Use of polarization modulation Fourier transform infrared spectroscopy

Within the past decade, polarization modulation has been successfully combined with Fourier transform infrared interferometry. The majority of measurements have employed the double modulation technique to measure vibrational circular dichroism.1 With this approach, polarization modulation is effected by a photoelastic modulator operating at a frequency well above the interferometric Fourier frequencies. Demodulation is first carried out for the polarization modulation followed by Fourier transformation of the band of Fourier frequencies. Sensitivity to polarization difference spectra as small as 6 orders relative to the parent absorption spectra has been achieved by this approach. More recently, a second approach to polarization modulation interferometry has been proposed2 termed interferometric modulation. In this method, the conventional beam splitter is replaced by a polarizing grid beam splitter after the design of Martin and Puplett.3 The output of such an interferometer is pure polarization for each wavelength at its own Fourier frequency without, in principle, any intensity modulation. Conversion to intensity modulation is achieved by insertion of a polarizer from whence phase correction can be obtained. Relative to this phase correction, without the polarizer in place, linear dichroism is obtained by the real part of the complex Fourier transform, and circular dichroism is obtained from the corresponding sine transform.