Paul Burrow

Electron transmission study of the temporary negative ion states of selected benzenoid and conjugated aromatic hydrocarbons

Electron transmission spectroscopy is utilized to determine the vertical electron affinities and to characterize the temporary anion states of a series of alternant hydrocarbons including benzene, naphthalene, anthracene, tetracene, styrene, and cis‐ and trans‐stilbene. The vibrational structure present in the low lying resonances is interpreted in light of the charge distributions of the temporarily occupied orbitals. The energies of the anion states are compared with the predictions of PPP, PPP‐CI, and HAM calculations, the pairing theorem and with the results from optical absorption measurements on the ground state anions in rigid glasses.

Temporary negative ions in the chloromethanes CHCl2F and CCl2F2: Characterization of the σ* orbitals

The electron transmission spectra of chlorometers CHCl2F and CCl2F2 are presented. The electron affinities are evaluted compared with thoses calculated using self‐consistent field methods. (AIP)

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.

On the electron affinities of ethylene and 1,3-butadiene

Abstract Temporary negative ion formation in ethylene and 1,3-butadiene has been studied using high resolution, low energy electron scattering. Sharp structure in the total electron scattering cross section allows the adiabatic electron affinity of each molecule to be determined leading to values of −1.55 ± 0.1 eV for ethylene and −0.62 ± 0.05 eV for 1,3-butadiene.

Temporary anion states of selected amino acids

Vertical attachment energies for the formation of low-lying temporary anion states of glycine, alanine, phenylalanine, tryptophan, and proline in the gas phase are reported using electron transmission spectroscopy. Electron attachment into the empty π* orbital of the –COOH group was observed in all the compounds. Temporary anion states associated with the side groups in phenylalanine and tryptophan are found to be stabilized with respect to those in the reference compounds toluene and indole, respectively, by approximately 0.2 eV. We attribute this to electrostatic effects and explore, using simple theoretical models, the extent to which such anion states could be further stabilized if these amino acids were in zwitterionic form.

temporary Σ and Π anions of the chloroethylenes and chlorofluoroethylenes

Abstract Electron transmission spectroscopy is employed to characterize the low-lying temporary anions of the chloroethylenes and several mixed chlorofluoroethylenes. Experimental evidence for both Σ and Π anion states is presented and supported by calculations. The effect of fluorine substitution on these states is examined. The relationship between the present data and dissociative attachment processes in these molecules is discussed briefly.

Temporary anions of the fluoroethylenes

Abstract Electron transmission spectroscopy is employed to measure the gas phase electron affinities of the fluoroethylenes. Fluorination is found to destabilize the π* anions with respect to that of ethylene. The role of bond length changes as well as inductive and resonance effects is examined.

Dissociative attachment of electrons in the chloromethanes

Abstract The yield of negative ions from CH 3 Cl, CH 2 Cl 2 , CHCl 3 and CCl 4 has been examined with electron beam methods. The relative cross sections at low electron energy vary by six orders of magnitude over this series, consistent with earlier swarm experiments, but at variance with other electron beam measurements. We provide evidence in CH 3 Cl suggesting that impurities severely affect measurements of the dissociative attachment cross section.

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.

Scattered-electron rejection in electron transmission spectroscopy

Abstract The rejection of scattered electrons in magnetically collimated transmission spectrometers which employ trochoidal monochromators is discussed. Two rejection mechanisms are described. We show that at low impact energies electrons backscattered into the monochromator contribute substantially to the rejection. At higher energies, rejection at the retarding plate following the collision chamber is the dominant mechanism. Under conditions in which the scattered electrons are only partly rejected, artifacts can appear in the measured spectra. We derive a simple model which qualitatively reproduces these features.