Enhanced Raman spectroscopy at dielectric interfaces

The examination of the merits of a new type of sampling structure for performing enhanced Raman scattering is presented here. These structures typically consist of 50 A of SiO$\sb2$ sputtered on Ag or Au island films, all coated on a lithographically defined opening in a masked SiO$\sb2$ substrate and use internal reflection in the SiO$\sb2$ substrate to excite plasma resonances in the island films with higher incident field strengths than possible with external excitation. Enhanced scattering from adsorbate molecules adsorbed on top of the SiO$\sb2$ overlayer is driven by the long ranged electromagnetic enhancement provided by the resonantly driven island film particles. Evidence for the long range enhancement comes from chemical passivation, scanning Auger, and distance dependence experiments, as well as comparing enhanced Raman scattering signals obtained from coated and uncoated Au island films. The dependence of the scattering on the excitation angle on incidence can be interpreted in terms of the strengths of the evanescent field components at the substrate-metal interface. Evidence suggesting that $p$-nitrobenzoic acid (PNBA) photodegrades to an azodibenzoate species when adsorbed on top of the SiO$\sb2$ overlayers as a result of laser illumination is obtained.The ability to obtain polarized Raman scattering for monolayer adsorbates on these structures has been examined. The relationship of the relative intensities of the in-plane enhanced electric field components to the totally symmetric vibrations of p-nitrobenzoic acid and phthalazine was found to indicate a constant depolarization on the in-plane electric field components induced by the island film particles themselves. With this information and polarized Raman scattering data from non-totally symmetric phthalazine vibrations, the ability to determine the average surface molecular orientation of phthalazine monolayers at sputtered SiO$\sb2$ surfaces is reported.The ability to utilize these structures for sensitive vibrational spectroscopic analyses of different types of monolayer adsorbate systems is addressed. Systems which have been examined include molecular self assembled films, Langmuir-Blodgett assemblies, and adsorbed protein layers. Implications for extending the scope of these experiments will also be discussed.U of I OnlyETDs are only available to UIUC Users without author permissio