Infrared and Raman spectroscopy are used to examine effects of solid structure on vibrational line shapes in thin solid films of CO 2 and N 2 0 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.

Mixing of near-degenerate vibrational modes by long-range dipole–dipole interactions can lead to complicated band shapes in the infrared spectra of clusters and microcrystals, particularly when the bands are inhomogeneously broadened as a result of distributions of crystal shapes and orientations in the sample. For the special case in which the clusters have isotropic crystal structures, the equivalence of amplitude vectors for all IR-active vibrational phonon modes permits the band shapes of dipole-mixed modes to be predicted from those of isolated modes. This analysis is successfully applied to experimental infrared spectra of thin films containing weakly interacting microcrystals of CO2 or N2O, as well as to CO2 clusters prepared by condensation in the gas phase. The results prove that these gas-phase clusters of CO2 have an isotropic crystalline structure.

Dipole–dipole interaction between molecular vibrations is the most important source of inhomogeneous line broadening in the strongest infrared absorption bands of amorphous films of N2O and O3 prepared by vapor deposition at cryogenic temperatures. This type of broadening occurs in all strongly allowed absorption bands of amorphous solids. Comparison of spectra obtained for amorphous and polycrystalline ozone show that the crystal structure is anisotropic, and most likely uniaxial. The absorption line shape of amorphous films is satisfactorily predicted by a computational model in which orientationally disordered molecules on a regular lattice interact through long-range dipole interactions. It is shown that the normal vibrations are delocalized throughout the solid, and the absorption line shape depends on the macroscopic shape and orientation of the sample.

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