Kayvan Aflatooni

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.

Observation of the Kapitza–Dirac effect

In their famous 1927 experiment, Davisson and Germer observed the diffraction of electrons by a periodic material structure, so showing that electrons can behave like waves. Shortly afterwards, Kapitza and Dirac predicted that electrons should also be diffracted by a standing light wave. This Kapitza–Dirac effect is analogous to the diffraction of light by a grating, but with the roles of the wave and matter reversed. The electron and the light grating interact extremely weakly, via the ‘ponderomotive potential’, so attempts to measure the Kapitza–Dirac effect had to wait for the development of the laser. The idea that the underlying interaction with light is resonantly enhanced for electrons in an atom led to the observation that atoms could be diffracted by a standing wave of light. Deflection of electrons by high-intensity laser light, which is also a consequence of the Kapitza–Dirac effect, has also been demonstrated. But the coherent interference that characterizes wave diffraction has not hitherto been observed. Here we report the diffraction of free electrons from a standing light wave—a realization of the Kapitza–Dirac effect as originally proposed.

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.

Total cross sections for dissociative electron attachment in dichloroalkanes and selected polychloroalkanes: The correlation with vertical attachment energies

Electron attachment into the lowest unoccupied molecular orbital of a typical polychloroalkane in the gas phase forms a temporary negative ion in which the impinging electron resides on a combination of local C–Cl σ* orbitals. Because of the antibonding character of these orbitals, these anions may dissociate, producing Cl− with cross sections that vary enormously over the chloroalkane family. In this work, we present absolute total dissociative electron attachment (DEA) cross sections for 33 of these compounds, and we show that the peak values of the cross sections correlate strongly with the vertical attachment energies (VAEs) for formation of the lowest anion states at the equilibrium geometries of the neutral molecules. This behavior is a consequence of the remarkably monotonic dependence of the resonance widths of the temporary anion states on VAE over the range 0.42<VAE<3.45 eV. Finally, we note also the strong connection between the s-wave attachment process at 0 eV in these compounds and the VAEs ...

Dissociative electron attachment near threshold, thermal attachment rates, and vertical attachment energies of chloroalkanes

The peaks appearing near zero energy in the dissociative electron attachment cross section of 18 chloroalkanes are studied by electron beam methods. Fits to the experimental data are made using model cross sections having appropriate energy dependences and inclusion of the broadening due to the electron energy distribution. The magnitudes of the zero peaks are found to be well correlated with the vertical attachment energies (VAE) associated with occupation of the lowest empty orbitals of the compounds. The magnitudes rise exponentially by more than five orders of magnitude as VAE decreases from 2 eV to a slightly negative value. This dependence is a consequence not only of the thermal population of vibrational levels, but also of an approximately linear relationship between VAE and the energy of the crossing between the neutral and anion potential curves. Franck–Condon factors for the transition to the anion curve are computed for model potential curves, and the nature of the attachment from vibrational ...

Dissociative electron attachment in chlorofluoromethanes and the correlation with vertical attachment energies

Abstract The total dissociative electron attachment (DEA) cross sections of the chlorofluoromethanes are determined in an electron beam experiment and correlated with the vertical attachment energies (VAEs) for formation of the lowest temporary anion states of these compounds. The latter are determined independently by electron transmission spectroscopy and correspond to the energies of the anions at the equilibrium geometries of the neutral molecules. As we observed previously in the chloroalkanes, the peak DEA cross sections are well correlated with VAEs. For values of VAEs between 0.9 to 3.0 eV, the peak DEA cross sections vary by more than four orders of magnitude. This is attributable in part to the remarkably monotonic variation of the temporary anion widths, arising from their finite lifetimes, over this range of VAE.

Dissociative electron attachment in chloroalkanes and the correlation with vertical attachment energies

Abstract Measurements of dissociative attachment (DA) cross sections and temporary negative ion energies in a series of dichloroalkanes show an exponential decrease of the peak DA cross section with increasing vertical attachment energy. Including similar data for monochloroalkanes, the exponential behavior spans five orders of magnitude in the peak cross section.

Dissociative electron attachment to molecules in the gas phase and in rare gas solids

Measurements of dissociative electron attachment (DEA) cross sections in chloroalkanes and chlorofluoromethanes have shown strong correlations between the peak DEA cross sections and the vertical attachment energies (VAEs) of these compounds. We explore the extent to which these gas phase data can be used to predict such cross sections for molecules embedded within or on the surface of a Kr solid. Effective VAEs are computed that include polarization of the solid by the anion and effects due to electron motion in the lattice. Comparisons are made with recent surface and bulk measurements and show good agreement for CF 3 Cl both within and on the surface. Satisfactory agreement is found for CH 3 Cl in the bulk but not on the surface.

Temporary anion states of three herbicide families.

Electron scattering studies are used to locate the energies of temporary negative ion states of three chloro-substituted molecular families of herbicidal importance: salicylic and phenoxyacetic acids and acetamides. The correlation between these energies and the computed virtual orbital energies of the compounds is examined and used to put the latter on an absolute energy scale. Such scaling of orbital energies permits the anion states of other members of these families, for which experimental data may not be available, to be estimated from the calculated orbital energies. Studies of electron reduction rates often rely on calculated LUMO energies as molecular descriptors. The use of measured anion energies as well as appropriately scaled orbital energies should serve to improve such studies in these and in related herbicides.