Infrared spectroscopy Michelson interferometer

Figure Bl.2.6. Schematic representation of a Michelson interferometer. From Griffiths P R and de Flaseth J A 1986 Fourier transfonn infrared spectroscopy Chemical Analysis ed P J Hiving and J D Winefordner (New York Wiley). Reprinted by pemiission of Jolm Wiley and Sons Inc.

In the mid-IR, routine infrared spectroscopy nowadays almost exclusively uses Fourier-transform (FT) spectrometers. This principle is a standard method in modem analytical chemistry45. Although some efforts have been made to design ultra-compact FT-IR spectrometers for use under real-world conditions, standard systems are still too bulky for many applications. A new approach is the use of micro-fabrication techniques. As an example for this technology, a miniature single-pass Fourier transform spectrometer integrated on a 10 x 5 cm optical bench has been demonstrated to be feasible. Based upon a classical Michelson interferometer design, all... 

A Fourier transform infrared spectroscopy spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitized. A digital computer initially records and then processes the interferogram and also allows the spectral data that results to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

One of these devices that is typically used in infrared spectroscopy is the Michelson interferometer (Fig. 6.22). This device works by splitting the beam into two components perpendicular to each other. Then each beam gets reflected by minors in such a way that the reflected beams recombine again at the beam splitter. In one of these beams a path difference is introduced by moving the mirror on which it reflects. This... 

Fourier transform infrared (FTIR) spectroscopy has been extensively developed over the past decade and provides a number of advantages. The main part of FTIR spectrophotometer is the Michelson interferometer. Radiation containing all IR wavelengths (e.g., 4000-400 cm 1) is emitted by source of infrared radiation (Globar) and is split into two beams. One beam is of fixed length, and the other is of variable length (movable mirror). 

Figure 3.4-1 Optical diagram of a commercial Michelson interferometer for infrared and Raman spectroscopy (Bruker IFS 66 with Raman module FRA 106). CE control electronics, D1/D2 IR detectors, BS beamsplitter, MS mirror scanner, IP input port, S IR source, AC aperture changer, XI — X3 external beams, A aperture for Raman spectroscopy, D detector for Raman spectroscopy, FM Rayleigh filter module, SC sample compartment with illumination optics, L Nd.YAG laser, SP sample position.

Between the source and the detector is put either monochromators used in dispersive instruments or interferometers used in Fourier transform infrared (FT-IR) instruments. In a dispersive instrument the intensity at each wavenumber is measured one by one in sequence and only a small spectral range falls on the detector at any one time. In a FT-IR instrument the intensities of all the wavenumbers are measured simultaneously by the detector. Fourier transform infrared spectroscopy offers some advantages compared to dispersive instruments, namely (i) higher signal-to-noise ratios for spectra obtained under conditions of equal measurement time, and (ii) higher accuracy in frequency for spectra recorded over a wide range of frequencies. Therefore we will give below a brief picture of the principle of FT-IR spectroscopy, based on a Michelson interferometer (Fig. 2). 

Summarizing the results of our discussion of the practice of Fourier transform spectroscopy, we start with the presumption that the equipment for most routine spectroscopic investigations consists of a Fourier spectrometer with a Michelson interferometer and a digital computer. In other words, the advantages of the lamellar grating used as a two-beam interferometer, and of phase modulation, for example, have been utilized only for certain special applications in the extreme far-infrared. All commercial Fourier spectrometers are available with a computer attached, which in most cases not only performs the Fourier transform but is also programmed to control the instrument. Commercial instruments have a remote switch for the selection of the different spectral ranges, and the filters and beams... 

Fourier transform infrared spectroscopy (FTIR) had its origins in the interferometer developed by Michelson in 1880 and experiments by astrophysicists some seventy years later. A commercial FTIR instrument required development of the laser (1960, by Theodore H. Maiman [1927- ], Hughes Aircraft), refined optics, and computer hardware and software. The Fourier transform takes data collected in time domain and converts them to frequency domain, the normal infrared (IR) spectrum. FTIR provided vasdy improved signal-to-noise ratios allowing routine analyses of microgram samples. 

In Fourier transform infrared spectroscopy the instrument consists of a Michelson interferometer, cf. Fig. 16.1. The light entering into the interferometer is split by a semi-silvered mirror into two beams. Each beam goes a different path, finally the beams being recombined again. The recombined beam is directed to a detector. In the case of monochromatic and coherent light, the intensity of the recombined beam is dependent entirely on the difference of the path length, because of interference. 

The modern spectrometers [7] came with the development of the high p>erformance Fourier Transform Infrared Spectroscopy (FT-IR) with the application of a Michelson Interferometer [10]. Both IR spectrometers classical and modern give the same information the main difference is the use of Michelson interferometer, which allows all the frequencies to reach... 

Figure 2.4 Schematic of a Michelson interferometer. From Stuart, B., Modern Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1996. UnivCTsity of Greenwich, and reproduced by permission of the University of Greenwich.

Fourier Transform Infrared Spectroscopy (FTIR) is based on the Michelson interferometer (Figure 17.87). A monochromatic source is passed through a beam splitter to give... 

The Michelson interferometer does not measure the infrared spectrum directly. Rather, an interfero-gram is measured, and converted to a single-beam spectrum via Fourier transformation. Because of the critical role of this transformation, the method is generally referred to as Fourier-transform infrared spectroscopy, or FT-IR . Instruments using this... 

Infrared spectroscopy has been applied to the characterization of molecules. Conventional dispersive infrared spectrometers have been replaced by Fourier-transform infrared equipment, which incorporates a Michelson interferometer and presents a series of advantages over dispersive systems, such as an improvement of energy and the simultaneous measurement of the whole spectral range. 

It must be emphasized that IR spectroscopy has undergone major improvements in terms of light sources, detectors, and data systems, and particularly in developments of the Fourier transform technique which utilizes interferometers instead of salt prisms and grating monochromators. Modern commercial spectrometers operating with the aid of a Michelson interferometer produce interfero-grams which, upon mathematical decoding by means of Fourier transformation, deliver simultaneously whole absorption spectra that are commonly referred to as Fourier transform infrared FTIR) spectra. 

To measure an interferogram using a Michelson interferometer the mirror is moved back and forth once. This is called a scan. The interferograms measured while scanning are Fourier transformed to yield a spectrum, hence the term Fourier Transform Infrared (FTIR) spectroscopy. 

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