Dipole-dipole-interaction-induced line narrowing in thin-film vibrational-mode spectra

Under certain circumstances the electric dipole interaction can reduce or eliminate the effects of spectral inhomogeneous broadening on an infrared absorption line of a thin film. The oscillating electric dipole moment must be oriented perpendicular to the film surface, and the dipole interaction, as measured by the frequency shift that it induces, must be larger than the width of the inhomogeneous frequency distribution. An example of this interaction-induced line narrowing is presented for a vibrational mode of a KReO4 film, and the relevance of this mechanism to two-dimensional electron-gas subband transitions and adsorbate vibrational modes is considered.