Analysis of polarized Raman spectra of liquids at liquid-solid interfaces

Polarized Raman spectra of liquids within a solid—liquid interface were analysed. The liquid, deposited on the solid surface, is illuminated by an evanescent electromagnetic wave; the penetration depth is l ≤ 100 nm. Interface liquid spectra are compared with the polarized spectra of the isotropic liquid. It is found that the changes observed in the spectra cannot be explained merely by structural changes in the molecules; the influence of changes in the main characteristics of the exciting electric field, arising from defects of the solid surface, must also be considered. With rutile surfaces, the experimental spectra are linear combinations of the isotropic liquid spectra; the electric field, partially depolarized by the surface defects, is still a long‐range field: no structural changes for the molecules are found within the illuminated part of the interface. With thin silver film‐coated surfaces, however, the spectra cannot be reduced to linear combinations of the liquid spectra. Structural changes for the molecules are also involved. The electric field is enhanced by the excitation of the surface plasmon of silver but, because of the surface defects, the field, partially depolarized, spreads over a range much shorter than l, and is strongly dependent on the defect geometry. The short‐range enhancement can mask a possible further orientation of the molecules, preventing any determination of the range of the solid—liquid interaction. Time dependence of the spectra related to an increase in the silver surface roughness leads to surface‐enhanced Raman scattering (SERS).