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Ultraviolet photoemission study of oligothiophenes: π-band evolution and geometries (1990)

Fujimoto, H. ; Nagashima, U. ; Inokuchi, H. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 92 (1990), S. 4077-4092

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4–8 thiophene rings. In a series of α-linked oligomers (αn with n being the number of rings), a systematic evolution of the π band is observed. Several peaks which correspond to the π band are observed in the region of 0.7–3 eV below the Fermi level (EF), and the bandwidth becomes broader with increasing n. The nonbonding π band is observed at 3.5 eV below EF and its energy is almost independent of the number of thiophene units. UPS spectra of α7 and α8 are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compounds of the polymer. The ionization threshold energy of α7 and polythiophene was observed to be 5.3 eV. The effect of irregularity on the π-electron system was also studied by using oligomers which contain a β linkage or a vinylene group at the middle of the molecule. The UPS spectra showed that the β linkages significantly affect the electronic structure of polythiophene, while the vinylene group does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometries of these oligomers are calculated by the semiempirical MNDO-SCF-MO (modified neglect of diatomic overlap self-consistent-field molecular orbital) method. Theoretically simulated spectra of these oligothiophenes derived from the obtained orbital energies by Gaussian broadening are compared with the observed ones. The agreement between the observed and calculated spectra is very good, particularly in the π region. It is shown from the optimized geometry that (1) αn 's have planar structure and π electrons are delocalized, (2) the oligomer with β linkages has nonplanar structure leading to limited delocalization of π electrons, and (3) the oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.458561

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Systematic behavior of mass-yield curves in low-energy fission of actinides (1990)

Nakahara, H. ; Ohtsuki, T.

Springer

Journal of radioanalytical and nuclear chemistry 142 (1990), S. 231-251

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ISSN: 1588-2780

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering

Notes: Abstract Mass yield curves of spontaneous fissions, thermal-neutron induced fissions, monochromatic-neutron and proton induced fissions of actinides are reviewed. Mean masses of light and heavy asymmetric mass yield peaks are listed. The subtle change of the shape of the asymmetric heavy peak is expressed in terms of the parameters of two Gaussians that fit the observed data well. One Gaussian is narrow in width and always peaks at A=133–136 while the other is broader and peaks at A=140−144. The FWHM of the heavy asymmetric peak becomes minimum for the fissioning mass A=240−245. An analysis of the projectile energy dependence of mass yield curves indicated, at least, two distinctively different behaviors, one for asymmetric products and the other for symmetric products. From the energy dependence of the peak-to-valley rations, the “extra-energy” required for symmetric mass division, or the difference in the fission barrier heights, in the nomeuclature of the two-mode hypothesis, is deduced for a wide range of fission nuclides.