We measured NEXAFS spectra on both C and N K-edges of an ordered monolayer of lead phthalocyanine adsorbed onto an InSb(001) 4 x 2/c(8 x 2) substrate. We compared the spectra with the calculated density of unoccupied states of an isolated molecule. We demonstrate that molecules lay flat on the surface, determine the energy position of the lowest excited state, and show that it is site dependent. Finally, we compare the core exciton structure with the valence exciton structure previously measured by two different valence spectroscopic techniques and discuss their differences in terms of electron-nuclear and electron-hole Coulomb interactions.
