In this lecture, we started with the concept of a surface enhanced raman spectroscopy. It was observed that it is a powerful measurement technique, but the Raman scattering is a very weak effect. We see that the surface plays a role in this detection method as it is metal that has a huge abundance of electrons. The electric field of coherent light oscillates the electrons as a collective group. The surface is also rough which allows the electrons to oscillate in a direction parallel to the surface. The result is a buildup of extremely large electromagnetic fields at "hot spots" on peak tips. This produces the electromagnetic enhancement effect in which a molecule experiences a greater electromagnetic field than it would ordinarily. It also produces the chemical enhancement effect in which there is a transfer of electrons between the metal and molecule. When the molecule comes into close physical contact with the surface it can be adsorbed or chemically bonded. The electromagnetic enhancement theory is then looked at in relationship to the sphere model and we see that the greatest Raman coupling occurs when the molecule is adsorbed on the surface and oriented with its main axis perpendicular to the surface. The scattering strength and wavelength is strongly dependant on the size of the metal sphere. The enhancement factor is then looked at and how the plasmon Electric field induces a dipole in an adsorbed molecule at the laser wavelength. The molecule emits a Raman-shifted electromagnetic field at a new wavelength. The new wavelength also induces a second dipole in the sphere at the Raman-shifted wavelength. After that examples and are shown in relationship to nanospheres and the Surface Enhanced Raman Spectroscopy.
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University of Illinois, Urbana-Champaign, IL
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