Angle-resolved UPS of ultrathin films of functional organic molecules with synchrotron radiation: Determination of molecular orientation by quantitative analysis of photoelectron angular distribution

This paper describes the determination of the molecular orientation of functional organic molecules in their ultrathin films deposited on crystal surfaces by the quantitative analysis of photoelectron angular distribution observed with synchrotron radiation. The examples of the results are presented for phthalocyanine and bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole), a new type of organic semiconductor. Further, the first theoretical simulation of the valence-band spectrum is also shown for oriented thin films of naphthacene, a conventional organic semiconductor. These analyses were based on the independent-atomic-center approximation (IAC) and single scattering theory (SS) combined with molecular orbital (MO) calculation. It is shown that even the simple IAC/MO model can simulate the photoelectron angular distribution very well for a pi band at near flat-lie orientation of planer molecules. Further we found that the SS/MO calculations can give the full valence band spectra of large organic molecules, which are in excellent agreement with observed ones.