Optical absorption spectra of alkali halide crystals containing H⁻ ions (U-centres) are calculated using a two-parameter model to describe the defect. It is shown that this model gives a satisfactory account of the observed anharmonic sideband of the main U-centre absorption band in the infrared, and also of the impurity-induced far infrared absorption for the two host crystals considered, potassium bromide and potassium iodide. The broadening of localized modes and resonances is also discussed. In particular, the anharmonic broadening is calculated for the main U-centre line in KI and KBr and for a localized gap mode in KI:KH. / Thesis / Doctor of Philosophy (PhD)