Local environment of molecules adsorbed on colloids: A high-pressure-surface-enhanced raman study

Surface-enhanced Raman spectroscopy is used to study molecules adsorbed at the metal/liquid interface under conditions of high hydrostatic pressures (up to 4000 bar). Three different but related colloidal preparations were investigated: a concentrated interfacial silver colloidal film, a dilute silver organosol, and a dilute silver hydrosol. The results are discussed in terms of the local environment at the adsorption site. We find large blue shifts of the frequencies of vibration of adsorbed species with pressure, as compared to shifts observed for molecules in the bulk. We also find vibrational modes which are screened from the effect of the external pressure by the rigid and nonisotropic nature of the adsorbed layer. Sensitivity to the precise conditions near the surface was observed, enabling a study of the initimate details of adsorbed layers.