General Description

A Fourier Transform Infrared (FTIR) Spectrometer uses the technique of Michelson interferometry. A beam of radiation from the source, S, is focused on a beam splitter constructed such that half the beam is reflected to a fixed mirror. The other half of the beam is transmitted to a moving mirror which reflects the beam back to the beam splitter from where it travels, recombined with the original half beam, to the detector, D.

Figure 1: Schematic of a Michelson interferometer. S - source; D - detector; M1 - fixed mirror; M2 - movable mirror; X - mirror displacement.

The IR intensity variation with optical path difference (interferogram) is the Fourier transform of the (broadband) incident radiation. The IR absorption spectrum can be obtained by measuring an interferogram with and without a sample in the beam and transforming the interferograms into spectra.
The method of FTIR is fast and overcomes the disadvantage of the measurement of one resolution element at a time (Multiplex or Fellgett Advantage) and avoids the drawback of the energy entering and leaving the monochromator being limited by a narrow slit which reduces the signal to noise ratio (Throughput or Jacquinot Advantage).

FTIR Experiments in our Laboratory

FTIR can be used as a powerful tool for investigating molecular conformation and the hindered rotation of lateral groups around the molecular long axis in chiral SmC liquid crystals. With polarized incident radiation, the IR spectrum yields orientational information about selected chemical groups.

We are currently investigating the rotational hindrance of carbonyl and chiral groups by measuring the IR absorption spectra of various FLC materials as a function of incident polarization and the sign of the applied electric field. The resultant spectra allow us to calculate the orientational distribution of lateral groups around the molecular long axis. An example showing hindered rotation of the carbonyl group is illustrated in Fig.2.

Figure 2: IR absorbance vs. polarizer rotation angle with respective to rubbing direction for a FLC from Ajinomoto Co. Ltd.for both signs of applied electric field. At 0 degrees the IR polarization is parallel to the rubbing direction. The carbonyl absorbance profiles (1756 and 1716 inverse cm) show quite different dichroic ratios and behavior under electric field. Note that the absorbance profile of the carbonyl peak at 1716 inverse cm is not symmetric with respect to that of phenyl ring stretching (1608 inverse cm), whose dipole moment direction is nearly parallel to the molecular long axis.

We are also studying the switching dynamics of selected functional groups using time-resolved FTIR with nanosecond resolution.

For more information on this project, contact Noel Clark.

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