RESONANT AND NONRESONANT X-RAY-SCATTERING SPECTRA OF SOME POLY(PHENYLENEVINYLENE)S

The electronic structure of some poly(phenylenevinylene)s have been in vestigated by resonant and nonresonant x-ray inelastic scattering spec troscopies. The nonresonant as well as all resonant spectra for each p olymer demonstrate benzene-like features, indicating a local character of the x-ray emission in which the phenyl ring acts as a building blo ck. Theoretical simulations of x-ray energies and intensities taking t he repeat unit as a model molecule of the polymer agree with the exper imental spectra fairly well. The edges of the occupied bands have been identified in the nonresonant spectra of each polymer. By subtracting the emission energy of the highest occupied molecular orbital in the nonresonant spectrum from the core excitation energy in the resonant s pectrum an alternative way to determine the optical band gap is obtain ed. As for free benzene the outer pi band in the polymer spectra show a depletion of the emission going from the nonresonant to the resonant x-ray emission spectra. It is demonstrated that this transition, whic h is strictly symmetry forbidden for free benzene, becomes effectively forbidden in the polymer case as a result of strong interference effe cts, and it is argued that this is the general case for resonant x-ray emission of conjugated polymers as far as the frozen orbital approxim ation holds. (C) 1998 American Institute of Physics.