Effective mass and inner potential of hot electrons in organic thin films.

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Low-energy-electron transmission (LEET) spectra were measured for thin films of copper phthalocyanine (CuPc) with different crystalline order using electron energies below 15 eV. The LEET features were highly dependent on the crystalline order, indicating that they do not originate from the electronic states localized in the individual molecules, but from those which reflect the crystal structure. For oriented polycrystalline films of alpha-form CuPc with the crystal bc plane parallel to the surface, the energy positions of the LEET minima were found to be in excellent agreement with those of the band gaps obtained by assuming a parabolic conduction band in the direction perpendicular to the crystal bc plane. Further we estimated the effective mass of hot electron m^* to be 2.2_2m_0, where m_0 is the free-electron mass, and the inner potential v_0 to be -1.3_3 eV in the direction perpendicular to the bc plane of alpha-form CuPc. The results show that the specular reflection of the incident electron at the band gap is a main factor for the LEET features and the hot-electron states can be well described by the parabolic dispersion even near the vacuum level.