Surface enhancement of optical fields

Abstract

The enhancement of electromagnetic processes on metallic surfaces is discussed in view of applications in surface photochemistry and surface analytics. The strongest enhancements are found on substrates that consist of discrete metal particles. A theoretical description for plasmon resonances of such particles is developed, which includes the effects of finite particle size and of dipolar interactions between the particles. Spheroidal particles with dimensions in the range 10–100 nm are found to produce the strongest enhancements; the role of particle interactions in shifting and broadening the plasmon resonances is discussed.

Potential applications of the amplified local fields for chemical processes include photochemistry, chemical vapour deposition, and etching. Storage of excitation energy in the bonds to be transformed is necessary for these reactions. The enhanced excitation rate is competing with radiationless energy transfer to the substrate. This rate must be reduced by a spacer layer between metal and adsorbate, in order to obtain a net increase in the population rate of excited states from the enhanced local intensity.

Surface enhanced Raman scattering is developing into a powerful analytical tool. The method combines submonolayer sensitivity with the inherent advantages of Raman spectroscopy, i.e. molecular specificity and compatibility with reaction pressure environments. Applications in heterogeneous catalysis for the in situ observation of catalyst surfaces and the detection of surface intermediates are discussed. The method is used to study dehydroamination reactions on copper catalysts; spectroscopic results on the bonding of amines to the surface are presented.