ANGLE-RESOLVED ULTRAVIOLET PHOTOELECTRON-SPECTROSCOPY OF ORIENTED FILM OF MODEL-COMPOUND OF POLY(P-PHENYLENE) - WAVE NUMBER(K)-CONSERVATIONAND K-BLURRING IN A SYSTEM WITH SMALL NUMBER OF REPEATING UNITS

The electronic structure of oriented thin films of sexiphenyl (6P) as a short model compound of poly(p-phenylene) (PPP) was investigated by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using sy nchrotron radiation. The sample film of 6P was prepared by vacuum evap oration onto a heated Ag substrate. Infrared reflection-absorption spe ctroscopy, x-ray diffraction, and the observed selection rules for ARU PS indicated that the 6P molecules are highly oriented aligning its mo lecular axis parallel to the surface normal. In the normal emission sp ectra of ARUPS, the band intensity of the HOMO and the next HOMO was e nhanced at h nu=28 similar to 40eV. This enhancement can be explained in terms of the wave number(k)-conservation rule in the one-dimensiona l intramolecular band with some initial state broadening of k (or rela xation of k-conservation rule) due to the small number of repeating un its. This indicates that wave number k along the molecular axis serves as a reasonably good quantum number even in a system consisting of on ly six repeating units.