Unoccupied electronic states in ordered PTCDA and H2Pc thin films grown on InAs and InSb: a resonance HREELS study

High resolution electron energy loss spectroscopy has been used to investigate the unoccupied electronic states of ordered thin films of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and free-base phthalocyanine (H2Pc) grown on InAs and InSb substrates. Measurement of the vibrational excitation functions over a wide range of electron beam energies shows that both molecules exhibit strong resonant behaviour due to temporary negative ion formation. For PTCDA/InAs(1 1 1)A, there is no mode selectivity and all the vibrational modes of the molecule are resonantly enhanced at 2.2 eV. By contrast, two resonances are observed for H2Pc/InSb(1 0 0) at 0.8 and 7.0 eV respectively, with the higher energy resonance demonstrating mode selectivity. Identification of the resonant states is assisted by comparison to previous near edge X-ray absorption fine structure measurements for PTCDA and Pc films.