Electron Attachment Cross Sections Research Articles

Total dissociative electron attachment cross sections of selected amino acids

Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binary-encounter-Bethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the -COOH pi* orbital. The authors discuss evidence that direct attachment to the lowest sigma* orbital may instead be responsible. A close correlation between the energies of the core-excited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H-.

Interpreting electron transmission spectroscopy and negative ion mass spectrometry data using a spherical potential well model

Experimental data obtained using electron transmission spectroscopy and negative ion mass spectrometry based on resonance electron capture are interpreted within the framework of a spherical potential well model in application to a series of chloro-and bromoalkane molecules. Allowance for the scattering of a single partial p-wave of the incoming electron makes possible (i) reproduction of the ratio of a resonance peak width to the electron energy observed in the electron transmission spectra and (ii) establishment of a relation between the total cross section of electron scattering on a molecule and the dissociative electron attachment cross section. The proposed model offers a radical simplification of the approach developed previously based on the Fashbach-Fano resonance theory.

Measurements of partial cross sections and photoelectron angular distributions for the photodetachment ofFe−andCu−at visible photon wavelengths

Photodetachment cross sections and the angular distributions of photoelectrons produced by the single-photon detachment of the transition metal negative ions ${\mathrm{Fe}}^{\ensuremath{-}}$ and ${\mathrm{Cu}}^{\ensuremath{-}}$ have been measured at four discrete photon wavelengths ranging from 457.9 to $647.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $(2.71--1.92\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ using a crossed-beams laser photodetachment electron spectrometry (LPES) apparatus. Photodetachment cross sections were determined by comparing the photoelectron yields from the photodetachment of ${\mathrm{Fe}}^{\ensuremath{-}}$ to those of ${\mathrm{Cu}}^{\ensuremath{-}}$ and ${\mathrm{C}}^{\ensuremath{-}}$, which have known absolute photodetachment cross sections. Using the measured photodetachment cross sections, radiative electron attachment cross sections were calculated using the principle of detailed balance. Angular distributions were determined by measurements of laboratory frame, angle-, and energy-resolved photoelectrons as a function of the angle between the linear laser polarization vector and the momentum vector of the collected photoelectrons. Values of the asymmetry parameter have been determined by nonlinear least-squares fits to these angular distributions. The measured asymmetry parameters are compared to predictions of photodetachment models including Cooper and Zare's dipole approximation theory [J. Cooper and R. N. Zare, J. Chem. Phys. 48, 942 (1968)], and the angular momentum transfer theory developed by Fano and Dill [Phys. Rev. A 6, 185 (1972)].

Electron photoemission from charged films: Absolute cross section for trapping –5eV electrons in condensed CO2

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.

Total dissociative electron attachment cross sections for molecular constituents of DNA

Total cross sections for the dissociative electron attachment process are presented for the DNA bases thymine, cytosine, and adenine and for three compounds used as surrogates for the ribose and phosphate groups, tetrahydrofuran, 3-hydroxytetrahydrofuran, and trimethylphosphate, respectively. Cross section magnitudes are obtained by observation of positive ion production and normalization to ionization cross sections calculated elsewhere using the binary-encounter-Bethe method. The average cross section of the three bases is 3-10 times smaller than the effective cross section per nucleotide reported for single strand breaks in surface-bound supercoiled DNA. Consequently, damage to the bases alone does not appear to account for the major portion of the strand breaks. The presence of an OH group on the ribose surrogate considerably enhances its cross section. Model compounds in which protonation or OH groups are used to terminate bonds may therefore display larger cross sections than in DNA itself.

Vibrational Feshbach resonances in uracil and thymine

Sharp peaks in the dissociative electron attachment (DEA) cross sections of uracil and thymine at energies below 3 eV are assigned to vibrational Feshbach resonances (VFRs) arising from coupling between the dipole bound state and the temporary anion state associated with occupation of the lowest sigma* orbital. Three distinct vibrational modes are identified, and their presence as VFRs is consistent with the amplitudes and bonding characteristics of the sigma* orbital wave function. A deconvolution method is also employed to yield higher effective energy resolution in the DEA spectra. The site dependence of DEA cross sections is evaluated using methyl substituted uracil and thymine to block H atom loss selectively. Implications for the broader issue of DNA damage are briefly discussed.

Electron Capture Detector Response and Dissociative Electron Attachment Cross Sections in Chloroalkanes and Chloroalkenes

Electron capture detectors (ECDs) are widely used in gas chromatography to detect electronegative compounds. In this work, we examine the connections between the ECD response and the cross sections for dissociative electron attachment (DEA) determined from low energy electron beam studies in the chloroalkane family, stressing in particular the role of temporary anion state energies. We show that attachment rate coefficients computed from these cross sections are well correlated with ECD response, and that the latter decreases exponentially with increasing energies of the lowest anion states. ECD measurements are also carried out in monochloroalkanes substituted with unsaturated ethenyl and phenyl moieties, and the response is shown to depend strongly on the mixing between the unsaturated pi* and the C-Cl sigma* temporary anions as exhibited by the vertical attachment energies (VAEs) of these states. The results show good correlations between the chloroalkene and phenyl chloride ECD responses and the VAEs for the mixed states.

Structures in dissociative electron attachment cross-sections in thymine, uracil and halouracils

The low energy parts (0–3 eV) of the dissociative electron attachment (DEA) cross-sections in uracil, thymine, and the halouracils 5-BrU, 5-ClU, 6-ClU, 5-FU, have been revisited with an improved energy resolution, focussing onto the puzzling structures observed on most cross-sections of the various anions fragments versus electron energy. The elastic electron scattering cross-sections near zero energy have also been recorded. In most cases they present a few features (cusps) related to the low energy peaks found in DEA cross-sections. Substantial differences are found in the detailed spectra of the fragments produced in 5-ClU and 6-ClU. Several interpretations already proposed to explain the previous observations, in terms of different thresholds and/or contributions of vibrational Feshbach resonances, and their limits, are discussed in the light of our new results. We expect that our more detailed results will stimulate theoretical work for a better understanding of the features observed.

Isotope effects in vibrational excitation and dissociative electron attachment of DCl and DBr

The results of calculations of vibrational excitation and dissociative electron attachment cross-sections of DCl and DBr are reported. The calculations are based on the nonlocal resonance model for electron-HCl/HBr scattering. The cross-sections for many initial rovibrational target states were calculated both for the hydrogenated and the deuterated compounds. The calculations reveal an unexpected result: the vibrational excitation cross-section of the deuterated molecule may in some cases be (significantly) larger than that of the hydrogenated compound. This effect is observed when the target molecule is initially excited to a vibrational state the energy of which is close to the threshold of dissociative attachment. Rotational excitation of the target molecule plays a similar role. Isotope effects in dissociative electron attachment are also discussed.

Empty Level Structure and Dissociative Electron Attachment Cross Section in (Bromoalkyl)benzenes

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for the series of (bromoalkyl)benzenes C6H5(CH2)nBr (n = 0-3), where the bromine atom is directly bonded to a benzene ring or separated from it by 1-3 CH2 groups, and the dihalo derivative 1-Br-4-Cl-benzene. The relative DEA cross sections (essentially due to the Br- fragment) are reported, and the absolute cross sections are also evaluated. HF/6-31G and B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral state molecules. The pi* VOEs, scaled with empirical equations, satisfactorily reproduce the corresponding experimental vertical electron attachment energies (VAEs). According to the calculated localization properties, the LUMO (as well as the singly occupied MO of the lowest lying anion state) of C6H5(CH2)3Br is largely localized on both the benzene ring and the C-Br bond, despite only a small pi*/sigma*C-Br interaction and in contrast to the chlorine analogue where the LUMO is predicted to possess essentially ring pi character. This would imply a less important role of intramolecular electron transfer in the bromo derivative for production of the halogen negative fragment through dissociation of the first resonant state. The VAEs calculated as the anion/neutral energy difference with the 6-31+G* basis set which includes diffuse functions are relatively close to the experimental values but do not parallel their sequence. In addition the SOMO of some compounds is not described as a valence MO with large pi* character but as a diffuse sigma* MO.

Electron Attachment Step in Electron Capture Dissociation (ECD) and Electron Transfer Dissociation (ETD)

We have made use of classical dynamics trajectory simultions and ab initio electronic structure calculations to estimate the cross sections with which electrons are attached (in electron capture dissociation (ECD)) or transferred (in electron transfer dissociation (ETD)) to a model system that contained both an S-S bond that is cleaved and a -NH(3)(+) positively charged site. We used a Landau-Zener-Stueckelberg curve-crossing approximation to estimate the ETD rates for electron transfer from a CH(3)(-) anion to the -NH(3)(+) Rydberg orbital or the S-S sigma* orbital. We draw conclusions about ECD from our ETD results and from known experimental electron-attachment cross sections for cations and sigma-bonds. We predict the cross section for ETD at the positive site of our model compound to be an order of magnitude larger than that for transfer to the Coulomb-stabilized S-S bond site. We also predict that, in ECD, the cross section for electron capture at the positive site will be up to 3 orders of magnitude larger than that for capture at the S-S bond site. These results seem to suggest that attachment to such positive sites should dominate in producing S-S bond cleavage in our compound. However, we also note that cleavage induced by capture at the positive site will be diminished by an amount that is related to the distance from the positive site to the S-S bond. This dimunition can render cleavage through Coulomb-assisted S-S sigma* attachment competitive for our model compound. Implications for ECD and ETD of peptides and proteins in which SS or N-C(alpha) bonds are cleaved are also discussed, and we explain that such events are most likely susceptible to Coulomb-assisted attachment, because the S-S sigma* and C=O pi* orbitals are the lowest-lying antibonding orbitals in most peptides and proteins.

Nonlocal model of dissociative electron attachment and vibrational excitation of NO

We present the results of a study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy $(0--2\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ region where the cross sections are dominated by resonance contributions. The $^{3}\ensuremath{\Sigma}^{\ensuremath{-}}$, $^{1}\ensuremath{\Delta}$, and $^{1}\ensuremath{\Sigma}^{+}$ $\mathrm{N}{\mathrm{O}}^{\ensuremath{-}}$ resonance lifetimes are taken from our earlier study [Phys. Rev. A 69, 062711 (2004)], but the resonance energies used here are obtained from different configuration-interaction studies. Here we employ a more elaborate nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model we employed in our earlier study, and gives cross sections in better agreement with the most recent experiments. We also present cross sections for dissociative electron attachment to NO leading to ground-state products, ${\mathrm{O}}^{\ensuremath{-}}(^{2}\mathrm{P})+\mathrm{N}(^{4}\mathrm{S})$. The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small $(\ensuremath{\sim}{10}^{\ensuremath{-}20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2})$, the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to $\ensuremath{\nu}=10$ and above.

Electron and positron scattering from perfluorocyclobutane(c−C4F8)molecules

Total cross sections (TCSs) are experimentally investigated for 0.8--600-eV electron and 0.7--600-eV positron scattering from $c\text{\ensuremath{-}}{\mathrm{C}}_{4}{\mathrm{F}}_{8}$ molecules using a linear transmission time-of-flight method, and a comparative study of the results is carried out in this paper. Electron-scattering differential cross sections (DCS) measurements carried out for energies 1.5--100 eV are used for a combined TCS and DCS discussion. These DCS results help to offer a better understanding of some of the structures observed in the TCSs. Fingerprints of a low-energy resonance, consistent with large electron attachment cross sections near 0 eV, have been observed below 2 eV where TCSs show a continually rising trend. Another resonance peak at $\ensuremath{\sim}8.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, attributed to dissociative electron attachment with the production of fragmented ions, and a broader one at 16--40 eV, have also been observed. Except for the pronounced shoulder at $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, there is good qualitative agreement between the TCS result and the integrated cross-section results. Although electron TCSs are found to be generally larger than positron TCSs in the 3--120-eV ranges, these two TCSs, however, show a tendency towards merging above 120 eV.

Total cross sections for dissociative electron attachment to ozone caused by the 2A2 Feshbach resonance

Total resonant cross sections for electron collision with ozone are presented. The results are based on ab initio calculations for the X1A1 target ground state and the 2A2 negative-ion state potential energy surfaces close to the Franck–Condon region. The fixed-nuclei resonance width as a function of collision energy and internuclear distances has been obtained from R-matrix calculations. The nuclear motion has been taken into account employing the Franck–Condon reflection principle. In order to determine the information which is significant for the parametrization of resonant scattering cross sections, the calculations have been carried out at different levels of accuracy. The lifetime of the anionic state with respect to electron detachment was found to be long compared with the fragmentation process. Therefore, the fragmentation probability can be assumed to be equal to 1 and the total cross sections for the formation of the ion can be compared with the experimental cross sections for dissociative electron attachment (DEA). The results confirm the assumption that the prominent structure in the DEA cross sections at 1.3 eV is caused by a metastable 2A2 state of O−3.

Inelastic electron interaction (attachment/ionization) with deoxyribose.

We have investigated experimentally the formation of anions and cations of deoxyribose sugar (C(5)H(10)O(4)) via inelastic electron interaction (attachment/ionization) using a monochromatic electron beam in combination with a quadrupole mass spectrometer. The ion yields were measured as a function of the incident electron energy between about 0 and 20 eV. As in the case of other biomolecules (nucleobases and amino acids), low energy electron attachment leads to destruction of the molecule via dissociative electron attachment reactions. In contrast to the previously investigated biomolecules dehydrogenation is not the predominant reaction channel for deoxyribose; the anion with the highest dissociative electron attachment (DEA) cross section of deoxyribose is formed by the release of neutral particles equal to two water molecules. Moreover, several of the DEA reactions proceed already with "zero energy" incident electrons. In addition, the fragmentation pattern of positively charged ions of deoxyribose also indicates strong decomposition of the molecule by incident electrons. For sugar the relative amount of fragment ions compared to that of the parent cation is about an order of magnitude larger than in the case of nucleobases. We determined an ionization energy value for C(5)H(10)O(4) (+) of 10.51+/-0.11 eV, which is in good agreement with ab initio calculations. For the fragment ion C(5)H(6)O(2) (+) we obtained a threshold energy lower than the ionization energy of the parent molecular ion. All of these results have important bearing for the question of what happens in exposure of living tissue to ionizing radiation. Energy deposition into irradiated cells produces electrons as the dominant secondary species. At an early time after irradiation these electrons exist as ballistic electrons with an initial energy distribution up to several tens of electron volts. It is just this energy regime for which we find in the present study rather characteristic differences in the outcome of electron interaction with the deoxyribose molecule compared to other nucleobases (studied earlier). Therefore, damage induced by these electrons to the DNA or RNA strands may start preferentially at the ribose backbone. In turn, damaged deoxyribose is known as a key intermediate in producing strand breaks, which are the most severe form of lesion in radiation damage to DNA and lead subsequently to cell death.

Empty Level Structure and Dissociative Electron Attachment Cross Sections in Saturated and Unsaturated Bromohydrocarbons

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for nine (normal, secondary, and tertiary) bromoalkanes and ten bromoalkenes where the bromine atom is directly bonded to an ethene carbon atom or separated from the double bond by 1−4 CH2 groups. The relative DEA cross sections (essentially due to the Br- fragment) are reported and compared with those of the corresponding chlorides. B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral states of the bromohydrocarbons. The calculated VOEs satisfactorily reproduce the trends of the vertical electron attachment energies (VAEs) measured in the ET spectra. Electron attachment to the σ*C-Br MO of the saturated bromides occurs at about 1.2 eV, the energy of the resonance being slightly stabilized with increasing branching. The corresponding peak in the DEA cross section occurs at about 0.6 eV in the normal bromoalkanes and 0.9 eV in the secondary and tertiary bromoalkanes. In vinyl bromide, the lowest resonance is associated with the ethene π* LUMO, whereas in the CH2CH(CH2)nBr alkenes with n > 2, the LUMO is the σ*C-Br MO. Consistently, in the latter compounds the energy at the peak of the DEA cross section and the magnitude of the latter are comparable to those of the normal bromoalkanes.

Dissociative-electron-attachment cross sections: A comparative study ofNO2andO3

Absolute cross sections for dissociative electron attachment (DEA) to NO 2 have been measured and compared with that of O 3 . A striking similarity is observed in the dissociation channels and the relative cross sections between the two molecules. This similarity is interpreted in terms of a common mechanism for electron capture to the two molecules and is exploited to characterize the nature of the NO 2 resonances in analogy with O 3 , using specific quantum chemical calculations as well as experimental data. This experimental observation and its interpretation seeks to direct attention to a previously uncommented on aspect of the DEA phenomenon in relation to molecules that are isoelectronically unrelated. The observation also tends to highlight the dominance of electronic polarizability in the electron-molecule scattering potential leading to capture.

Dissociative electron attachment to gas-phase glycine.

By using a high-resolution electron energy monochromator low-energy electron attachment to gas-phase glycine (H2NCH2COOH, or G) has been studied by means of mass spectrometric detection of the product anions. In the same way as for several other biologically relevant molecules no stable parent anion was formed by free electron attachment. The largest dissociative electron attachment (DEA) cross-section, approximately 5x10(-20) m2, was observed for (G-H)-+H at an electron energy of 1.25 eV. Glycine and formic acid (HCOOH) have several common features, because a precursor ion can be characterized by electron attachment to the unoccupied pi* orbital of the -COOH group. At higher incident electron energies several smaller fragment anions are formed. Except for H-, which could not be observed in this study, there was good agreement with an earlier investigation by Gohlke et al.

Low energy electron attachment to CH3CN

Low energy electron attachment cross sections for acetonitrile (CH3CN) are reported in the energy range from about 0 up to 10 eV determined with an energy resolution of 140 meV. Electron attachment is shown to be a purely dissociative process with the production of the five anionic fragments: CH2CN−, CHCN−, CCN−, CN− and CH3− observed in two energy regions, the first between 1 and 4 eV, the second in excess of 6 eV. Quantum chemical and trajectory calculations have been carried out to complement the experimental results.

Structures in elastic, vibrational, and dissociative electron attachment cross sections inN2Onear threshold

Absolute differential cross sections were measured at 135° forthe elastic and the vibrationally inelastic electron scattering from threshold to 12eV, with emphasis on the threshold region. In addition, relative dissociativeelectron attachment spectra were measured from 0.1 to 3.5 eV. Structures ofvibrational origin were observed at energies below 1 eV, well below the 2Π shaperesonance, in the cross sections for the excitation of vibrational overtones and for dissociativeattachment. They are generally narrower and deeper than similar structures in CO2.The structures are absent when the N≡Nstretch is co-excited. The structures are interpreted in terms of vibrationalFeshbach resonances supported by a state of the anion where an electron istemporarily loosely bound in a spatially diffuse cloud by a combination of dipolarand polarization forces around a molecule with an excited bending and/or N–Ostretch vibration.