Modulated interferogram

Figure 6. SNIFTIR spectra of the adsorbed intermediates involved in the oxidation of 0.1 M CHjOH in 0.5 M HCIO4 on a smooth Pt electrode (p-polarized light modulation potential AE = 0.3 V averaging of 128 interferograms). Electrode potential (mV/RHE) (1) 370, (2) 470, (3) 570, (4) 670, (5) 770.

A Fourier transform infrared spectroscopy spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitized. A digital computer initially records and then processes the interferogram and also allows the spectral data that results to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

The sample is submitted to radiation modulated by the interferometric device. Temperature variations within the sample are translated into pressure variations within the cell. A microphone is used to detect the sound (this is called photoacoustic detection). The acoustic interferogram obtained from the pressure waves is converted into a classical spectrum. 

The cosine Fourier transform given by Eq. (2) is only applicable if the interferogram is perfectly symmetrical about <5 = 0. In practice additional wavenumber-dependent phase shifts are present, owing to beamsplitter characteristics or refraction effects, and cause the interferogram to appear partially asymmetric. The modulated part of Eq. (1) then becomes... 

For a finite sampling interval A<5, more than one superposition of cosine/sine waves can give rise to the recorded interferogram. For the transformed spectrum to be unique, the sampling interval A<5 must be sufficiently small to detect modulations in the interferogram due to the shortest wavelength present in the spectrum, the so-called Nyquist criterion [66] ... 

In most pulsed NMR experiments, the rf field is applied off-resonance. Modulated pulse interferograms (Fig. 2.4(e), 2.5(a), 2.6(a), and 2.7(a)) arise because the vectors of transverse magnetization do not precess with a constant phase shift of itj2 relative to the vector Bj. This is demonstrated in Fig. 2.8. The transverse magnetization is then a resultant of two components, t>(f) with a phase shift of n/2 relative to B, and u(r) in phase with Bt ... 

Bouwman and Freriks (18.19) have also noted the advantages of FT spectroscopy and have used this technique to study the adsorption of CO on a silica-supported nickel catalyst at temperatures between 70 and 180°C. These authors point out that FT spectroscopy is particularly advantageous for in situ observation of heated samples since the radiation emitted by the sample is not modulated by the interferometer and hence does not contribute to the fluctuating portion of the interferogram. 

In FTIR, the incident radiation is automatically modulated in an acoustic frequency, so no special devices (such as choppers) are necessary. The modulation frequency of the interferogram is given by ... 

The next class of VCD instruments to be developed was centered around a Fourier transform infrared (FT-IR) spectrometer. The idea was to design the sample compartment to be the same as in a dispersive VCD instrument, including a photoelastic modulator. To measure VCD, the detector signal is first sent to a lock-in amplifier to demodulate the high-frequency polarization modulation. The output of the lock-in is a VCD interferogram which is Fourier transformed in much the same way as the ordinary transmission interferogram. 

If the detector signal bypasses the lock-in and is Fourier transformed directly, the ordinary transmission is obtained. Since there are two interferograms generated simultaneously by the instrument, this general approach to measurements with a FT-IR spectrometer is called double modulation [5,6,11]. The concept of carrying... 

Spectrometers can be devided into two groups (a) scanning spectrometers, where the frequency (wavelength) of the radiation is continuously scanned and the radiation is simultaneously measured, and (b) Fourier spectrometer, where all frequencies (wavelengths) are modulated by an interferometer, and simultaneously detected. The interferogram is Fourier transformed to generate the spectrum. Scanning spectrometers are usually... 

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