Interferometric infrared

Previously, we referred to the FFT. Its development was a giant step that enabled the efficient computation of discrete Fourier transforms. The FFT algorithm and its variations have revolutionized signal analysis and made interferometric infrared spectroscopy practical. Both NMR spectra and mass spectra are also now computed from data that are acquired in their Fourier transform domain. The rediscovery of this algorithm by Cooley and Tukey (1965) is responsible for its current widespread use. Summaries of its properties and pitfalls are provided by Bergland (1969), Brigham (1974), and Bracewell (1978). 

Hitherto, in the form of reflection-absorption infrared spectroscopy (RAIRS), the infrared method had been capable of detecting single monolayers only in the exceptionally favorable (strong absorption) cases of carboxylate ions [Francis and Ellison (14)] or carbon monoxide [Chesters, Pritchard, and Sims (15)] adsorbed on flat metal surfaces. The new challenge from VEELS provided the motivation for a search for improvements in RAIRS sensitivity, and this was very successfully achieved by M. A. Chesters and his colleagues through the introduction of Fourier-transform-based interferometric infrared spectroscopy (16). 

The basic concepts of dispersive and interferometric infrared spectroscopy are dealt with in this chapter. An historical approach is taken in which many of the problems encountered in the development of both techniques are discussed along with the modifications used to solve them. The benefits, drawbacks, and limitations of both techniques are discussed. The materials and instrumentation used in infrared spectroscopy are outlined and discussed, but specific implementation is left to other sources, and sample-handling techniques should be studied in more comprehensive texts on sampling. A short bibliography is included which will allow the investigator to research selected topics more thoroughly. 

In this chapter will be presented an introduction to infrared spectroscopy. It is by no means intended to be all-inclusive whole books which have been written on highly specialized aspects of infrared spectroscopy do not even claim to be exhaustive in their own specialty. It is meant to provide a base for the beginning spectroscopist, and it is also meant to provoke a host of questions. The questions usually will not be difficult to answer it is merely beyond the scope of one chapter to provide all the answers. A novice does require at least a vague notion of where to begin the quest it is hoped that this chapter will provide stimuli and point the novice toward the most useful path. Several basic texts on both dispersive and interferometric infrared spectroscopy that have proved to be invaluable to the authors are included as references. Further chapters will also serve to explain more detailed concepts of and applications for infrared spectroscopy. 

When the spectral characteristics of the source itself are of primary interest, dispersive or ftir spectrometers are readily adapted to emission spectroscopy. Commercial instmments usually have a port that can accept an input beam without disturbing the usual source optics. Infrared emission spectroscopy at ambient or only moderately elevated temperatures has the advantage that no sample preparation is necessary. It is particularly appHcable to opaque and highly scattering samples, anodized and painted surfaces, polymer films, and atmospheric species (135). The interferometric... 

Rousselet-Perraut, K., Haguenaue, P, Petmezakis, P, Berger, J., Mourard, D., Ragland, S.D., Huss, G., Reynaud, F, FeCoarer, E., Kern, P.Y., Malbet, R, 2002, Qualification of IONIC (integrated optics near-infrared interferometric camera), SPIE 4006, 1042... 

Figure 8.1 (a) Block diagram of the femtosecond near-infrared laser microscope system, (b) Spectrum ofthe light pulse from the Cr F laser, (c) Interferometric autocorrelation trace of SHG signal with envelope curve calculated assuming a chirp-free Gaussian pulse with 35 fs fwhm. 

J.A. de Haseth and T.L. Isenhour, Reconstruction of gas chromatograms from interferometric gas chromatography/infrared spectrometry data, Anal. Chem., 49, 1977-1981 (1977). 

For readers interested in greater detail, Fourier transform techniques are treated in the following references (a) Marshall, A.G. Verdun, F.R. Fourier Transforms in NMR, Optical, and Mass Spectrometry Elsevier Amsterdam, 1986 (b) Griffiths, P.R., DeHaseth, J.A. Fourier Transform Infrared Spectrometry Wiley-Interseience New York, 1986 (c) Chamberlain, J. The Principles of Interferometric Spectroscopy Wiley-Interscience Chichester, 1979 (d) Bell, R. J. Introductory Fourier Transform Spectrometry Academic Press New York, 1972. 

In contrast to the relatively limited number of experimental approaches utilized to determine electron collisional information for C02 laser species, many different types of experiments have been employed in the determination of heavy particle rates as a function of temperature, for temperatures slightly below room temperature up to several thousand degrees. At room temperature, measurements have been obtained using sound absorption and/or dispersion as well as impact-tube and spectrophone techniques. High temperature rate data have been obtained primarily from shock tube experiments in which electron beam, infrared emission, schlieren, and interferometric diagnostic techniques are employed. For example, as many as 36 separate experiments have been conducted to determine the relaxation rate of the C02 bending mode in pure C02 [59]. The reader is referred to the review by Taylor and Bitterman [59] of heavy-particle processes of importance to laser applications for a detailed description and interpretation of available experimental and theoretical data. 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

Fig. 2.2. Far infrared spectrum of oxetanone determined interferometrically. P = 48 torr, pathlength = 1 m. Absorption is plotted downward in this spectrum.

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