Knowledge Bank

The near-normal-incidence infrared reflectivity of an intense fundamental vibrational mode in a crystal is large between the transverse and longitudinal optic mode frequencies and usually exhibits no fine structure. However, in a few cases anomalous reflectivity minima are observed in the transverse-longitudinal band gap. These may be due to surface defects, surface modes, or resonant interactions between the fundamental and an overtone, combination, or a multi-phonon process. Data are presented for several crystals with anomalous reflectivity minima, including $KBrO3,Na14NO3$, and $Na15NO4$. The spectra are discussed in terms of the possible explanations described above.