Enhanced infrared absorption of C₆₀ on thin evaporated Pd Island films

Infrared transmission spectra of C₆₀ multilayers on thin Pd films deposited onto surface-oxidized Si(100) and hydrogen-terminated Si(111) substrates are reported. In both cases, the spectra in the 1300-1100 cm ^-1 region exhibited bands at 1444, 1429, and 1182 cm^-1 due, respectively, to the A_g (2), T_lu (4), and T _lu (3) modes. The appearance of the A_g (2) mode, which is originally infrared inactive (Raman active), reveals electron transfer from the metal to chemisorbed C₆₀. Indeed, increasing the thickness of C ₆₀, the A_g (2) mode intensity saturated more rapidly than the T_lu (4) and T_lu (3) modes. The originally infrared active T_lu (4) and T_lu (3) modes were enhanced in intensity depending upon the Pd thickness. Actually, while both substrates gave nearly the same magnitude of enhancement, the optimum Pd thickness was smaller on the hydrogen-terminated surface than on the surface-oxidized surface. On the other hand, the A_g (2) mode was less intense on the hydrogen-terminated surface than on the oxidized surface, suggestive of a shortage of chemisorbed C₆₀ and thus pointing out the importance of the metal film morphology. Indeed, Pd films deposited on the two substrates gave rise to quite different AFM images. We also show that, regardless of the substrate, the A_g (2) mode is an order of magnitude smaller than for Ag deposition, though no remarkable intensity differences were observed with respect to the T_lu (4) and T_lu (3) modes.