Dichroic infrared absorption of dipole centers in cadmium halide crystals

Dichroic infrared (IR) absorption measurements on CN− or OH− centers in cadmium halide crystals were carried out at 6 K with a high spectral resolution of 0.025 cm−1 at 2000 cm−1, by using a FT-IR spectrometer. Several sharp absorption lines with widths less than 0.1 cm−1 are observed in the energy region of the stretching vibration, i.e. 2000–2250 cm−1 for CN− and 2500–4500 cm−1 for OH−. These lines are classified into three groups attributed to: (1) an isolated center simply substituted for a halogen ion, (2) an interstitial center located between the cadmium and halogen ion sheets and (3) a coupled center with an anion vacancy. A doublet structure is observed in CdI2 which comes from different halogen-ion sites in the 4H-polytype crystal. The isolated CN− centers in CdCl2 and CdBr2 align toward Cd2+ ions, away from the direction of the c-axis. Almost all of the coupled center with a vacancy lies in a halogen-ion plane. The doublet structure of the coupled center in CdCl2 and CdBr2 is connected to the tunneling splitting of a vibrational level.