An instrument for the measurement of circular dichroism (CD) between 5000 cm−1 and 750 cm−1 is described. This instrument also can perform high precision linear dichroism measurements in the same wavelength range. A germanium photoelastic modulator having a large linear and angular aperture is used to vary the polarization of monochromatic ir light between left- and right-circularly polarized at 8 kHz. This polarization-modulated light traverses the sample and falls onto a cooled InSb or HgCdTe detector. Phase-sensitive demodulation yields the CD of the sample to a precision of 4 × 10−6 o.d. units near 3000 cm−1. The instrument has been used to study ir circular dichroism arising from molecular vibrations in cholesteric liquid crystals, crystals, and fluids. CD arising from low-lying electronic transitions of metal complexes and linear dichroism of polymer films have also been studied.
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