Infrared circular dichroism and linear dichroism spectrophotometer.

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 x 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.