Molecular orientation in thin films of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) on graphite studied by angle-resolved photoelectron spectroscopy

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Abstract
Angle-resolved ultraviolet photoelectron spectra using synchrotron radiation were measured for oriented thin films of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) on a cleaved highly oriented pyrolytic graphite (HOPG) surface. The observed takeoff angle dependence of the photoelectron intensity was analyzed by using the independent-atomic-center approximation and modified neglect of diatomic overlap molecular-orbital calculations. The calculated results agree well with the experimental ones. From the comparison between these results, the molecules in the thin film are estimated to lie flat with the inclination angle β≤10° to the HOPG surface. This analysis method is useful as a first step to a quantitative analysis for angular distribution of photoelectrons from thin films of large and complex organic molecules.