Polarized electronic infrared spectra of CoCl2 single crystals

Polarized infrared absorption spectra of single crystal CoCl₂ from 8 to 300 °K with light propagated parallel and perpendicular to the trigonal axis are reported. At 8°K spectra are presented in magnetic fields from 3 to 43 kG. Three polarized lines arising from magnetic dipole transitions are identified as transitions to electronic levels within the lower ⁴T_1g manifold of a Co²⁺ ion. A line at 956 cm⁻¹ (3 kG) and 960 cm⁻¹ (43 kG) of oscillator strength ${\text{1.65× 10}}^{\text{−8}}$ , and a second line at 977 cm⁻¹ (3 kG) and 980 cm⁻¹ (43 kG) of oscillator strength ${\text{2.5× 10}}^{\text{−8}}$ , are transitions to components of a Kramers doublet. Their separation of 21 cm⁻¹ measures the splitting of an excited electronic level of a Co²⁺ ion in the exchange field of ground state neighbors. A third line at 1007 cm⁻¹ (3 kG) and 1010 cm⁻¹ (43 kG) is the transition to the next higher Kramers doublet whose exchange splitting is not resolved. Calculations of positions, polarizations, and oscillator strengths of magnetic dipole transitions are in accord with the observations. A broad band is observed at 8 °K between 1100 and 1500 cm⁻¹. A structured absorption between 300 and 600 cm⁻¹ is reported in the unpolarized axial spectrum.