ANGLE-RESOLVED PHOTOELECTRON SPECTROSCOPIC STUDY OF ORIENTED P-SEXIPHENYL - WAVE-NUMBER CONSERVATION AND BLURRING IN A SHORT MODEL-COMPOUND OF POLY(P-PHENYLENE)

The electronic structure of oriented thin films of sexiphenyl (6P), which is a short model compound of poly(p-phenylene), was studied by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. Oriented thin films of 6P were prepared by vacuum evaporation on a heated Ag substrate. Infrared reflection-absorption, x-ray diffraction, and the observed selection rules for ARUPS indicated that the axes of 6P molecules are oriented perpendicular to the substrate surface. The dependence of normal emission intensity of the highest occupied molecular orbital (HOMO) and the next 4OMO in these films on photon energy hv, with a maximum at hv=28-40 eV, was explained in terms of the wave number (k) conservation rule in the one-dimensional intramolecular band with some initial state broadening of k (or relaxation of the k-conservation rule) due to the small number of repeating units. This indicates that the wave number k along the molecular axis serves as a reasonably good quantum number even in a system of only six repeating units. The quantitative simulation for the hv dependence of normal emission using the independent-atomic-center (IAC) approximation combined with MO calculations gave a good description of the observed experimental data.