INFRARED REFLECTION-ABSORPTION SPECTROSCOPIC STUDY ON ADSORPTION STRUCTURES OF ACRYLIC ACIDS ON EVAPORATED SILVER FILMS UNDER ULTRA-HIGH-VACUUM CONDITIONS

Infrared reflection absorption spectra were measured under ultra-high vacuum conditions for acrylic acid adsorbed on evaporated silver films. The spectral features especially associated with out-of-plane vibrations including an OH out-of-plane bending (gamma(OH)) vibration were found to be very useful to identify the adsorption structures. The features at 90 K clearly indicate that, upon increasing exposure levels, the adsorbates form type-A, -B and -C states successively. The type-A state coordinates directly to the surface through the C=O group; upon forming a few monolayers the adsorbate occurs mainly in the type-B state and on a further increase of monolayers the adsorbate is converted to the type C. Both type-B and -C states form a hydrogen-bonded cyclic dimer with the molecular plane parallel to the surface. The dimeric structure of type B is perturbed by an interaction with the surface, as judged from appreciable spectral changes of the out-of-plane vibrations compared to those of the type C. The type C forms a state similar to acrylic acid in a crystalline state. The gamma(OH) band of the type C, however, is observed at 916 cm-1, which is higher than that of the crystalline state by 21 cm-1, indicating that stacking of the dimeric units in the type-C adsorbate exerts a stronger restraining force on the OH group than stacking in the crystalline state.