Inhomogeneous broadening of infrared and Raman spectral bands of amorphous and polycrystalline thin films

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Abstract
Infrared and Raman spectroscopy are used to examine effects of solid structure on vibrational line shapes in thin solid films of CO2 and N2O prepared by vapor deposition. A new method is described for forming films consisting of weakly‐interacting microcrystals by deposition of the solute in an argon matrix followed by complete evaporation of the argon. The infrared absorption bands of these films exhibit inhomogeneous spectral broadening in the range between the transverse and longitudinal frequencies of the bulk crystal. The origin of this broadening is shown to be the dependence of vibrational frequencies on size and shape of the microcrystals due to long‐range dipole interactions. In contrast, the spectra of polycrystalline films formed by direct vapor deposition are the same as for single crystals because the long‐range dipole interactions effectively couple vibrational motions across the grain boundaries. An analysis shows that inhomogenously broadened spectra are obtained whenever the separation between the individual crystal grains is comparable to or greater than their dimensions.