Time-resolved FTIR techniques

This section describes the state of the art of fast time-resolved FTIR techniques, together with the advantages and limitations of each method. Sec. 6.6.3 discusses the application of the described techniques to actual problems. 

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In this section it is hoped to provide useful experimental details which demonstrate the simplicity of both the time-resolved FTIR technique and its incorporation into an experiment. Two major implementations of the technique have emerged, namely stopped and continuous-scan they are dealt with separately below in Sections III.A and III.B. A detailed comparison of the two methods is then presented in Section III.C. 

As demonstrated, different time-resolved FTIR techniques allow to study the complete photocycle of bacteriorhodopsin in the entire range from picoseconds to several milliseconds. Infrared difference spectra trace reactions which take place in different parts of the protein molecule. Isotopically labeled proteins or proteins with mutations at specific sites... 

The chapter is set out in the following way. Section II contains elements of the theory of Fourier transformations which, rather than being exhaustive (and exhausting), aims to cover the details and limitations of the technique which are of importance for the experimentalist to understand. Section III contains descriptions and comparisons of the SS and CS methods and outlines the advantages and pitfalls of each, together with recommendations for their suitability for specific applications. Section IV presents recent results from time-resolved FTIR emission experiments, emphasizing photochemical applications. 

The following sections are not intended to give an exhaustive account of the application of time-resolved FTIR emission spectroscopy, but highlight particular problem areas in chemical physics to which the technique is particularly suited and focus on photochemical reactions, and results which have emerged since the previous reviews on the subject [26,27]. 

The discussion of the recent results concerning the ethene polymerization on the Phillips catalyst demonstrates that temperature- and time-resolved FTIR spectroscopy (where both temperature and time change simultaneously during the experiment), together with an accurate control of the pressure conditions, has been decisive in clarifying the nature of the adsorbed species and of some of the precursor species present in the first stages of the polymerization reaction. Further advances in this direction may be achieved by increasing the sensitivity of the technique. 

The overall diagram of evolution of the excited states and reactive intermediates of a photoinitiating system working through its triplet state can be depicted in Scheme 10.2 [249]. Various time resolved laser techniques (absorption spectroscopy in the nanosecond and picosecond timescales), photothermal methods (thermal lens spectrometry and laser-induced photocalorimetry), photoconductivity, laser-induced step scan FTIR vibrational spectroscopy, CIDEP-ESR and CIDNP-NMR) as well as quantum mechanical calculations (performed at high level of theory) provide unique kinetic and thermodynamical data on the processes that govern the overall efficiency of PIS. 

Thus, this paper gives tantalising glimpses of the possible applications of in-situ rapid-scan time-resolved FTIR spectroscopy. Unlike the digital time-resolved technique, the rapid-scan method does not require reaction reversibility and may thus have wider application. The technique does have its limitations, however. 

NH3, it should be emphasized that a number of other techniques have been used to determine the energy distribution in the NH2 fragment, including LIF and time-resolved FTIR, and these have revealed even more detailed aspects of the dynamics. In particular, work with PTS has revealed a number of interesting vector correlations. A good summary of these findings can be found in a review by Lee (2003). 

The reorientation of liquid crystals in an external electric field is the basis of their technical application in liquid-crystal displays (LCDs) and has been treated in numerous books and reviews [1-3, 12, 38, 46 50]. This section is intended to demonstrate the application of time-resolved FTIR spectroscopy for a better understanding of the electro-optical performance of liquid-crystalline systems. Due to the reversible character of the orientational and relaxational motions of liquid crystals under the perturbation of an electric field, the time-resolution of the FTIR measurements can be extended down to the microsecond range by the implementation of the step-scan technique (see Section 2.2). Thus, an insight on a molecular level... 

The formation and disappearance of intermediate states can be followed by spectroscopic techniques and their fitting to a kinetic model yields a set of rate constants. The consensus model is based on the photocycle shown in Figure 16.5 in which all proton transfer steps are reversible except the O bR transition, which accounts for the fact that bR is the only stable state in the dark. A first set of rate constants was deduced from visible absorption experiments carried out in the 5-30 °C range between pH 4.5 and 9 (see Table 16.1). More recent time-resolved FTIR experiments were designed to... 

Another field of time-resolved FTIR spectroscopy relates to polymeric liquid crystals, with the rate of electric field reorientation of nematic liquid crystals (100] and the orientation of strands in liquid crystals [101] each having been monitored using this technique. 

Kazarian et al. [281-283] have used various spectroscopic techniques (including FUR, time-resolved ATR-FHR, Raman, UV/VIS and fluorescence spectroscopy) to characterise polymers processed with scC02. FTIR and ATR-FTIR spectroscopy have played an important role in developing the understanding and in situ monitoring of many SCF processes, such as drying, extraction and impregnation of polymeric materials. 

The use of FTIR techniques in studies of time-resolved IR emission has been a relatively recent development, and two of the major practitioners in the field have provided excellent reviews of progress up to 1989 [26,27]. This chapter does not attempt a historical survey of the method, but instead describes progress since 1989, suggests possible further areas of study, and most importantly tries to provide the experimentalist with a practical guide to the use of the technique for studying a wide range of photochemical systems. 

Besides the continuous improvements of FTIR-VCD instruments described above, some exciting new developments related to VCD measurements have been reported in recent years. These include the developments of matrix isolation FTIR-VCD instruments and of laser based real time VCD spectrometers. These new developments are associated with brand new applications and research directions, such as combining the matrix isolation technique with VCD spectroscopy to probe conformationally flexible chiral molecules and H-bonded chiral molecular complexes, and using femtosecond laser VCD instruments to record time resolved VCD spectra for monitoring fast chemical reactions or folding and unfolding events of peptides and proteins in solution. These will be discussed in more detail in Sects. 4.5 and 4.6. 

B.2.1. Ethene, Propene, and Butene Oligomerization in Zeolites with Three-Dimensional or Two-Dimensional Sets of Interconnected Channels. Spoto et al. (6) employed the FTIR technique to investigate the interaction of ethene with HZSM-5. The sequence of spectra shown in Fig. 16 was obtained when dosing a fixed pressure of ethene at room temperature and recording a spectrum every 6.8 s. It is evident that in these short-time intervals there was a dramatic change of the spectral features that would be completely lost by operating under the conventional (non-time-resolved) acquisition conditions (note that spectrum 19 in Fig. 16 corresponds to a total contact time of only about 130 s). 

While the majority of the studies of carotenoid radicals have been based on monitoring the strong near infrared absorption bands, other techniques, including time resolved resonance Raman spectroscopy (Jeevarajan et al., 1996), FTIR spectroscopy (Noguchi et al, 1994), EPR (Grant et al., 1988), ENDOR (Piekarasady et al., 1995), and cyclic voltametry (Grant et al, 1988) have also been used. 

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