Model to explain the effects of halide ions on the increase in surface enhanced Raman spectral intensity over time

Understanding the mechanisms responsible for the large increase in spectral intensity when molecules are adsorbed to nanoparticle surfaces such as occurs during surface enhanced Raman (SER) spectroscopy will allow scientists to probe ever smaller scales, even allowing single molecule detection. One particular scenario that increased the SER scattering efficiency was the addition of halide ions to Rhodamine 6G (R6G)-ethanol solution. This thesis presents a theoretical model explaining the effects of halide ions on the SER spectral intensity of the Rhodamine 6G (R6G) molecule when co-adsorbed to a silver nanoparticle surface. Glaspell et al. 2005, found a linear correlation between the increase in spectral intensities of selected vibrational normal modes of R6G over time and the polarizabilities of co-adsorbed halide ions. When the R6G molecule co-adsorbs to the silver nanoparticle surface with the halide ions, the molecule is exposed to three external electric fields that add vectorially, creating a total external electric field. Modelling the fields from the halide ions and the silver nanoparticles as electric dipole fields introduces the polarizability of the halide ion linearly into the Raman spectral intensity equation. This model also shows that there is a necessary interaction between the halide ions and the silver nanoparticle surface in order to see the effects as described by Glaspell et al. Furthermore, we will present experimental results that show that there is a necessary interaction between the halide ions and the nanoparticle surface. Without this interaction there was no increase in the SER spectral intensity of R6G or pyridine molecules in solution with the halide ions but without the silver nanoparticles.