Electron Attachment Processes Research Articles

Electron Attachment: A New Approach to ${\rm H} _{2}$ Fluxless Solder Reflow for Wafer Bumping

Wafer bumping, via fluxless solder reflow, can be performed using electron attachment (EA) with nonflammable mixtures of hydrogen (= 4 vol%) in nitrogen. The EA process facilitates dissociation of molecular hydrogen at ambient pressure and a temperature significantly lower than that of thermal dissociation. Our studies suggest that the EA process promotes the formation of atomic hydrogen anions, which reduce solder oxides and facilitate fluxless solder reflows at temperatures close to the solder melting point.

Collective mode properties in a paired fullerene-ion plasma

A pair-ion plasma without electrons consisting of C60+ and C60- is generated through the processes of electron-impact ionization, electron attachment, and magnetic filtering. Properties of electrostatic modes propagating along magnetic-field lines are experimentally investigated by externally exciting them with two types of electrodes. It is found that four kinds of wave modes exist and a frequency spectrum of phase lag between the density fluctuations of C60+ and C60- is unique in comparison with ordinary electron-ion plasmas. One of the modes is an ion acoustic wave which is divided into two branches at around the ion cyclotron frequency in the presence of a backwardlike mode joining them. The phase lag of the ion acoustic wave strongly depends on the frequency, while those for the other ion plasma and intermediate-frequency waves are constant at pi independent of the frequency.

What can we learn from two‐center three‐electron bonding with the topological analysis of ELF?

AbstractIn this paper, a review is presented of the abundant literature on the two‐center three‐electron (2c‐3e) bonding, which plays a crucial role in electron transfer and radical chemistry. Important questions regarding this peculiar type of interaction are (1) Is a three‐electron bonded complex a molecule or an assembly of molecules?, (2) Since an unpaired electron is involved, where is the spin density located?, and (3) Is there a descriptor of electron fluctuation, which is a central phenomenon in this type of bonding? We demonstrate that the topological analysis of the electron localization function (ELF), which provides a convenient mathematical framework to study chemical bond in molecules and solids, is able to answer these questions. First, examples of potentially 3e‐bonded complexes proposed by Pauling are reinvestigated. Second, the electron attachment process on molecules of the HnXYHmtype with X,Y = Cl, S, P, Si andn, m= 0–2 is considered. Finally, the ELF‐based topology of some prototypical radicals containing the SO bond is examined. From these studies, several topological signatures of the 3e bonding have been elaborated. No disynaptic basin, well known as the topological signature of a covalent shared‐electron pair interaction, is found between two 3e‐bonded atoms. Therefore, to distinguish 3e bonds from ionic or hydrogen bonds, a core valence bifurcation (CVB) index has been introduced, similar to the one previously defined by Fuster and Silvi (Theor Chem Acc 2000,104,13) to differentiate weak and medium H‐bonds. Moreover, in 3e‐bonded systems, the spin density is mainly localized within the lone‐pair basins of the heavy atoms. To quantify the electron fluctuation, a topological delocalization index has been defined. An unambiguous characterization of the 3e bonding is thus obtained, which is in agreement with other theoretical approaches, such as the valence bond method. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:135–160, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20325

Errors in the application of the JWKB method to calculating the survival factor in dissociative electron attachment using the local complex potential

The accuracy of the JWKB method for determining the survival factor defined for dissociative electron attachment (DEA) processes is examined for a range of electronic resonance lifetimes within the local complex potential approximation. The author concludes that the accuracy is inadequate for molecules with properties commonly found for shape resonance induced DEA. More accurate methods using the uniform Airy function approximation give much better results, but the direct numerical integration of Schrodinger's equation appears simpler still.

Low energy electron stimulated desorption of ions from whole human blood

We present the results of experiments on positive and negative ion desorption, from whole dried samples of human blood and red blood cell (RBC), induced by the impact of electrons with energies below 70 eV. Such bombardments induce a rich fragmentation pattern of cations in comparison to that for negative ions. The threshold for desorption of cations is observed around 20 eV, whereas anions can be formed with electron energy as low as 2 eV. The electron energy dependence of ion yields for all detected anions (H −, CH 3 −/NH −, O −/NH 2 − and OH −) exhibits resonant structures attributed to the dissociative electron attachment process. The similarity between the line shapes of the ion yield functions of O − desorption from the blood sample and a pure oxygen molecular film suggests that this anion originates from the (Fe–O 2) unit in hemoglobin. Additionally, from the observed shift of peaks in the O − yield, the binding energy of the oxygen molecule to iron is estimated to be approximately 1.2 eV.

Low-Energy (3−24 eV) Electron Damage to the Peptide Backbone

We report the mass spectrometric measurement of anions desorbed by 3-24 eV electron impact on thin films of formamide-1-d (DCONH2) and on the self-assembled monolayer (SAM) of two different Lys amide molecules used as a molecular model of the peptide backbone. In the present SAM configuration, the amides are elevated from a gold substrate by hydrocarbon chains to remove the effects of the metal substrate. Electron irradiation produces H- and D- from the formamide-1-d film and H-, CH3-, O-, and OH- from the SAM Lys amides. Below 13 eV, the dependence of the anion yields on the incident electron energy exhibits structures indicative of the dissociative electron attachment process, which is responsible for molecular fragmentation via the initial formation of core-excited anions. Above 13 eV, anion desorption is dominated principally by non-resonant dipolar dissociation. Our results suggest that the sensitivity of the peptide backbone to secondary electrons produced by ionizing radiation depends on the chemical environment (i.e., the amino acids sequence).

Dissociative electron attachment to theH2Omolecule I. Complex-valued potential-energy surfaces for theB12,A12, andB22metastable states of the water anion

We present the results of calculations defining global, three-dimensional representations of the complex-valued potential-energy surfaces of the $^{2}B_{1}$, $^{2}A_{1}$, and $^{2}B_{2}$ metastable states of the water anion that underlie the physical process of dissociative electron attachment to water. The real part of the resonance energies is obtained from configuration-interaction calculations performed in a restricted Hilbert space, while the imaginary part of the energies (the widths) is derived from complex Kohn scattering calculations. A diabatization is performed on the $^{2}A_{1}$ and $^{2}B_{2}$ surfaces, due to the presence of a conical intersection between them. We discuss the implications that the shapes of the constructed potential-energy surfaces will have on the nuclear dynamics of dissociative electron attachment to ${\mathrm{H}}_{2}\mathrm{O}$.

Analysis of electron interactions in dielectric gases

We present and discuss results concerning electron interactions processes of dielectric gases and their relationship with the macroscopic behavior of these gases, in particular, with their dielectric strength. Such analysis is based on calculating energies of reactions for molecular ionization, dissociative ionization, parent negative ion formation, and dissociative electron attachment processes. We hypothesize that the estimation of the required energy for a reduced number of processes that take place in electrically stressed gases could be related to the gas’ capability to manage the electron flow during an electrical discharge. All calculations were done with semiempirical quantum chemistry methods, including an initial optimization of molecular geometry and heat of formation of the dielectric gases and all of species that appear during electron interaction reactions. The performance of semiempirical methods Austin model 1 and Parametric model 3 (PM3) was compared for several compounds, PM3 being superior in most cases. Calculations performed for a sample of nine dielectric gases show that electron attachment and detachment processes occur in different energy bands that do not overlap for any value of the dielectric strength. We have also analyzed the relationship between dielectric strength and two physical properties: electron affinity and ionization energy. Calculations performed for 43 dielectric gases show no clear correlation between them, although certain guidelines for the qualitative estimation of dielectric strength can still be assessed.

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.

Remote bond breaking by interacting temporary anion states

The cross section for bond breaking at the site of a dissociative temporary negative ion state through the dissociative electron attachment process can be considerably enhanced by the presence of a second longer-lived temporary negative ion state elsewhere in the molecule, even one quite remote from the first. In a series of chloroalkenes possessing both C-Cl and C==C bonds separated by various distances, we show that the cross sections are determined by the lifetime of the lower anion state created by the mixing of the anion states of these two moieties, with the wave function's coefficients giving the probability that the electron is located at the dissociative site. Furthermore, the lifetime of the composite anion state can be expressed in terms of these same coefficients and the lifetimes of the unmixed resonances. We also discuss how these results may give insight into the means by which strand breaks are induced in DNA by the attachment of slow electrons.

Cross sections for electron trapping by DNA and its component subunits I: Condensed tetrahydrofuran deposited on Kr

We report cross sections for electron capture processes occurring in condensed tetrahydrofuran (THF) for incident electron energies in the range of 0-9 eV. The charge trapping cross section for 6-9 eV electrons is very small, and an upper limit of 4 x 10(-19) cm2 is estimated from our results. This latter is thus also an upper bound for the cross section for dissociative electron attachment process that is known to occur at these energies for condensed THF. At energies close to zero eV electron trapping proceeds via intermolecular stabilization. The cross section for this process is strongly dependent on the quantity of deposited THF. Since THF may model the furyl ring found in deoxyribose, these measurements indicate that this ring likely plays little role in either initiating or enhancing strand break damage via the attachment of the low energy secondary electrons produced when DNA is exposed to ionizing radiation.

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.

Development of a Time Projection Chamber using CF 4 gas for relativistic heavy ion experiments

A prototype Time Projection Chamber (TPC) using pure CF 4 gas was developed for possible use in heavy ion experiments. Basic characteristics such as gain, drift velocity, longitudinal diffusion and attenuation length of produced electrons were measured with the TPC. At an electric field of 900 V/cm, the drift velocity and longitudinal diffusion for 1 cm drift were obtained as 10 cm / μ s and 60 μ m , respectively. The relatively large gain fluctuation is explained to be due to the electron attachment process in CF 4. These characteristics are encouraging for the measurement of the charged particle trajectories under high multiplicity conditions at RHIC.

Resonant electron-CFcollision processes

Electronic structure methods are combined with variational fixed-nuclei electron scattering calculations and nuclear dynamics studies to characterize resonant vibrational excitation and electron attachment processes in collisions between low-energy electrons and $\mathrm{CF}$ radicals. Several low-lying negative ion states are found which give rise to strong vibrational excitation and which are expected to dominate the low-energy electron scattering cross sections. We have also studied several processes which could lead to production of negative ions (${\mathrm{F}}^{\ensuremath{-}}$ and ${\mathrm{C}}^{\ensuremath{-}}$); however, in contrast to other recent predictions, we do not find $\mathrm{CF}$ in its ground state to be a significant source of negative ion production when interacting with thermal electrons.

1080 PUBLICATION Surgical treatment craniofacial malignant tumors

The combustion of methanol was studied in a conical methanol—oxygen flame of fuel-lean composition (equivalence ratio φ = 0.25) burning at atmospheric pressure surrounded by a flowing argon shield. Oxygen bubbled through liquid methanol contained in two gas saturators provided a premixed flame of excellent stability and reproducibility. The flame gas was sampled into a mass spectrometer which was used to detect a variety of naturally occurring ionic species, both positive and negative. Although the maximum level of ionization in this flame had the rather low value of 5 × 108 ions ml−1, nevertheless concentration profiles were drawn as a function of distance along the flame axis at 17 individual mass numbers (m/z) for positive ions and 11 for negative ions; that is, whenever an appreciable signal could be detected below 100 amu. The positive profiles in the reaction zone are dominated by proton transfer processes which indicate that CHO+ is the primary ion. A variety of protonated combustion intermediates and products were detected including both stable molecules (H2O, HCHO, the CH3OH fuel, CH2CO, CH3CHO, HCOOH and possibly CH3OCH3/C2H5OH) and radicals (C, CH, CH2, HCO, CH2OH or CH3O, CH3CO or CH2CHO, and possibly CH3OO). Chemical ionization by charge transfer is certainly operative for two species (O2 and NO as an impurity) and perhaps others (CH3, O). As far as the negative ions are concerned, a number of species can be ionized in the reaction zone by three-body electron attachment and subsequent charge transfer processes (O2 initially, and then O, OH, HO2, HCOO and possibly HCOOO). Some of these negative ions are sufficiently strong bases to abstract protons from other species in proton transfer reactions (CH3OH, possibly H2O2, HCOOH, and perhaps the peroxidic intermediates HCOOO and HCOOOH). The remaining negative ions detected can be interpreted as cluster ions (OH− · H2O, HO2− · O2 or HO2− · CH3OH, CHO3− · H2O). All of this ion chemistry was compared with that observed previously for an analogous fuel-lean methane—oxygen flame. Many similarities were noted, particularly downstream, for both positive and negative ions. However, the combustion path leading to formic acid is more prominent in the methanol flame.

Topology of the adiabatic potential energy surfaces for the resonance states of the water anion

The potential energy surfaces corresponding to the long-lived fixed-nuclei electron scattering resonances of ${\mathrm{H}}_{2}\mathrm{O}$ relevant to the dissociative electron attachment process are examined using a combination of ab initio scattering and bound-state calculations. These surfaces have a rich topology, characterized by three main features: a conical intersection between the $^{2}A_{1}$ and $^{2}B_{2}$ Feshbach resonance states; charge-transfer behavior in the $\mathrm{OH}\phantom{\rule{0.2em}{0ex}}(^{2}\ensuremath{\Pi})+{\mathrm{H}}^{\ensuremath{-}}$ asymptote of the $^{2}B_{1}$ and $^{2}A_{1}$ resonances; and an inherent double-valuedness of the surface for the $^{2}B_{2}$ state the ${C}_{2v}$ geometry, arising from a branch-point degeneracy with a $^{2}B_{2}$ shape resonance. In total, eight individual seams of degeneracy among these resonances are identified.

Low energy electron attachment to C60

Low energy electron attachment to the fullerene molecule (C60) and its temperature dependence are studied in a crossed electron beam–molecular beam experiment. We observe the strongest relative signal of C60 anion near 0 eV electron energy with respect to higher energy resonant peaks confirming the contribution of s-wave capture to the electron attachment process and hence the absence of threshold behavior or activation barrier near zero electron energy. While we find no temperature dependence for the cross-section near zero energy, we observe a reduction in the cross-sections at higher electron energies as the temperature is increased, indicating a decrease in lifetime of the resonances at higher energies with increase in temperature.

Effect of CO2 on Nonthermal-Plasma Reactions of Nitrogen Oxides in N2. 2. Percent-Level Concentrations

Both NOx conversion and CO2 conversion decrease with increasing percent-level CO2 concentration in nonthermal nitrogen plasma. The rate constants of electron collision reactions of both N2 and CO2 decrease with increasing CO2 concentration because electronegative CO2 reduces electron concentrations in the reactor due to the electron attachment process. The rate constant of CO2 dissociation through electron collision is 1−2 orders of magnitude higher than that of N2 dissociation because of low dissociation energy of CO2. Model data for reactor outlet NOx and COx concentrations agree well with experimental data. The effect of CO2 on NO, NO2, N2O, and CO concentrations can be explained on the basis of the proposed reaction mechanism and kinetic modeling.

Properties of Pair-Ion Plasmas using Fullerenes

We have developed a novel method for generating a pair-ion plasma which consists of only positive and negative ions with equal mass. The experimental setup is improved step by step, being suitable for plasma generation through the processes of electron-impact ionization, electron attachment, and magnetic filtering. Density modulations are actively excited by a thick annulus in order to investigate collective electrostatic-modes. Three modes propagating along magnetic-field lines are experimentally found in the pair-ion plasma. The ion acoustic wave and ion plasma wave among them are theoretically predicted, but the new mode is only experimentally found in an intermediate-frequency band between the waves for the first time.

Resonant Capture of Low-Energy Electrons by Gas-Phase Glycine: A Quantum Dynamics Calculation

We present quantum calculations for the low-energy dynamics of electrons interacting with glycine molecules in the gas phase. The scattering equations are formulated within a symmetry-adapted, single-center expansion of both continuum and bound electrons, and the interaction forces are obtained from a combination of ab initio calculations and a nonempirical modeling of exchange plus correlation effects. Several open-channel (shape) resonances are obtained, and the features of the excess electron resonant wave functions are related to the experimental findings about the dissociative electron attachment (DEA) processes that follow the initial formation of those transient negative ions.