Rapid-scanning mode

Since modern FTIR spectrometers can operate in a rapid scan mode with approximately 50 ms time resolution, TRIR experiments in the millisecond time regime are readily available. Recent advances in ultra-rapid scanning FTIR spectroscopy have improved the obtainable time resolution to 5 ms. Alternatively, experiments can be performed at time resolutions on the order of 1-10 ms with the planar array IR technique, which utilizes a spectrograph for wavelength dispersion and an IR focal plane detector for simultaneous detection of multiple wavelengths. ... 

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. 

The principle of the rapid scan mode is simple after taking a reference spectrum of the sample in its ground state, one activates the reaction (e.g., by a laser flash) and obtains an interferogram in a much shorter time than the half life time of the reaction (e.g., the photocycle). 

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. 

Because modem FTIR spectrometers can operate in a rapid scan mode with approximately 10 ms time resolution, TRIR experiments in the millisecond... 

Figure 7.45. Potential-difference unpolarized ATR-SEIRA spectra of 4-mercaptopyridine (PySH) SAM on 20-nm-thick (80-nm-size particles) Au evaporated electrode in 0.1 M HCIO4. Reference potential was -0.1 V (SCE). Arrows show changes of peaks for positive shift of electrode potential from -0.3 to -1-0.4 V. Spectra were recorded using Bio-Rad FTS 60A/896 FTIR spectrometer equipped with dc-coupled MCTdetector and bandpass optical filter transmitting between 4000 and 1000 cm". Spectrometer was operated in rapid-scanning mode and spectra were collected sequentially during potential sweep at 5 mV s". Sixty-four interferograms were coadded to record each spectrum, which required about 10 s. Reprinted, by permission, from K. Ataka, Y. Hara, and M. Osawa, J. Electroanal. Cham. 473, 34 (1999), p. 37, Fig. 3. Copyright 1999 Elsevier Science S.A.

The difference spectrum was measured immediately after the off-set of actinic light by rapid scanning mode. 

In routine FTIR spectroscopy the spectrometer is operated in continuous scan mode. In this mode of operation, the moving mirror is scanned at a constant velocity, v (cms ), with the light beam path difference at any time, t being given hy 6 = 2 1 (cm). An internal HeNe laser beam is also passed through the interferometer and, since it is essentially monochromatic (I5,798cm ), it is used to accurately calibrate the positions of Mm for data sampling. Continuous scan FTIR is most commonly used to monitor stable samples, but can also be used in rapid-scan mode to monitor time-dependent processes on timescales down to ca. 20 ms. 

When V is so small that the HeNe laser frequency is less than 1 kHz, many interferometers become unstable. For these measurements, the slowest scan was actually taken in the step-scan mode at 200 steps per second, with the spectrum being acquired just as one would in the rapid-scan mode. 

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