Picosecond‐excited surface‐enhanced hyper‐Raman scattering (SEHRS) spectra have been obtained for several non‐centrosymmetric oxa‐ and thia‐carbocyanine molecules adsorbed on pre‐aggregated colloidal silver. To facilitate interpretation of the SEHRS results, a comprehensive vibrational study of these dyes by employing Fourier transform (FT) IR spectroscopy, surface‐enhanced Raman scattering (SERS) and the newly developed techniques of near‐IR‐excited FT‐Raman and FTSERS is also reported. The SERS spectra of these carbocyanine dyes are dramatically different from their bulk FT‐Raman and SERS spectra, but exhibit similarities to their FTIR spectra. Many SEHRS lines appear to arise from vibrational modes of the central π‐conjugated chain, in contrast to the appearance of predominantly end‐chromophore vibrations in the Raman spectra. Some of the intense surface hyper‐Raman lines are found to be experimentally undetected or unresolvable by Raman scattering and IR absorption. The results thus conclusively demonstrate that, for non‐centrosymmetric molecules, surface‐enhanced hyper‐Raman scattering is capable of providing important vibrational information that is unobtainable through the Raman and IR methods.
ASJC Scopus subject areas
- Materials Science(all)