Polarization effects in the valence and inner-shell spectra of poly(di-n-hexylsilane)

Abstract

The polarized absorption spectra of oriented films of phase-I and phase-II poly(di-n-hexylsilane), PDHS, have been determined in the visible/UV and in the near-edge regions of the C 1s and Si 1s x-ray ionization potentials. The phase-I absorption band at 360 nm is strongly polarized parallel to the direction of the Si chain, whereas the analog of this band in phase II (317 nm) is only weakly polarized in the same direction; this is consistent with partial trans→gauche isomerization of the Si chain in phase II and delocalization of the excitation in part onto disordered n-hexyl groups. Polarized absorption in the vicinity of the Si 1s ionization limit reveals strong polarization of the discrete and continuum transitions in both phase I and phase II, whereas the equivalent spectra taken in the vicinity of the C 1s limit show strong polarization in phase I but little or no polarization in phase II. The x-ray spectra are interpreted as showing that the Si chain and the n-hexyl groups of the polymer are ordered in phase I and are essentially perpendicular to one another, whereas in phase II the n-hexyl chains are disordered while the Si backbone remains relatively fixed in space. The Si 1s electron-yield spectra also display EXAFS interferences which have been transformed to yield Si-Si and Si-C distances in the polymer of 2.37 ± 0.02 and 1.81 ± 0.03 Å, respectively. The various discrete and continuum transitions are assigned according to an orbital model, and the visible/UV thermochromism earlier reported for PDHS is now observed in the Si 1s x-ray near-edge spectrum as well. © 1988 American Institute of Physics.