Bond- and site-selective loss of H- from pyrimidine bases

Electron attachment to gas phase thymine and uracil leads to H- loss within a broad and structured feature in the energy range between about 5 and 12 eV consisting of 4 overlapping resonances. By using thymine and uracil methylated at the N1 and N3 positions, respectively, and taking into account recent results from partly deuterated thymine, we find that by tuning the electron energy, H- loss turns out to be not only bond selective, i.e., (C-H) versus (N-H) bonds, but also site selective (N1 versus N3 site). Such a bond and site selectivity by energy has not been observed before in dissociative electron attachment. Implications for the mechanism of strand breaks observed in plasmid DNA are considered.     

Low energy electron attachment to C<Subscript>60</Subscript>

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.     

Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters

Using a high resolution ( meV) laser photoelectron attachment method, we have studied the formation of (CO ₂) _q ⁻ ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO₂) _N () clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists of several narrow vibrational Feshbach resonances of the type [(CO ₂) _{N −1}CO which involve a vibrationally-excited molecular constituent ( denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at energies below those of the vibrational excitation energies of the neutral clusters [(CO ₂) _{N −1}CO ]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies. The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster. Received 11 January 2000 and Received in final form 10 April 2000     

Selective resonance population in high resolution electron energy loss spectroscopy of 4-ethylbenzenethiol self-assembled monolayers on Au(1 1 1)

We have investigated the vibrational frequencies and excitation cross-section of self-assembled monolayers (SAMs) of 4-ethylbenzenethiol (CH₃CH₂(C₆H₄)SH) on Au(1 1 1) by high resolution electron energy loss spectroscopy (HREELS). Negative ion resonances were observed in the energy loss intensities as a function of the incident electron energy. Analysis of the C–H stretching modes indicates resonances of different energies are localised in both the ethyl and phenyl functional groups of the SAM molecules, which regulate the observed vibrational lineshape.     

Generation of cold electrons in the downstream region of a microwave plasma source with near boundary resonances for production of negative ions

A microwave driven multicusp plasma based volume negative ion source equipped with a magnetic filter is developed. Instead of employing any electrodes or current carrying filaments, microwaves of frequency 2.45 GHz is used to generate plasma by resonance heating mechanisms namely the electron cyclotron resonance (ECR) and upper hybrid resonances (UHR), occurring near the boundary plasma layers. The principal process of negative ion production in hydrogen is dissociative attachment of low energy (0.5–1.0 eV) electrons to vibrationally excited neutral molecules generated from high energy (15–20 eV) electron impact. The source therefore necessitates two distinct spatial regions (a) production and (b) attachment chambers; which would contain electrons with optimum cross section for the aforementioned processes. A biased grid after the magnetic filter further helps to lower down the electron temperature to ≤1 eV which is favorable for the dissociative attachment process.     

Inelastic electron scattering and energy-selective negative ion reactions in molecular films on silicon surfaces

Careful control of plasma chemistry, particularly the negative ion production and reaction channels, could lead to nanoscale patterning, growth, and etching strategies. Since the energies of anion and electron surface collisions are relatively low (<10 eV) in plasmas, these interactions are essentially damage free with respect to the buried interface and subsurface region. Thus, renewed interest has arisen that takes advantage of negative ion surface chemistry in film deposition and nanopatterning. We review (i) the transient negative ion states such as shape and dissociative electron attachment resonances produced during electron molecule scattering, (ii) the many body interactions and substrate-effects on the resonance energies, widths and cross sections and (iii) examples of post-dissociation interactions of the reactive fragments and anions that may lead to controlled etching or growth. Specifically, we summarize past and recent studies on electron scattering with gas- and condensed-phase H₂O, CF₄, SiCl₄ and O₂ targets. We then discuss recent examples of energy selective oxidation and fluorination of hydrogen terminated silicon surfaces and comment on the general applicability of low-energy electrons and negative ion surface chemistry in film deposition, nanopatterning and growth strategies.     

Crossed beam measurement of binding energies of positronium compounds: A proposal

We describe an experiment to establish the chemical stability and measure the binding energies of compounds containing positronium atoms. The method consists of crossing a monoenergetic beam of low energy (1–25 eV) positrons with a supersonic adiabatic expansion molecular beam, and, after a dissociative attachment reaction takes place, detecting the resulting ¡gnature ion while measuring its kinetic energy. Except for the use of a positron beam instead of an electron beam, this is similar to electron impact MS/IKES (mass spectrometry/ion kinetic energy spectrometry). The start signal for the IKES measurement is provided by the remoderation of the positron beam. Among the benefits of the proposed research are the study of: (1) a new class of resonances, those involving temporarily bound positrons to molecules in excited electronic states; and (2) the surprising sub-threshold fragmentation observed by Surko and Hulett in positron-molecule scattering.     

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.     

QED effects in Cu-like Pb recombination resonances near threshold

In an electron-ion recombination study with Pb53+ dielectronic recombination resonances are found for as low as approximately 10(-3)-10(-4) eV relative energy. The resonances have been calculated by relativistic many-body perturbation theory and through comparison with experiment the Pb53+(4p(1/2)-4s(1/2)) energy splitting of approximately 118 eV is determined with an accuracy comparable to the position of the first few resonances, i.e., approximately 10(-3) eV. Such a precision provides a test of QED in a many-body environment at a level which can still not be reached in calculations.