Step-Scan 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... 

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]. 

Time-resolved Spectroscopy Finally in this section, the advent of step-scan interferometers has opened up exciting opportunities to study fast, reversible surface processes. Details on step-scan interferometry may be foxmd elsewhere [144] briefly in conventional mode (see in previous text), the mirror moves essentially continuously, with intensity measurements taken at regular intervals (Fig. 12). In step-scan mode, the mirror is paused at each position (retardation), allowing the exploitation of the time-resolved spectroscopy option ofthe spectrometer. Once the mirror has settled at a particular position, a reference point can be taken, after which a reaction can be triggered, that is via a light pulse or potential step, and the intensity measured at regular intervals (Fig. 13). 

A step-scan interferometer provides a means to maintain constant optical retardation for an arbitrarily large time. This large time is required when the FPA coadds collected frames at each optical retardation point to give reasonable signal to noise ratio images, which is similar to in-scan co-addition for regular step-scan spectroscopy. An electric pulse triggers each interferometer step, which may also... 

The movable mirror can either be moved at a constant velocity (a continuous-scan interferometer) or be held at equally spaced points for fixed short periods and stepped rapidly between these points (a step-scan interferometer). When the mirror of a continuous-scan interferometer is moved at a velocity greater than 0.1 cm s (the usual case for most commercial instmments), the interferometer is often called a rapid-scan interferometer. 

Figure 2.21. Retardation versus time for a step-scanning interferometer. The velocity of the moving mirror is zero at each step and the mirtOT moves very quicMy between steps.

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. 

The SNR of spectra measured interferometrically is determined in part by how accurately the position of the moving mirror is known (see Eq. 7.12). Many measurements made using a step-scan interferometer require the OPD to be held constant to better than 1 nm (just a few atomic diameters ). To achieve this goal, the interference record from the HeNe laser must be measured. The points that correspond to the zero crossings of the laser interferogram are the points where the slope... 

No single technique will compensate completely for the effects of fluctuation noise. The best way to minimize this effect is to use a scan speed that causes the infrared wavelengths to be modulated at frequencies outside the frequency range of the perturbations. Atmospheric scintillation is rarely observed at frequencies much above 500 Hz, so these effects are rarely seen in the mid-infrared spectrum when the mirror velocity is greater than about 1 cm s On the other hand, fluctuation noise can be a major problem when the scan speed is very low, as it is with FT-Raman spectrometry (see Chapter 18) and step-scanning interferometers (see Section 5.5). The possible causes of fluctuation noise in step-scan FT-IR spectrometry have been discussed in some detail by Manning and Griffiths [6]. 

Hyperspectral Imaging with a Step-Scanning Interferometer... 

Figure 14.7. Typical FT-IR imaging spectrometer incorporating a step-scan interferometer and a focal plane array detector. (Reproduced from [21], by permission of CRC Press copyright 2003.)...

ADC would have necessitated a data acquisition frequency of over 20 MHz 20-MHz ADCs with the high dynamic range needed for FT-IR spectrometry are still not available. In the first imaging FT-IR spectrometers, the data acquisition frequency was reduced to a manageable level through the use of a step-scan interferometer (see Section 5.5). 

Many of the problems associated with the manufacture of FPAs have now been overcome, and most contemporary instruments no longer suffer from the bad pixel problem. Instruments based on the combination of step-scanning interferometers and FPA detectors are now available from Varian (the Stingray), Thermo Electron (the Continupm XL), and Bruker Optics (the Hyperion 3000). 

TIME-RESOLVED MEASUREMENTS USING STEP-SCAN INTERFEROMETERS... 

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