Slow-scanning interferometers

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... 

Another example of an fax-infrared Fourier spectrometer is the Bruker IFS 114 (cf. Fig. 50). In contrast to the Poly tec FIR 30, the Coderg FS 4000 and the Beckman FS 720, this spectrometer is a rapid-scan instrument. But the consequences of rapid and slow scanning will be discussed later. Here we shall concentrate on the optical layout. From Figs. 49 and 50, it is evident, that the interferometers are constructed as a modular system and that, by inserting some windows, the compartments of the several modules can be evacuated separately. This is of particular interest for the sample chamber. There, it is an advantage to be able to exchange samples without breaking the vacuum of the whole instrument and to have to reevacuate only the sample chamber. The different modules of the... 

The last quantity to be discussed in this section is maximum path of the movable mirrors. In the slow-scan instruments, usually a lead-screw is employed to drive the mirror with the synchronous motor. And a maximum scan length of the movable mirror of 5—10 cm is achieved without any problems. In the case of rapid-scan instruments however, the customer has to pay for a larger maximum scan length. From a comparison of this quantity for various instruments in Table 3 e.g. No. 4a—d and No. 5a, b) with the prices of the instruments in Table 2, we learn that with increasing prices also the maximum mirror path is increased which determines maximum resolution of the instruments as limited by the mechanics of the interferometer. We recall from our considerations in Sections 2.3, 3.2, 4.6 and 5.1 that, in Fourier spectroscopy, the resolution width is... 

The high sensitivity of SEIRAS (see above) allows measurements in real time during a slow electrode potential scan [303-305] for particularly fast acquisition, step-scan interferometers may be used [306]. A series of time-resolved SEIRA spectra recorded during reduction of heptyl viologen to HV + at a silver electrode in an aqueous solution of 0.3 M KBr is displayed in Fig. 5.60 [274]. 

Other detectors that are useful in the near- and mid-infrared regions are bolometers and pyroelectric detectors. Both these detectors have very large bandwidths and can operate at room temperature however, they have long response times compared to the photodetectors and they have low D s. Pyroelectric detectors are useful in the far-infrared region with rapid-scanning spectrometers whereas Golay cell detectors are often used with slow scanning far-infrared interferometers. These cells are modulated at or below 20 Hz. 

Successful concepts for "reaction-modulated" IR difference spectroscopy use the multiplex advantage of FTIR spectroscopy or the availability of high-intensity laser IR sources. A kinetic photometer using tunable IR diode lasers as sources for the mid-infrared has been developed in our laboratory and will be described elsewhere [6]. It covers the time-domain from approx. 500 nsec to some seconds. A second approach is time-resolved FTIR spectroscopy using a rapid-scanning interferometer, several scans can be recorded per second and the time-domain of slow reactions thus be covered [7]. The following schemes illustrate both concepts ... 

Figure 2.19. (a) Retardation versus time for a unidirectional rapid-scanning interferometer where the mirror speed is relatively slow during data collection, then resets rapidly for the next scan (b) velocity scan that corresponds to (a). The speed in the reset direction is greater than that for scanning. —A and +A indicate the limits of retardation at the beginning and end of travel, respectively. 

It is always advisable to adhere closely to the manufacturer s instructions, especially in regard to the use of a dry purge gas. Failure to do this will result in data dominated by the spectrum of water vapor. Photoacoustic measurements are based on thermal processes, which are intrinsically slow, and in order to maximize sensitivity, slow interferometer scan rates are recommended for best performance. Note that the sample size is normally limited to a few millimeters. Always docmnent the purge gas that is used and the scan speed of the instrument because these impact the intensity and quality of the final spectrum. 

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