Abstract
Normal-incidence infrared absorption has been observed for silver overlaid C<sub>60</sub> thin films formed on surface-oxidized Si(111) substrates as a function of the silver or C<sub>60</sub> film thickness. The absorption spectra exhibit bands at 1429 and 1180 cm<sup>-1</sup> due, respectively, to the infrared active T<sub>lu</sub> (4) and T<sub>lu</sub> (3) modes of C<sub>60</sub> in multi-layers. Additionally, two bands appear at 1442 and 1370 cm<sup>-1</sup>. The former band is caused by activation of the infrared inactive (Raman active) A<sub>g</sub> (2) mode via electron transfer from the silver to adsorbed C<sub>60</sub>, and the latter is assigned to the T<sub>1u</sub> (4) mode red-shifted by the charge transfer. These bands are all enhanced in intensity dependent either upon the silver or C<sub>60</sub> thickness, i.e., the largest absorption enhancement is obtained for 25-monolayers-thick silver and 12-nm-thick C<sub>60</sub>. Under atomic force microprobe inspection, the average size and height of the islands in the overlaid 25-monolayers-thick silver change with underlying C<sub>60</sub> film thickness. The influence of the C<sub>60</sub> film structure upon the silver film and in turn the absorption intensity is strongly suggested.
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