Enhanced Infrared Absorption of C60 on Thin Evaporated Pd Island Films
Infrared transmission spectra of C 60 * 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 1500-1100 cm-' region exhibited bands at 1444, 1429, and 1182 cm-' due, respectively, to the A 8 (2), T 1u (4), and T 1u (3) modes. The appearance of the A 8 (2) mode, which is originally infrared inactive (Raman active), reveals electron transfer from the metal to chemisorbed C 60. Indeed, increasing the thickness of C 60, the A g (2) mode intensity saturated more rapidly than the T 1u (4) and T 1u (3) modes. The originally infrared active T 1u (4) and T 1u (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 C 60 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 1u (4) and T 1u (3) modes.
Infrared absorption enhancement of monolayer species on thin evaporated Ag films by use of a Kretschmann configuration: Evidence for two types of enhanced surface electric fields
Infrared absorption enhancement of m- and p-nitrobenzoic acid deposited on thin-evaporated silver films has been investigated using the Kretschmann's ATR coupling method. The absorption spectra provide direct evidence that enhancement is prominent only for vibrations of the first monolayer adsorbed on the Ag surface. It is shown that all of the vibrational modes observed obey the normal dipole selection rule. Moreover, it was found that there exist two types of absorption enhancement; the first is enhanced (∼300) by both p- and s-polarized radiation, and the second is enhanced (∼60) by p-polarized radiation alone. The Ag film thickness optimum is about 50 Å in the former case and about 200 Å in the latter. The enhancement insensitive to the polarization state of radiation can well be explained by the excitation of the transverse collective electron resonance of the Ag islands, whereas that obtained by p-polarized radiation may be due to the excitation of delocalized surface plasmons modified by surface roughness. The image-dipole effect may also be significant in the mechanisms.
Chemisorption on Copper
Although it is much less active than the transition metals copper does catalyse reactions of hydrogen and carbon monoxide. The nature of the weak adsorptions of these gases has been investigated by surface potentials, low energy electron diffraction, and infrared spectroscopy. Hydrogen is dissociatively adsorbed by copper but the adsorption is activated. Surface potential measurements have provided isotherms and isosteric heats of adsorption. Carbon monoxide is adsorbed in two distinct stages giving positive and negative surface potential changes. The positive stage is accompanied by a sharp infrared absorption band near 2100 cm−1 which is sufficiently strong to permit reflection spectroscopic studies on single crystal surfaces. Frequency shifts occur during the negative stage. Direct comparisons of surface potentials, diffraction patterns, and vibrational frequencies have been made and will be discussed in relation to the chemisorption bond.