Electronic properties of polymers. Anisotropic light absorption and photoelectron emission of oriented polyethylene films in the vacuum ultraviolet region

Abstract

Anisotropic optical absorption of oriented ultrathin films, (down to 50 nm thickness) of low density polyethylene (LDPE) was measured in the vacuum ultraviolet (VUV) region between 6 and 12 eV. Clear anisotropy was found in the observed spectra. The lowest-energy absorption feature at 9 eV was polarized perpendicular to the chain axis, and the second at about 11.5 eV parallel to the chain axis. The edge of the lowest perpendicular absorption was determined to be 8.05±0.1 eV. The usefulness of the anisotropic absorption data in testing two theories was verified; the anisotropy predicted by the independent system model disagrees with the data, but the energy band model gives some promising results. The photoemission threshold of the oriented LDPE was determined to be 8.5±0.1 eV. The spectral dependence of the anisotropy ratio of the photoemission quantum yield was found to coincide with that of the optical absorption in the energy range above 9.5 eV. This coincidence was explained in terms of the three-step model of photoemission.