Step-scan investigation

Laminates have been extensively studied by PA-FTIR and are the subject of extensive investigations by the new much more complex step-scan PA-FTIR techniques. Kapton film is often used to illustrate the analysis of this type of sample. Kapton 200FN919 film is a DuPont product which consists of a 25 pm layer of polyimide laminated between 12.7 pm layers of Teflon. If a high mirror velocity is used then a spectrum of only the Teflon layer is obtained. On the other hand if a slow mirror velocity is used a composite spectrum of the Teflon and polyimide layer is obtained. The difference spectrum (low mirror velocity spectrum minus high mirror velocity spectrum) represents that of the polyimide layer. As illustrated in Figure 2.16 the difference spectrum closely resembles the PA FTIR spectrum of polyimide except for the regions where the strong C-F bands of the Teflon are present. 

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. 

Fig. 2. Island growth of PPy at an HOPG step (scan size 606.3x606.3 n l height 100 mn) investigated by in situ electrochemical AFM (-100 -350 mV vs. Ag/AgCl at 20 mV/s).

Turro, George, and co-workers [70] have also observed and studied an acyl radical in solution by TRIR methods in their investigation of the photochemistry of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (Scheme 2.4). More recently, Vasenkov and Frei [71, 72] reported the generation of the acetyl radical by photolysis of 1-naphthyl acetate (Scheme 2.5) or pinacolone (Scheme 2.2 R = Me, R = f-Bu) in zeolite NaY. Through the use of step-scan FTIR spectroscopy, they observed a carbonyl stretching mode at 2125 cm , interestingly... 

Experimentation with step-scan interferometry in electrochemistry began in the early 1990s (cf Ref. [23]), and interest has grown steadily ]24, 29-31, 51-54]. Step-scan FTIR spectroscopy provides a means to investigate time- and frequency-dependent processes. Measurements are hmited to reversible systems. However, a great deal of insight can be gained into the molecular transformations that accompany the external perturbation [181-183]. 

Although we have focused on the photochemistry of Rh(Cp )(CO)2 (Cp =Cp or Cp ) so far, the most complete understanding of C-H reactions came in parallel TRIR studies on Rh(Tp )(CO>2 (Tp = HB(3,5 -Me2pyrazol-l-yl)3, and extensive reviews cover this work." We choose this example since this investigation built upon matrix-isolation and liquid Xe studies. A combination of ultrafast TRIR and ns-step-scan FTIR spectroscopy was used to unravel the mechanism of alkane C-H activation by Rh(Tp An alkane complex was formed within the earliest... 

However, TRIR has also been applied to more classical coordination compounds. Ford and co-workers have used a combination of ns-TRIR and time-resolved UV/vis spectroscopy to investigate the mechanism of hydrocarbon C—H bond activation with the rhodium complex, trans-RhCl(CO)(PR3)2 (R = Ph, />-tolyl, or Me). Upon photoexdtation, each of these species was found to undergo CO dissociation to form the transient solvated complex, tra 5-RhCl(Sol)(PR3)2 (Sol = solvent). The solvated complexes reacted with added CO to regenerate the parent complex, and also underwent competitive unimolecular C H activation to form the Rh products of hydrocarbon oxidative addition. These were identified from the step scan FTIR spectra, which showed a positive shift in /(CO) relative to the parent complex, which is consistent with oxidation of the metal center. 

The excited states of coordination compounds that do not contain CO or CN ligands have also been investigated using TRIR by probing peripheral organic j/(CO) reporter groups, such as carboxylic acids, esters, and amides, on substituted diimine ligands. For example, step scan FTIR was used to probe the ester /(CO) bands in the MLCT states of [Ru(bpy)2(4,4 -(COOEt)2-bpy)] + and [Ru(bpy)2(4-COOEt-4 -CH3-bpy)] " ". Such an approach has been used with ps-TRIR spectroscopy to probe the excited state dynamics in the Pt chromophore, Pt(4,4 -(COOEt)2-bpy)Cl2. The lifetime of the MLCT excited state was found to be 8.7 ps. ... 

Figure 2-20. Step-scan FTIR spectra of the investigated nematic liquid-crystalline guest host solution before (0.0 ms) and after (19.5 ms) reorientation in the electric field.

Melting Step scan DSC has been used to study a number of melting processes. Papageorgiou et al. (2006) investigated the effects of annealing at different temperatures for different molecular masses of poly(l,3-propylene terephthalate) samples. Step ramp rates of 5 °C/min were utilized... 

Fischer et al. [101] investigated the simultaneous quantification of the content of several additives in PVC with an in-line diffuse reflectance probe. The signal from diffuse reflectance can be affected by a number of physical properties of the sample, rather than just its chemical make up. This makes obtaining quantitative data very difficult. Chemometric analysis showed the possibility of detecting even small amounts of additives (3%) with an absolute prediction error of 0.3%. Step-scan PA-FTIR spectroscopic studies were used to study surfactant exudation and film formation in PS-nBA latex films [102]. 

Real time experiments on non-reproducible phenomena are then possible on the time scale of milliseconds, while in the so called step-scan mode much faster processes can be investigated, but only if the phenomena under study are cyclic and can be triggered. ... 

The Bruker FT-IR imaging system (Bruker Optik GmbH, Ettlingen, Germany) used for the investigations described in this chapter is shown in Figure 8.3. The system consists of an IFS66/S step-scan/rapid-scan FT-IR spectrometer (left), which... 

As noted above, there have been many reports of rapid time-resolved spectroscopy achieved using a step-scan interferometer, most of which involve the investigation of rapid photolysis reactions. We will give two examples of this type of measurement, one of a relatively ligand-exchange reaction and the second of a more complex biochemical system. 

Several workers have investigated the photochemistry and photophysics of proteins and other biomolecules by time-resolved FT-IR spectroscopy with a step-scan... 

In summary, we have described in this chapter how the dynamics of three of the types of reversible chemical and physical changes can be investigated using a step-scan interferometer by modulating some property of the sample. It can be expected that several more reports of analogous processes will be reported in the future. 

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