Interferometers step-scan type

It should be mentioned here that a different type of interferometer, called the step-scan type, is also commercially available. In this type, the movable mirror is stopped stepwise at equal intervals at specified positions on the OPD axis, and the interference of the two beams is measured at each position. This type has the advantage that the sampling of interferogram can be carried out independently of the travel of the movable mirror. Because of this advantage, the step-scan type is particularly useful for photoacoustic, time-resolved, and two-dimensional correlation spectroscopic measurements, which will be discussed in Chapters 14, 20, and 21 respectively. (Historically, the step-scan type had existed before the continuous-scan type was developed.)... 

In its basic design, the equipment is similar to a 2-D TL glow-curve system as described previously, but with the addition of a modified Twyman-Green, Michelson type, interferometer between the oven and the photomultiplier. As the sample is heated, the TL signal is recorded while the movable mirror of the interferometer is scanning a given optical path difference in a preset number of steps. The interference pattern corresponding to each one-way scan... 

Another major Hmitation arose from the need to employ a step-scan interferometer. This necessity arose from the relatively slow read-out rates of these first-generation FPAs, which were of the order of only a few hundred Hertz. The read-out rate (or frame rate) of a FPA detector determines the type of interferometer that must be used for FT-IR imaging, as the FPA cannot be triggered (for data transfer) any faster than its maximum read-out (frame rate) speed. As the first-generation FPAs were only capable of frame rates in the hundreds of Hz, and rapid-scanning interferometers required a faster frame rate, the use of step-scan... 

The schematic in Figure 7.3 shows a simplified layout for an FTIR microspectrometer based on a staring-type FPA detection system. The FTIR spectrometer is similar (if not identical) to that used in the scanning instrument, except that it may require step-scan to allow time for reading out all pixels of the FPA at each optical retardation (scanning mirror position) of the interferometer. The system also uses Schwarzschild objectives, but the apertures for constraining the microscope s sensitive location are left open, i.e. they do not provide any spatial discrimination. Thus, the microscope s first objective... 

There are several types of measurements for which standard rapid-scanning interferometers may be inappropriate. These include hyperspectral imaging (Section 14.5), high-speed time-resolved spectrometry (Section 19.2), photoacoustic spectroscopy (Section 20.3), and sample modulation spectroscopy (Chapter 21). For these measurements it is necessary to hold the optical path difference constant while a measurement is made, after which the OPD is rapidly advanced to the next sampling position and then held constant once again for the next measurement. This process is repeated until all the data needed to obtain the interferogram are acquired. Such interferometers are called step-scan interferometers. 

As noted above, there have been many reports of rapid time-resolved spectroscopy achieved using a step-scan interferometer, most of which involve the investigation of rapid photolysis reactions. We will give two examples of this type of measurement, one of a relatively ligand-exchange reaction and the second of a more complex biochemical system. 

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. 

Rapid reversible processes can be studied by FT-IR spectrometry in at least four ways, two using rapid-scan interferometers and two using step-scan interferometers. Three of these approaches, asynchronous sampling and stroboscopic measurements with a rapid-scan interferometer and time-resolved spectroscopy with a step-scan interferometer, were described in Sections 19.2 and 19.3. The fourth approach involves the use of a step-scan interferometer and some type of sample modulation. We have seen one application in the earlier part of this chapter, and two other applications will now be described. The reorientation of liquid crystals induced by rapid switching of the electric field to which they are being subjected has been studied by at least three of these approaches. Results have been summarized in an excellent article by Czamecki [17]. In this section we discuss the application of sample-modulation FT-IR spectrometry to this problem. 

In summary, we have described in this chapter how the dynamics of three of the types of reversible chemical and physical changes can be investigated using a step-scan interferometer by modulating some property of the sample. It can be expected that several more reports of analogous processes will be reported in the future. 

In time-resolved measurements with a step-scan FT-IR spectrometer (see Section 5.2.1.1 for description of a step-scan FT-IR spectrometer), the timing of the two types of signal processing described above is rendered compatible by scanning the movable mirror of the interferometer step by step and sampling the interferogram signals at each step [1,9]. The... 

---