Phase-modulated step-scan interferometer

No matter what modulation frequency is used for PA/FT-IR spectrometry, the bands from the upper layers of the sample always dominate the spectrum. In addition, the fact that the thermal wave decay length varies as when the spectra are measured with a rapid-scanning interferometer has always led to suboptimal results. The variation of L with wavenumber may be circumvented through the use of a phase-modulated step-scan interferometer, and most contemporary PA/FT-IR spectra are now measured with this type of instrument. 

In Chapter 19 we saw that kinetic processes cannot be studied with a conventional rapid-scanning interferometer when the reaction rate is so fast that the reaction is essentially complete by the time just one or two interferograms have been measured. Instead, very fast reactions must be repeated at each retardation step of a step-scan interferometer. A different but related approach to the measurement of reversible dynamic systems can also be made with step-scan interferometers. In this case, however, very small changes in the state of the sample are introduced by subjecting it to a modulated perturbation of some type. Dynamic information can be obtained when the phase of the induced signal lags behind the phase of the perturbation by several degrees. An example of a reversible modulated process is when a polymer film is subjected to a modulated uniaxial strain, and this measurement is discussed in some detail in the first four sections of this chapter. 

In Chapter 20 we saw how photoacoustic (PA) spectra could be measured with a step-scan interferometer no matter whether the PA signal was demodulated with a lock-in amplifier or by digital signal processing (DSP). For DSP, a Fourier transform (FT) has the same function as the lock-in amplifier. Manning et al. [14] showed that the same approach is feasible in DIRLD spectrometry with a step-scan FT-IR spectrometer but without a PEM. Consider the case where the detector signal contains components caused by simultaneous sinusoidal phase modulation at frequency /pm, and sample modulation at frequency fs. The phase- and sample-modulated components of the signal can be demodulated either with a... 

Step-scanning interferometers can enhance the photoacoustic technique, because the variable modulation frequency of the scanning interferometer is decoupled. Thus, a constant modulation frequency is applied to all wavelengths in the spectral range. One can modulate the IR beam via phase modulation in which an interferometer mirror dithers to oscillate the retardation about the set point of each interferometer step. With this approach, all of the frequencies are modulated synchronously, and lock-in modulation can be used to extract the phase-modulation [53]. When phase-... 

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