FT-spectroscopy

The introduction of commercial Fourier transform (FT) spectrometers in the early 1960 s has made it possible, in part, to overcome the limitations associated with dispersive instruments and has helped to broaden the scope of problems amenable to investigation by infrared spectroscopy. The purpose of this review is to compare the performance of FT and dispersive spectrometers and to illustrate areas of application in which FT spectroscopy has proven advantageous for the study of adsorbed species. In view of these objectives only a limited treatment of the theory underlying FT spectroscopy will be presented here. 

More detailed discussions of this subject and examples of the applications of FT spectroscopy to other fields of chemistry can be found in references (4, 5, , 7, J5). ... 

Low and coworkers (12-17) have utilized FT spectroscopy extensively to characterize species adsorbed on the surface of strongly absorbing and scattering solids such as CaO and MgO. Their work has shown that spectra exhibiting good resolution can be recorded in 5 to 10 min. To obtain similar spectra using a grating spectrometer would require one to three hours. 

Bouwman and Freriks (18.19) have also noted the advantages of FT spectroscopy and have used this technique to study the adsorption of CO on a silica-supported nickel catalyst at temperatures between 70 and 180°C. These authors point out that FT spectroscopy is particularly advantageous for in situ observation of heated samples since the radiation emitted by the sample is not modulated by the interferometer and hence does not contribute to the fluctuating portion of the interferogram. 

Specular and Diffuse Reflectance. Reflectance techniques have been applied primarily to samples which do not permit observation by transmission. Flat surfaces such as those of metal foils and single crystals can he studied by specular reflectance, whereas rough surfaces such as those of powders must be observed by diffuse reflectance. In both cases FT spectroscopy offers strong advantages in terms of the time required to acquire a spectrum. 

Diffuse reflectance spectroscopy has been widely used to characterize the surface of solids as well as films and coatings present on a solid substrate (6). The application of this technique to the study of adsorbed species has been much more limited (26) and, thus far, has not involved the use of FT spectroscopy. 

A further example of the use of FT spectroscopy for the observation of emitted radiation is provided by the work of Kember and Sheppard (31). By observing the radiation emitted by a copper surface heated in air, these authors were able to detect... 

Time Resolution. Time-resolved studies of surface species are of considerable interest in the field of catalysis since they offer a means for investigating the kinetics of adsorption and surface reaction and for distinguishing between species active and inactive in catalysis (32, 33, 34). Dispersive spectrometers can be used for this purpose (33, 35) but are restricted to the observation of either a single frequency or a narrow range of frequencies, unless the dynamics of the observed phenomenon are very slow compared to the time required for the acquisition of a spectrum. FT spectroscopy allows these limitations to be surmounted and opens up the possibility of recording complete spectra very rapidly. 

As illustrated by the examples discussed here, the use of FT spectrometers for the observation of surface structures is favored by situations in which the flux of radiation coming from the sample is very low or the data acquisition time is limited. Such cases arise in transmission spectroscopy using strongly absorbing or scattering samples, specular and diffuse reflectance spectroscopy from opaque samples, and emission spectroscopy from low temperature sources. FT spectroscopy is also well suited for observing the dynamics of surface species during adsorption, desorption, and reaction. 

These developments in Raman instrumentation brought commercial Raman instruments to the present state of the art of Raman measurements. Now, Raman spectra can also be obtained by Fourier transform (FT) spectroscopy. FT-Raman instruments are being sold by all Fourier transform infrared (FT-IR) instrument makers, either as interfaced units to the FT-IR spectrometer or as dedicated FT-Raman instruments. 

A quite different approach to radiofrequency, microwave and infrared spectroscopy is that known as Fourier transform (FT) spectroscopy. As we shall see, this method of recording the spectra of transient molecular species is particularly appropriate in combination with the use of pulsed gas nozzles. For this reason it has proved to be a powerftd technique for the study of weakly bound dimer complexes formed in supersonic gas expansions. It has, however, also been used for the study of diatomic molecules, both... 

The Fourier transform (FT) has revolutionised spectroscopy such as NMR and IR over the past two decades. The raw data are not obtained as a comprehensible spectrum but as a time series, where all spectroscopic information is muddled up and a mathematical transformation is required to obtain a comprehensible spectrum. One reason for performing FT spectroscopy is that a spectrum of acceptable signal to noise ratio is recorded much more rapidly then via conventional spectrometers, often 100 times more rapidly. This has allowed the development of, for example, 13C NMR as a routine analytical tool, because the low abundance of 13 C is compensated by faster data acquisition. However, special methods are required to convert this time domain ... 

We will mainly discuss studies of radicals to give an overview of this area of research. We briefly describe FT spectroscopy in the infrared and ultraviolet/visible and discuss a few of the experiments carried out in our laboratories. We compare... 

Regardless of the spectroscopic method used, the signal level generated by each spectral element, dcr, is dependent on the product of the spectral intensity in do, B(o) (with units of W/cm 1), and the observation time [9]. In FT spectroscopy, the whole spectrum is collected at once, so that each spectral element is sampled for the whole observation time, t. Then the signal from each spectral element is proportional to ... 

Recently there have been many studies of reactive molecules making use of FT spectroscopy in the UV/VIS (see Table 2). FT emission spectra have successfully characterized the electronic states of many radicals and other reactive species Produced in a variety of sources. Other workers have used FT emission and FT... 

The FT direct absorption spectra [28] of OCIO provide an example of the capabilities of FT spectroscopy in the visible and ultraviolet regions for the study of short-lived species. In Figure 14, part of the near-UV absorption spectrum is... 

That direct absorption spectra with high signal-to-noise can be obtained with such dilute expansions demonstrates the superior sensitivity of the FTUV technique as compared to a conventional dispersive method. Based on the OCIO A <— X absorption cross section of about 3000 L mol-1 cm-1, we calculated [28a that the FTUV method has 15 times better sensitivity and three orders of magnitude higher resolution than our best dispersive absorption, supersonic jet technique [3c. To date, systems with absorptivities as low as e = 200 L mol-1 cm-1 have been studied with FT spectroscopy in a free jet expansion with reasonable signal-to-noise at spectral resolutions as low as 0.1 cm"1. 

Vibrational spectroscopy has proven to be a powerful method of studying biological molecules. Continued technical improvement (FT spectroscopy, time resolved spectroscopy, etc.) open up new domains of investigation which help solve fundamental problems of structure-function correlation at the molecular level. Many domains are beginning to be explored, and results are expected in the fields of compatible biomaterials, intelligent drug development, and in vivo spectroscopic measurement. 

G Gellerstedt and T Josefsson. Use of FT Spectroscopy and Chemometrics for Analysis of Wood Components. 3rd European Workshiop on Lignocellusics and Pulp, Stockholm, 1994, pp. 179-182. 

A Fourier transform spectrometer is also a scanning wavemeter. It transfers the absolute frequency calibration of a reference laser to the frequencies of all lines in the FT spectrum. The I2, Te2, Br2, and H20 line atlases, with absolute accuracy on the order of 0.001cm-1, are measured by FT spectroscopy relative to the more accurately known frequency of the He-Ne laser. 

Figure 9.8 Femtosecond Transition State (FTS) spectroscopy of I2. By adjusting the center wavelength of the pump pulse (Ai) a wavepacket is launched on the repulsive wall of the weakly bound A3Ifiu state (Ai 700 nm, wavepacket labelled a), the Franck-Condon vertical excitation region of the B3If + state (Ai 620 nm, wavepacket labelled b), or on the repulsive wall of the B3If0+ state (and the repulsive 1Ifu state) near the I(2 P3/2) +1 P1/2)...

The advent of FT spectroscopy has made spectroscopists far more knowledgeable about dealing with spectral information in both FT and cw techniques and in both wide line and high resolution NMR. 

For wide line NMR, it is especially important to understand the fundamentals of FT spectroscopy because some of the aspects which can be ignored in high resolution FT spectroscopy are significant for wide lines. Even for the high resolution NMR spectroscopist, this section will provide additional understanding. 

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