Infrared (IR) spectroscopy of chemisorbed C
60 on Ag (111), Au (110) and Cu (100) reveals that a non-IR-active mode becomes active upon adsorption, and that its frequency
shifts proportionally with the charge transferred from the metal to the molecule by about 5 cm
-1 per electron. The temperature dependence of the frequency and the width of this IR feature have also been followed for C
60/Cu (100) and were found to agree well with a weak anharmonic coupling (dephasing) to a low-frequency mode, which we suggest
to be the frustrated translational mode of the adsorbed molecules.
Additionally, the adsorption is accompanied by a broadband reflectance change, which is interpreted as due to the scattering
of conduction electrons of the metal surface by the adsorbate. The reflectance change allows determination of the friction
coefficient of the C
60 molecules, which results in rather small values (∼2×10
9 s
-1 for Ag and Au, and ∼1.6×10
9 s
-1for Cu), consistent with a marked metallic character of the adsorbed molecules.
Pre-dosing of alkali atoms onto the metal substrates drastically changes the IR spectra recorded during subsequent C
60 deposition: anti-absorption bands, as well as an increase of the broadband reflectance, occur and are interpreted as due
to strong electron–phonon coupling with induced surface states.
Received: 6 June 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002
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