The infrared extinction spectra of molecules adsorbed to gold films over periodic particle arrays in the infrared have the potential to show enhanced extinction at wavelengths that are resonant both with vibrations of adsorbed molecules, and with the localized surface plasmon. A third type of resonance that can contribute to such extinction spectra are diffractive resonances. An experiment was carried out in which four distinct hexagonally packed arrays of polystyrene spheres, having diameters of 3, 4, 5 and 6 microns respectively, were prepared on calcium fluoride windows and coated with gold. FTIR microscopy was used to obtain spectra of each gold-coated array. In these spectra the envelope of the gas phase CO2 absorption at ~2350 cm-1 undergoes characteristic changes in shape associated with interference of multiple resonances. The interferences are not observed in the water vapor bands nor in the 670 cm-1 CO2 band. The interference effects were reproduced with a second set of arrays that were independently produced and analyzed. The periodicity of the interferences appears to scale with the sphere diameter, as the spectral line-shape at ~2350 cm-1 from the three micron arrays is replicated in the spectra from the six micron arrays. We present a preliminary model of the spectra that incorporates the infrared absorption of molecular carbon dioxide, as well as the localized surface plasmon resonance (LSPR), and diffractive resonance of the gold-coated film over microsphere array.

Interferences in reflected infrared extinction spectra from a gold-coated periodic particle array

251st National Meeting (2016)

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