HIGH-RESOLUTION ELECTRON-ENERGY LOSS SPECTROSCOPY OF EPITAXIAL-FILMS OF C-60 GROWN ON GASE

The growth of thin C60 films on GaSe(0001) has been studied by infrared High Resolution Electron Energy Loss Spectroscopy (HREELS) in specular reflection geometry. In contrast to previously studied disordered films on Si(100), it was found that deposition on a GaSe substrate heated to 120-degrees-C leads to well-ordered, epitaxial C60 films. The structural order, manifested by a sharp LEED pattern, was confirmed by the clear detection in the specular beam of all four T1u dipole-active vibrations of the C60 molecule. Several other infrared inactive modes, which were prominent in the C60/Si HREELS spectrum, are still seen in C60/GaSe but with reduced intensities. Application of the dielectric theory of reflection EELS allows a quantitative determination of the contribution of the dipole-active modes to the total spectrum. Although the relative oscillator strengths of the four infrared modes are in good agreement with optical absorption measurements, their absolute values, as determined by theoretical simulation, are found to be three times larger than the optical results.