Antiresonance in the Optical Spectra of Transition-Metal Ions in Crystals

When a sharp absorption line of a defect or impurity center is overlapped by a broad vibronic band, interference can occur between the two types of transition. We find several examples of this effect in the optical spectrum of V²⁺ in octahedral fluoride coordination. The spectral line shapes can be fitted well by the four-parameter theory of Fano, which was developed to account for the interference of a sharp intra-atomic transition with an overlapping ionizing continuum. We justify the application of this theory to the vibronic problem, and calculate some of the parameters in terms of other spectroscopic data, obtaining good agreement with experiment.