Valence bands of oriented finite linear chain molecular solids as model compounds of polyethylene studied by angle-resolved photoemission

Abstract

Angle-resolved photoemission spectra were measured using synchrotron radiation of two kinds of oriented model compounds of polyethylene with their molecular axes perpendicular to the substrate surface, i.e. evaporated films of hexatriacontane CH 3(CH 2) 34CH 3 and Langmuir-Blodgett films of Cd arachidate (CH 3(CH 2) 17COO) 2Cd. Both films show similar photoelectron energy distribution curves determined by the long-alkyl chain. The intramolecular energy-band dispersion of polyethylene was determined from the photon-energy dependence of the normal-emission spectra. This is the first direct observation of an energy-band dispersion in organic solids. The upper bands formed by C 2p and H 1s electrons extend from 8.8 to 15.5 eV below the vacuum level, and the deeper-lying bands originating from C 2s electrons lie from 17.5 to 24.7 eV. The band structure obtained is compared to results from XPS and ESR studies. Furthermore, the experimentally determined band structure is discussed in detail in view of theoretical calculations for polyethylene. Ab initio and extended Hückel calculations give a good description of the experimental results.