Far Infrared Reflection Spectra of AgCI, AgBr, and Agl at Low Temperatures

The reflection spectra of AgCl, AgBr, and AgI are measured at liquid hydrogen, liquid nitrogen, and room temperatures. For AgCl, they show distinctly two components. For AgBr and AgI, the structure, though complex, is not resolved. Nevertheless, in all cases, except for AgI, at low temperature, the Lorentz analysis needs two oscillators. The stronger one v(1) corresponds to the TO transition at point Gamma, the second one v(2), about an order of magnitude smaller, corresponds to a TA + TO addition band for the NaCl type structure, and to a TA + LA for the zinc blende or wurtzite structure, confirming preliminary measurements made by a completely different experimental approach (transmission of thin evaporated films instead of reflection on bulk crystals). They are, however, slightly distorted towards high frequencies. The proposed frequencies for the TO branch at point Gamma are, for AgCl: 105 cm(-1) (290 degrees K), 114 cm(-1), (80 degrees K), 119 cm(-1) (25 degrees K); for AgBr: 79 cm(-1) (290 degrees K), 84 cm(-1) (80 degrees K), 85 cm(-1) (25 degrees K), and for AgI: 103 cm(-1) (290 degrees K), 106 cm(-1) (80 degrees K), 106 cm(-1) (25 degrees K). The oscillator strengths and the damping of both oscillators are studied vs temperature. A large decrease of the damping is still observed between liquid nitrogen and liquid hydrogen temperatures. The refractive index and the absorption index are determined in the whole ir for the first time at liquid hydrogen, liquid nitrogen, and room temperatures, showing large variations, particularly a high transparency in the very far ir at liquid hydrogen temperature. The maximum values of n and k range from 2 to 10. The real and the imaginary parts of the dielectric constants are also computed for the whole ir range at room and lower temperatures down to liquid hydrogen temperature.