A Method for the Determination of the Zero-Path-Difference Position in a Fourier Transform Infrared Spectrometer: Application to Magnetic Vibrational Circular Dichroism

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Abstract
Some modern FT-IR spectrometers, without a white light source, use a pre-scan of the signal to determine the zero path difference for purposes of scan initialization. The interferograms in vibrational circular dichroism (VCD) and other low signal-to-noise ratio experiments have little or no centerburst, which can cause problems. This can lead to wasted data collections and to instabilities between successively collected blocks of scans. In this modification, a PbSe detector is used to detect frequencies above 2200 cm−1 extracted from a small fraction of the beam. This near-IR signal is then added to the demodulated differential signal to enable the instrument to initialize scans. Since, in this example, the VCD spectra are typically obtained below 2000 cm−1, there is no spectral interference between the two detector signals. However, use of two signals from different detectors can lead to phase problems, which have been solved with the use of polarization calibration methods. As an example, construction of such a modified spectrometer for magnetic VCD measurement is described in detail. At 0.5 cm−1 resolution, these modulated spectra evidence resolution of the rotational structure in the bands. Such data were unobservable with the unmodified instrument.