Step-scan Operation

In step-scan mode, the moving mirror of the interferometer is stopped at each data acquisition point and held for some time (seconds to minutes) during which data are acquired. In step-scan mode the collected interferograms contain the same information as in continuous-scan mode, only the time required for the complete experiment is much longer. Under stroboscopic measuring conditions, a time resolution of 100 ns can be achieved. This technique can be applied to processes which can repeatedly be started under highly reproducible conditions. The step-scan technique can also be applied for the acquisition of voluminous data. This 

---

In our operations, we often establish step-scan parameters as if we were continuously scanning. Because continuous scanning requirements are more demanding, no problems arise using this approach. 

P 26] Time-resolved FTIR spectroscopy was performed by operation of an infrared spectrometer in the rapid scan acquisition mode (see Figure 1.59) [110]. The effective time span between subsequent spectra was 65 ms. Further gains in time resolution can be achieved when setting the spectral resolution lower (here 8 cm4) or by using the step-scan instead of rapid-scan mode. 

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" . 

Such FPA detector setups were first used by the group of Lauterbach for the parallel characterization of solid samples and the product gas stream from catalytic reactors [18,19]. These authors also changed the mode of operation from the previously used step-scan mode to the rapid scan mode which made it possible to even record transient processes [20,21]. The group of Lauterbach was also the first to apply FPA IR spectroscopy to a problem from zeolite science, even if it was only in form of a feasibility study. They investigated the adsorption of CO on Cu-ZSM-5 and on Pt/Si02 in order to prove that it would be possible to detect the absorption bands of adsorbed species [19J. Since experiments were carried out at room temperature, bands for CO on the Cu-ZSM-5 would be expected to have very low intensity, and indeed, no spectra for CO on this solid were shown. The band of CO on the noble metal, on the other hand, could clearly be detected without problems, and a signal-to-noise ratio not much different from that obtained for a conventional experiment. 

A Michelson interferometer with the facility for step and integrate, and rapid scan operation has been used for Fourier transform PAS. The merits of the two methods were considered. The versatility of this technique for the measurement of the infrared spectra of powdered samples has been demonstrated. Some possible reasons for the distortion of such measurements have been discussed. " FT-PAS has enabled discrimination between K N03 and K NOj and has shown the technique to be suitable for quantitative analysis of solid mixtures. ... 

Fig. 13 Schematic representation of the data acquisition during the time-resolved spectroscopy operation of a step-scan FTIR spectrometer. S is the settlement time, and R, a reference data point.

Figure 7.46. In-phase (solid) and quadrature (dashed) potential-modulated ATR-SEIRA spectra of 4-mercaptopyridine (PySH) SAM on 20-nm-thick (80-nm-size particles) Au evaporated electrode in 0.1 M HCIO4. Modulation frequencies are shown. Amplitude of potential modulation was 400 mV, between -0.1 and 0.3 V (SCE). Spectra were recorded using Bio-Rad FTS 60A/896 FTiR spectrometer equipped with dc-coupled MCT detector and bandpass optical filter transmitting between 4000 and 1000 cm. Spectrometer was operated in step-scanning mode using setup shown in Fig. 4.56. Reprinted, by permission, from K. Ataka, Y. Hara, and M. Osawa, J. Electroanal. Chem. 473, 34 (1999), p. 39, Fig. 6. Copyright 1999 Elsevier Science S.A.

The essential difference between conventional FT instruments and the step-scan devices is that, for successful operation, it is necessary to control the retardation (mirror) velocity in the case of the former and the retardation (mirror) position for the latter. In both cases, the method used to control the retardation involves a collinear or parallel helium/neon laser interferometer. In continuous scan operations the laser interference fringes are used to generate feedback signals to maintain constant mirror velocity, and in the step-scan mode the laser interferogram provides the means for the control of the mirror position via a feedback signal. 

---