Spectroscopy step-scan

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

Seguin, E., Thibault-Starzyk, F. and Arnolds, H. (2006) Coupling step scan IR and pulsed laser for operando at 33ns in catalysis, proceedings of operando - II Second international congress on operando spectroscopy, Toledo (Spain), 23-27 April, 2006. 

Vasenkov, S. and Frei, H. (2000). Time-resolved study of acetyl radical in zeolite NaY by step-scan FT-IR spectroscopy. J. Phys. Chem. A 104, 4327 4332... 

Neubert, R. Collin, B. 8t Wartewig, S., Direct determination of drug content in semisolid formulations using step-scan FT-IR photoacoustic spectroscopy Pharm. Res. 1997, 14, 946-948. 

These conclusions were supported by transient absorption spectroscopy, which revealed signals corresponding to the formation of the diimine radical anion, with lifetimes in close agreement with the luminescence lifetimes. Time-resolved infrared spectroscopy of the acetylide C = C bonds provides further conclusive evidence for the MLCT assignment. Thus, in the ground state IR spectrum of 4, there are two v(C=C) bands at 2115 and 2124 cm-1, whilst the step-scan FTIR difference spectrum obtained 50 ns after irradiation at 355 nm reveals bleaching of the parent bands, and the formation... 

The sensitivity and detection limits of an analytical technique are determined by the SNR of the measurement, an important metric for assessing both the instrumental performance and analytic limits of the spectral measurement. Following typical analytical practices, 3 and 10 times the noise have been suggested as limits of detection and of quantification for IR spectroscopy, respectively. The performance of interferometers in the continuous-scan mode, which is simpler compared with that of the step-scan mode, has been analyzed well. The SNR of a spectrum measured using a Michelson interferometer is given by12... 

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. 

FT techniques have greatly broadened the scope of both IR and Raman spectroscopy. In the near future, these methods are expected to provide a considerable stimulant to studies of time dependent phenomena in complex systems. The stroboscope and step-scan technique are probably also u.seful for FT-Raman. This would be very convenient, since it is relative easy to back up an FTIR instrument with an FT-Raman module. 

Time-resolved, step-scan FT-IR spectroscopy has been used to monitor the v(CO) frequencies of rhenium(I) carbonyl polypyridine complexes and hence to study the excited-state electronic structures of these systems. The MLCT and IT character in the emissive states of [Re(CO)3(phen)(py-4-Me)]+ and [Re(CO)3(dppz)(PPh3)]+, respectively, has been studied by this technique. The presence of two close-lying states of MLCT and IL character for the complex [Re(CO)3(4,4 - NH2 2-bpy)(py-4-Et)]+ was also confirmed. 

Further recent developments in DSC and IR techniques, with respect to the study of SC barrier properties, include step-scan FT-IR photoacoustic spectroscopy [195] and combined microscopic differential calorimetry-Four-ier transform infrared (DSC-FTIR) spectroscopy [196]. The former allows depth profiling of the membrane the latter enables the simultaneous detection of calorimetric and structural modifications during a thermal transition. Technological advances in DSC and IR will, no doubt, continue to expand the application of these techniques to the study of skin barrier function. 

The challenge now is to perform time-resolved experiments and thus, to benefit from the huge potentialities of infrared spectroscopy to identify reaction mechanisms induced by irradiation. For example, in the LINAC-FTIR coupling, the Rapid Scan system of the spectrometer can be used with a resolution of 100 to 10 ms, and for reactions much faster it could be possible to use the Step Scan system. 

Radicals can be characterized by several techniques, such as mass spectro-metry ° or the characterization of alkoxycarbonyl radicals by Step-Scan Time-Resolved Infrared Spectroscopy. Another technique makes use of the magnetic moment that is associated with the spin of an electron, which can be expressed by a quantum number of or According to the Pauli principle, any two electrons occupying the same orbital must have opposite spins, so the total magnetic... 

IR and Raman spectra of copper(II) complexes of histamine gave evidence for the formation of [Cu2(L H)2]2+, CuL2 and CuL2+ at high pH, Cu(LH)2, CuL2- and CuL2+ at lower pH all with coordination through the imidazole moiety.251 Time-resolved step-scan FTIR spectroscopy was used to probe the... 

Rammelsberg, R., Boulas, S., Chorongiewski, H. and Gerwert, K. (1999) Set-up for time-resolved step-scan FTIR spectroscopy of noncyclic reactions. Vih. Spectrosc., 19, 143-149. 

As noted above, step-scan FT-IR can provide a better time resolution than PA-IR spectroscopy for time-resolved studies, as well as full spectra at the desired resolution. On the other hand, its major limitation is that the phenomenon under study must be perfectly repeatable-information which often is not available before an experiment is carried out. Another problematic aspect to consider is that sufficient relaxation time must be allocated for the sample to return to its initial state between consecutive perturbations. Unfortunately, this parameter is also often not known a priori before the experiment is performed, and may risk artifacts appearing in the data. In contrast, a single perturbation is required in a PA-IR experiment to record the time-resolved data, eliminating the requirements of repeatability and an a priori knowledge of the relaxation time. PA-IR spectroscopy was used to assess directly the repeatability of the orientation/reorientation cycles for 5CB [27]. Table 13.1 shows the switch-on and switch-off time constants determined individually for a series of 300 consecutive reorientation cycles. As expected for this well-studied LC, the time constants did not evolve systematically as a function of the number of cycles. In this case, however, the repeatability was demonstrated experimentally and not only assumed, as is often necessary in step-scan studies. 

To obtain IR spectra on a time scale of nanoseconds, the sample cell in conventional spectrometers is usually excited by an Nd YAG laser. Flow cells with a pathlength of at least 0.1 mm must be used for photoreactive samples and the pulse repetition frequency is then limited to 1 Hz. In step scan FTIR spectroscopy,211 the time evolution is collected at single points of the interferogram, which is then reconstructed point-by-point and subsequently transformed to time-resolved IR spectra. Alternatively, dispersive instruments equipped with a strong IR source can be used.212 The time resolution of both methods is about 50 ns. FTIR instruments provide a triggerable fast-scan mode to collect a complete spectrum within a few milliseconds.213... 

Rodig, C., Siebert, F., Error and Artifacts in Time resolved Step scan FT IR Spectroscopy, Appl. Spectrosc. 1999, 53, 893 901. 

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. 

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. 

Johnson T J, Simon A, Weil J M and Harris G W1993 Applications of time-resolved step-scan and rapid-scan FT-IR spectroscopy dynamics from ten seconds to ten nanoseconds Appl. Spectrosc. 47 1376-81... 

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