(-). The pseudohalogen OCN(-) is formed at comparatively lower intensity having a specific energy profile for each of the target molecules. In dibromocyanoacetamide (CBr(2)CNC(O)NH(2)) the situation changes completely as now comparatively intense CN(-) and OCN(-) signals appear already near zero eV. Electronic structure calculations predict that in dibromocyanoacetamide the extra electron resides in a molecular orbital (MO) which is strongly localized at the Br sites. For the other compounds, the relevant MOs are appreciably delocalized showing pi*(C=O) character.

Low-energy electron attachment to SF6. I. Kinetic modeling of nondissociative attachment

Low-energy electron-molecule collisions are analyzed by kinetic modeling within the framework of statistical unimolecular rate theory. Nondissociative electron attachment to SF(6) is used to illustrate the approach. An internally consistent representation is provided for attachment cross sections and rate coefficients in relation to detachment lifetimes, and both thermal and specific rate coefficients for detachment. By inspecting experimental data, the contributions of intramolecular vibrational redistribution and vibrationally inelastic collisions can be characterized quantitatively. This allows for a prediction of attachment rate coefficients as a function of electron and gas temperature as well as gas pressure over wide ranges of conditions. The importance of carefully controlling all experimental parameters, including the carrier gas pressure, is illustrated. The kinetic modeling in Part II of this series is extended to dissociative electron attachment to SF(6).

Low-energy electron attachment to SF6. II. Temperature and pressure dependences of dissociative attachment

Low-energy electron-molecule collisions, leading to dissociative attachment through metastable anionic states, are kinetically modeled within the framework of statistical unimolecular rate theory. The reaction e(-)+SF(6)-->SF(5)(-)+F is used as an illustrative example. The modeling is applied to new measurements of branching fractions for SF(5)(-) formation in the bath gas He between 360 and 670 K at 1 and 2 Torr, and between 490 and 620 K over the range of 0.3-9 Torr. The analysis of the data follows the previous kinetic modeling of the nondissociative electron attachment, e(-)+SF(6)-->SF(6)(-), from Part I of this series. Experimental results from the present work and the literature on branching fractions and total cross sections for anion formation as functions of electron energies, bath gas temperatures and pressures, as well as observation times are analyzed. The assumption of a participation of the electronic ground state of SF(6)(-) alone suffices to model the available experimental data. A value of the dissociation energy of SF(6)(-) into SF(5)(-)+F of E(0,dis)=1.61(+/-0.05) eV is determined, which may be compared to the electron affinity of SF(6), EA=1.20(+/-0.05) eV, such as derived in Part III of this series.

Low-energy electron attachment to SF6. III. From thermal detachment to the electron affinity of SF6

The thermal attachment of electrons to SF(6) is measured in a flowing-afterglow Langmuir-probe apparatus monitoring electron concentrations versus axial position in the flow tube. Temperatures between 300 and 670 K and pressures of the bath gas He in the range of 0.3-9 Torr are employed. Monitoring the concentrations of SF(6)(-) and SF(5)(-), the latter of which does not detach electrons under the applied conditions, an onset of thermal detachment and dissociation of SF(6) at temperatures above about 530 K is observed. Analysis of the mechanism allows one to deduce thermal detachment rate coefficients. Thermal dissociation rate coefficients for the reaction SF(6)(-)-->SF(5)(-)+F can only be estimated by unimolecular rate theory based on the results from Part I and II of this series. Under the applied conditions they are found to be smaller than detachment rate coefficients. Combining thermal attachment and detachment rates in a third-law analysis, employing calculated vibrational frequencies of SF(6) and SF(6)(-), leads to the electron affinity (EA) of SF(6)(-). The new value of EA=1.20(+/-0.05) eV is significantly higher than previous recommendations which were based on less direct methods.

Ion beam cancer therapy: Fundamental aspects of the problem

We discuss different physical processes which may be responsible for biological damage induced by an ion beam propagating through living tissue. The creation of free radicals and an electron plasma, dissociative attachment of low-energy electrons, and local heating mechanisms are considered. Numerical estimates are performed for the case of carbon ion beams. We conclude that all three mechanisms are capable of inflicting single and double strand breaks in DNA molecules and thus can be responsible for the observed enhanced biological effectiveness of ion beams in cancer therapy.

Mechanism for Electron-Induced SF5CF3 Formation in Condensed Molecular Films

A potent greenhouse gas, trifluoromethyl sulfur pentafluoride (SF5CF3), has been discovered in the Earth's upper atmosphere. There are no known natural sources of SF5CF3, and it has been reported that high-energy electron irradiation (5keV) of condensed mixtures of SF6 and CF4 forms SF5CF3. This paper focuses on a search for the electron-induced formation of SF5CF3 at low electron energies and for the mechanism of formation of SF5CF3. Temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD) are used to search for products formed under electron irradiation of ultrathin condensed films (∼3 ML) of SF6 and CF4. The major evidence for the formation of SF5CF3 via irradiation is the observation in ESD of ionic fragments containing S−C bonds, which are also seen in the mass spectra of SF5CF3. The maximum yield of SF5CF3 products in the condensed film is induced by ∼8 eV electron irradiation; the mechanism is associated with low-energy dissociative electron attachment (DEA) of SF6 and CF4 f...

Dissociative electron attachment to unstable molecules and slow fragmentation of metastable molecular anions

Recent experimental investigations of dissociative electron attachment to unstable molecules and free radicals are reviewed along with observations of metastable negatively charged ions. Measurements have been made with a time-of-flight mass spectrometer in Belfast and also, for metastable ions, with a double focussing twin field mass spectrometer in Innsbruck. Electron attachment to unstable CS, for example, was found to be similar to electron attachment to the valence isoelectronic CO molecule with observation of S- and C- ions just above the thermodynamic threshold for S- + C (3P) at 5.43 eV, C-+ S at 6.40 eV and S-+ C(1D) at 6.70 eV with peak cross sections of ~ 0.025 Å2, 0.002 Å2 and 0.003 Å2 respectively. Slow fragmentation of metastable SF-*6 formed in low energy electron attachment to SF6 has been observed on microsecond timescales in competition with autodetachment; processes SF-*6 → SF-5 + F and SF-*6 → SF6 + e- respectively. Fragmentation of metastable anions of benzene derivatives, such as 2,4-dinitro-toluene [CH3.C6H3(NO2)2], has also been observed on microsecond timescales.

Evolution of Negative Ion Resonances at Surfaces: Effect of Environment and Orientation

The influence of the environment and orientation on the evolution of negative ion resonances at surfaces following low energy (0-15 eV) electron attachment to adsorbed and condensed molecules is studied by means of electron stimulated desorption (ESD) of negative ions. In the low energy regime ESD of anions usually proceeds via dissociative electron attachment (DEA) to an individual molecule at or near the surface. Three prototypical cases are discussed in the light of the corresponding gas phase process. (i) ESD of F− from a multilayer film of NF3 shows desorption features which are considerably narrower compared to the corresponding gas phase DEA analogues. This is a mirror of the particular dynamics of the unimolecular reaction in relation to the energy constraints operative in ion desorption. In addition to the strong F− desorption it is demonstrated by applying surface vibrational spectroscopy that (resonant) dissociative electron attachment at low energies (0–5 eV) is responsible for chemical changes within the NF3 film, viz. formation of NF2 radicals and N2F4 molecules. (ii) In the ESD of Cl− from submonolayer Cl2 adsorbed on rare gas surfaces a strong dependence of both the signal intensity and shape of the desorption resonance on the angle of incidence of the electron beam with respect to the surface normal is observed. This behavior points towards a preferential orientation of the molecules perpendicular to the surface. (iii) The absolute DEA cross-section can appreciably be enhanced by the medium due to an increase of the autodetachment lifetime of the negative ion resonance. This is demonstrated in ClCN for an electronically excited resonance.

Influence of Functional Groups on the Site‐Selective Dissociation of Adenine upon Low‐Energy Electron Attachment

Influence of Functional Groups on the Site‐Selective Dissociation of Adenine upon Low‐Energy Electron Attachment

Decomposition of propionyl chloride triggered by slow electrons

Low energy electron attachment to propionyl chloride is studied by means of a crossed beam experiment. Two distinct resonances are observed located near 0 and 0.7 eV. The first resonance is attributed to a temporary occupation of the lowest unoccupied molecular orbital (LUMO) with predominant π C O ∗ character. It mainly decays into Cl −, but also the fragments (M–CH 2) −, COCl − and (M–Cl) − are observed. In addition the non-decomposed parent molecular anion M − is observed. The second resonance exclusively leads to Cl − formation. It is characterized as temporary occupation of a repulsive σ C – Cl ∗ MO.

Low-Energy Electron Attachment to 5‘-Thymidine Monophosphate: Modeling Single Strand Breaks Through Dissociative Electron Attachment

Mechanisms of low-energy electron (LEE) attachment and subsequent single-strand break (SSB) formation are investigated by density functional theory treatment of a simple model for DNA, i.e., the nucleotide, 5'-thymidine monophosphate (5'-dTMPH). In the present study, the C5'-O5' bond dissociation due to LEE attachment has been followed along the adiabatic as well as on the vertical (electron attached to the optimized geometry of the neutral molecule) anionic surfaces using B3LYP functional and 6-31G* and 6-31++G** basis sets. Surprisingly, it is found that the PES of C5'-O5' bond dissociation in the anion radicals have approximately the same barrier for both adiabatic and vertical pathways. These results provide support for the hypothesis that transiently bound electrons (shape resonances) to the virtual molecular orbitals of the neutral molecule likely play a key role in the cleavage of the sugar-phosphate C5'-O5' bond in DNA resulting in the direct formation of single strand breaks without significant molecular relaxation. To take into account the solvation effects, we considered the neutral and anion radical of 5'-dTMP surrounded by 5 or 11 water molecules with Na+ as a counterion. These structures were optimized using the B3LYP/6-31G** level of theory. We find the barrier height for adiabatic C5'-O5' bond dissociation of 5'-dTMP anion radical in aqueous environment is so substantially higher than in the gas phase that the adiabatic route will not contribute to DNA strand cleavage in aqueous systems. This result is in agreement with experiment.

Low-energy electron interaction with nitrobenzene: C 6H 5NO 2

Low-energy electron attachment to C 6H 5NO 2 (nitrobenzene) in the gas phase is reported in the electron energy range from about 0 up to 10 eV with an energy resolution of 120 meV. Dissociative and nondissociative electron attachment to nitrobenzene were observed. From the numerous ions observed, the two most abundant were NO 2 − and C 6H 5NO 2 −. Based on comparison of the abundance of studied ions with Cl − in the dissociative electron attachment to CCl 4 at 0.8 eV, estimates of cross sections for the all observed ions were obtained for the first time (e.g. σ(NO 2 −)=4.6×10 −20 m 2 and σ(C 6H 5NO 2 −)=3.8×10 −21 m 2).

Energy selective excision of CN−following electron attachment to hexafluoroacetone azine ((CF3)2CN–NC(CF3)2)

Low energy electron attachment (DEA) to hexafluoroacetone azine (HFAA) leads to a remarkable energy selective excision of CN(-) within a pronounced resonance located at 1.35 eV. The underlying dissociative electron attachment (DEA) reaction involves multiple bond cleavages and rearrangement within the neutral products. A series of further fragment ions (F(-), CF(3)(-), (CF(3))(2)C(-) and (CF(3))(2)CN(-)) are observed from resonant features above 2 eV and only (CF(3))(2)CN(-) is additionally formed within a narrow resonance below 1 eV. In contrast to CN(-) all the remaining fragment ions can be formed by simple bond cleavages with (CF(3))(2)CN(-) being the result of a symmetric decomposition of the target molecule by cleavage of the (N-N) bond with the excess charge localised on either of the identical fragments. Our ab initio calculations predict an adiabatic electron affinity of HFAA close to 2 eV with the geometry of the relaxed anion considerably distorted with respect to that of the neutral molecule.

Selective Excision of C5 from D‐Ribose in the Gas Phase by Low‐Energy Electrons (0–1 eV): Implications for the Mechanism of DNA Damage

Attachment of low-energy electrons to D-ribose triggers a series of complex decomposition reactions associated with the loss of neutral water molecules as well as the excision of C-containing units leading to the degradation of the cyclic structure of the sugar. This excision of C-containing neutral fragments involves C5 exclusively. The sugar unit is thought to play a key role in the mechanism of DNA damage by low-energy electrons.

Importance of Nonresonant Scattering in Low-Energy Dissociative Electron Attachment to Molecular Hydrogen

A central premise of nearly all theories of dissociative electron attachment is that this process is resonance driven. Neglect of nonresonant scattering, although appropriate for electron-molecule systems with narrow (long-lived) resonances, is problematic for the e-H2 system, which has one of the broadest known resonances. Using the nonadiabatic phase-matrix method we have found that at energies from threshold to 6 eV contributions from nonresonant scattering to cross sections to dissociative attachment to in its ground vibrational and electronic state exceed 60%. Comparison of theoretical and experimental cross sections argue strongly for further efforts to resolve the considerable remaining discrepancies over this most elementary rearrangement process.