Utilizing symmetry selection rules of the absorption of linearly polarized light one can determine the relative orientation of molecular orbitals with respect to the plane of polarization of the x-ray radiation. Thus, by variation of the angle of incidence of the radiation and measuring the intensity of the PI-resonances in the spectra, one can determine the direction of a molecular axis with respect to the surface. At monolayer coverage and room temperature rings around the substrate diffraction spots were observed. This indicated a certain long range order, but not a perfect superstructure. Figure 4 and Fig. 5 show STM images of heptahelicene molecules on the surface at two different coverages.
Every bright dot is representative for one molecule.
This conclusion is based on the linear dependence of the number of bright dots with time of exposure to the molecular beam. At sub monolayer coverage there is no lateral order of the molecules on the surface. The intermolecular distances are 1 nm, which is exactly in the desired range. Very interestingly, Fig. 5 shows another detail. Although atomic resolution on the subtrate is reached at the atomic steps of the terrace edges, no intramolecular details are revealed.