VIBRATIONAL STUDY OF C-60 OVERLAYERS ON H SI(111)-(1X1)/

We have studied the vibrational properties of thin C-60 overlayers dep osited on ideally H-terminated Si(111) surfaces, by multiple infrared reflection and high-resolution electron energy-loss spectroscopies. We find that the Si-H interface remains intact upon C60 deposition: the Si-H stretching mode frequency, initially at 2083.8 cm(-1), shifts dow nward to 2062 cm(-1), and broadens while keeping the same integrated i ntensity. Due to the symmetry-breaking effect at the surface on the ad sorbed C60 molecules, a new IR band is observed at 1445 cm(-1) which i s mainly s-polarised. No evidence for chemical interaction between C60 and H/Si(111)-(1 x 1) is found, pointing to van der Waals-type bondin g. From the relative intensities of the dipole-active and -inactive mo des in HREELS in the specular direction, and from the angular width of the scattered elastic peak, the growth of an ordered film is inferred .