IPES experiments were carried out in UHV at ~10 -10 mbar base pressure. The measurements were performed in the isochromatic mode, i.e., the incident electron kinetic energy was varied and emitted photons of fixed energy (9.7 eV) were collected by a band-pass photon analyzer consisting of a CaF2 entrance window and a Geiger-Müller detector. The incident electronic current was about 2 µA and the photon yield about 30-50 counts/s. The photon counts were normalized to the measured injected current. No significant degradation was observed in comparing the first and the last scan composing each spectrum. The spectra are referenced to the Fermi level measured on a cleaned Ta foil. The overall line shape is well reproduced by DFT, as expected for weakly-interacting units in a molecular film. On the other hand, the IPES reveals major differences across the three molecules.