Step-scan FT-IR spectrometer

A short time later, Levin s group modified step-scan FT-IR spectrometer to operate with a mid-IR MCT FPA detector. Unlike most MCT detectors used in FT-IR spectrometers, which operate in the photoconductive (PC) mode, the pixels of MCT FPA detectors operate in the photovoltaic (PV) mode. As noted in Section 1.2.2, the cut-off wavenumber of narrow-band PC MCT detectors is about 750 cm" . The PV detector elements used in MCT FPA detectors have the same high sensitivity as narrow-band PC MCT detectors, but the cut-off wavenumber is higher, at about 850 cm" . 

For imaging measurements using a step-scan FT-IR spectrometer, the interferometer mirror is held at a constant position (usually at the zero-crossing of the laser interferogram) while the signal from each pixel is recorded by the ADC. This... 

Figure 13.9 Comparison of the DIRLD in-phase spectra of isotactic polypropylene recorded in 20s with a PA-IR spectrograph and in 34min using a step-scan FT-IR spectrometer. The FT-IR spectrum has been Inverted for clarity. The dynamic quadrature spectrum and the static PA-IR spectrum of the bulk sample are also shown. Reproduced with permission from Ref [31].

If the system to be examined is reversible or repeatable, much faster time resolutions are achieved by using a step-scan FT-IR spectrometer [129]. The interferometer is moved stepwise in the step-scan mode as shown in Fig. 8.27 (left). When the moving mirror of the interferometer is fixed at a position (i.e., at a... 

Table 19.1. Matrix of Data Taken During Time-Resolved Spectroscopic Measurements with a Step-Scan FT-IR Spectrometer...

Similar experiments have also been carried out with a step-scan FT-IR spectrometer [28], and analogous results were obtained. Although this experiment was not performed with very short time resolution, it gives an idea of the quality of the data that can be obtained, and successful experiments with much shorter time resolution can be predicted. 

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... 

Time-Resolved Measurements with a Step-Scan FT-IR Spectrometer... 

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... 

Figure 203 Time-resolved Infrared difference spectra of photoexcited PYP (photoactive yellow protein) measured using a step-scan FT-IR spectrometer [141. Time is indicated on a common logarithmic scale after 50 ns from photoexcitation. AAbsorbance, absorbance difference a.u., absorbance unit. (Source Reprinted by permission from Macmillan Publishers Ltd Nature Structural and Molecular Biology [14]. Copyright 2001.)...

In a similar way as for time-resolved measurements by a step-scan FT-IR spectrometer, signals collected from different transient phenomena are processed to form time-resolved interferograms. If the repeatability of transient phenomena is not maintained, the quality of the time-resolved spectra will deteriorate rapidly. The sampling interval must be always the same, that is, the speed of the moving mirror must be constant. Unstable scans of the movable mirror will not only affect the accuracy of time resolution but also increase the noise level. 

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