The community development and education connection

Education - Economic aspects

Interactions of slow electrons with biomolecules

We report on results of computational studies of the interaction of slow electrons with the purine and pyrimidine bases of DNA, as well as with their associated nucleosides and nucleotides. The calculations focus on characterisation of the π* resonances associated with the bases and also provide general information on the scattering of slow electrons by these targets. High-level studies of the π* resonances in pyrazine, a close analogue of the pyrimidine bases, indicate that the higher-energy π* resonances in these bases may in fact contain large admixtures of core-excited character built on low-lying triplet states. Decay into such triplet states may provide a mechanism for damage to DNA

Vibrational branching ratios and shape resonant photoionization dynamics in N_2O

Vibrational branching ratios and photoelectron asymmetry parameters for alternative vibrational modes in the photoionization of N_2O(7σ^(−1)) have been studied using accurate photoelectron continuum orbitals. Earlier dispersed ionic fluorescence measurements [E. D. Poliakoff, M. H. Ho, M. G. White, and G. E. Leroi, Chem. Phys. Lett. 130, 91 (1986)] revealed strong non‐Franck–Condon vibrational ion distributions for both the symmetric and antisymmetric stretching modes at low photoelectron energies. Our results establish that these features arise from a σ shape resonance which, based on its dependence on internuclear geometry, must be associated with the molecular framework as a whole and not with either of its fragments, N–N or N–O. This behavior accounts for the more pronounced deviations of the vibrational branching ratios from Franck–Condon values observed in the symmetric than in the antisymmetric mode. The σ continuum also supports a second shape resonance at higher energy which does not influence the vibrational branching ratios but is quite evident in the photoelectron asymmetry parameters around a photon energy of 40 eV. These vibrationally resolved studies of the photoelectron spectra of this polyatomic system provide an interesting example of the rich shape resonant behavior that can be expected to arise in polyatomic molecules with their alternative vibrational modes

Resonant Channel Coupling in Electron Scattering by Pyrazine

Detailed investigation of the three low-energy resonances seen in electron scattering by the diazabenzene molecule pyrazine reveals that the first two are nearly pure single-channel shape resonances, but the third is, as long suspected, heavily mixed with core-excited resonances built on low-lying triplet states. Such resonant channel coupling is likely to be widespread in pi-ring molecules, including the nucleobases of DNA and RNA, where it may form a pathway for radiation damage

Low-energy electron collisions with gas-phase uracil

We have studied gas-phase collisions between slow electrons and uracil molecules with a view to understanding the resonance structure of the scattering cross section. Our symmetry-resolved results for elastic scattering, computed in the fixed-nuclei, static-exchange and static-exchange-plus-polarization approximations, provide locations for the expected pi* shape resonances and indicate the possible presence of a low-energy sigma* resonance as well. Electron-impact excitation calculations were carried out for low-lying triplet and singlet excitation channels and yield a very large singlet cross section. We discuss the connection between the resonances found in our elastic cross section and features observed in dissociative attachment

Interaction of low-energy electrons with the purine bases, nucleosides, and nucleotides of DNA

The authors report results from computational studies of the interaction of low-energy electrons with the purine bases of DNA, adenine and guanine, as well as with the associated nucleosides, deoxyadenosine and deoxyguanosine, and the nucleotide deoxyadenosine monophosphate. Their calculations focus on the characterization of the pi* shape resonances associated with the bases and also provide general information on the scattering of slow electrons by these targets. Results are obtained for adenine and guanine both with and without inclusion of polarization effects, and the resonance energy shifts observed due to polarization are used to predict pi* resonance energies in associated nucleosides and nucleotides, for which static-exchange calculations were carried out. They observe slight shifts between the resonance energies in the isolated bases and those in the nucleosides

Low-energy electron scattering by deoxyribose and related molecules

We apply first-principles computational methods to study elastic scattering of low-energy electrons by 2-deoxyribose and 2-deoxyribose monophosphate, which are of interest as components of the DNA backbone, and to tetrahydrofuran (THF), which has been studied as a deoxyribose analog. To investigate the dependence of the scattering process on the molecular conformation, we examine Cs and C2 conformers of THF as well as the planar C2v geometry imposed in earlier calculations. There is little difference between the elastic cross sections determined at the nonplanar geometries, but there are noticeable differences between those results and the cross sections computed using the planar ring. By comparing results for tetrahydrofuran obtained with and without inclusion of polarization effects, we obtain energy shifts that are applied to the computed resonance positions for deoxyribose and deoxyribose monophosphate

Elastic electron scattering by fullerene, C60

We report cross sections for elastic scattering of low-energy electrons by fullerene, C60, calculated within the static-exchange approximation. The calculations are carried out via the Schwinger multichannel (SMC) method, equivalent in this case to the standard Schwinger variational principle. Combining the high parallel efficiency of the SMC method with a quadrature specially adapted to the high symmetry of C60 facilitates the most demanding step of the calculation and so permits the use of a large basis set. We analyze the structure of the cross section with reference to a simple spherical-shell model, and we compare our results to prior measurements and calculations

Rotationally resolved photoelectron angular distributions in resonance enhanced multiphoton ionization of NO

We report calculated ionic rotational branching ratios and associated photoelectron angular distributions for (1+1′) resonance enhanced multiphoton ionization (REMPI) via the R_(21)(20.5), P_(21)+Q_(11)(25.5), and P_(11)(22.5) branches of the A ^2 Σ^+(3sσ) state of NO. The branching ratios are dominated by even angular momentum transfer peaks, in agreement with the ΔN+l=odd (ΔN≡N+−Ni ) selection rule. Whereas the calculated photoelectron angular distributions are very branch dependent alignment, the ionic branching ratios are found to be less so. The present calculated results agree well with the experimental results of Allendorf et al