Fourier Transform Infrared Spectrometer Interferometer

Fourier transform infrared (FT IR) spectrometry has been extensively developed over the past decade and provides a number of advantages. Radiation containing all IR wavelengths (e.g., 5000-400 cm1) is split into two beams (Fig. 3.5). One beam is of fixed length, the other of variable length (movable mirror). 

The varying distances between two pathlengths result in a sequence of constructive and destructive interferences and hence variations in intensities an inter-ferogram. Fourier transformation converts this interfer-ogram from the time domain into one spectral point on the more familiar form of the frequency domain. 

The varying distances between two pathlengths result in a sequence of constructive and destructive 

FIGURE 2.4. Optical system of double-beam IR spectrophotometer. 

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Fourier transform infrared spectrometers first appeared in the 1970s. These single beam instruments, which differ from scanning spectrometers, have an interferometer of the Michelson type placed between the source and the sample, replacing the monochromator (Figs 10.9c and 10.11). 

Advantages of Fourier transform infrared spectrometers are so great that it is nearly impossible to purchase a dispersive infrared spectrometer. Fourier transform visible and ultraviolet spectrometers are not commercially available, because of the requirement to sample the interferometer at intervals of S = l/(2Av). For visible spectroscopy, Av could be 25 000 cm 1 (corresponding to 400 nm), giving S = 0.2 im and a mirror movement of 0.1 xm between data points. Such fine control over significant ranges of mirror motion is not feasible. 

A Fourier transform infrared 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 digitised. A digital computer initially records and then processes the interferogram and also allows the spectral data that result to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

Although VCD and ROA are presently measured in entirely different ways, future configurations of the two kinds of instruments may draw them into closer correspondence. For instance, measurement of ROA using a Fourier transform infrared spectrometer, with polarization modulation developed within the interferometer, has been proposed as a possible way to measure ROA [43]. 

A Fourier transform infrared spectrometer (FT-IR) uses an interferometer,... 

Fig. 1. Scheme of a Fourier transform infrared spectrometer with a Michelson interferometer. BS, beam splitter FM, focussing mirror PM, parallelizing mirror. (Adapted from [16]). 

Fourier transform infrared spectrometers utilize an ingenious device called a Michelson interferometer, which... 

Pyroelectric transducers exhibit response limes thal are fast enough to allow them lo track the changes in the lime-domain signal from an interferometer. For Ihis reason, most Fourier transform infrared spectrometers use this type of transducer. 

The basic component of most Fourier Transform Infrared spectrometers is the Michel son interferometer. This is not the only interferometer used in FT-IR, but it is employed more often than other designs. A treatment of many other interferometer designs is available. The Michel son interferometer in a Fourier Transform Infrared spectrometer replaces the monochromator in a dispersive instrument, although the functions cannot be correlated. A monochomator divides a continuous bandwidth into its component frequencies, whereas an interferometer produces interference patterns of the bandwidth in a precise and regulated manner. It should be noted that this type of interferometer is not restricted to the infrared region and its use can be extended to the visible and millimeter regions of the electromagnetic spectrum. 

The function of the interferometer in a Fourier transform infrared spectrometer has been presented. An FT-IR spectrometer optical layout is now described and information is provided for each element of a typical spectrometer design. A schematic of a typical FT-IR optical design is given in Figure 6. 

The infrared spectra were produced using a Nicolet 170SX Fourier Transform Infrared spectrometer. This infrared spectrometer is equij ied with a laser-referenced Michelson interferometer with... 

A Fourier transform infrared spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference... 

For radiofrequency and microwave radiation there are detectors which can respond sufficiently quickly to the low frequencies (<100 GHz) involved and record the time domain specttum directly. For infrared, visible and ultraviolet radiation the frequencies involved are so high (>600 GHz) that this is no longer possible. Instead, an interferometer is used and the specttum is recorded in the length domain rather than the frequency domain. Because the technique has been used mostly in the far-, mid- and near-infrared regions of the spectmm the instmment used is usually called a Fourier transform infrared (FTIR) spectrometer although it can be modified to operate in the visible and ultraviolet regions. 

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... 

Figure 4.5 Schematic diagram of a Fourier transform infrared (FTIR) spectrometer. Infrared radiation enters from the left and strikes a beam-splitting mirror (BS) angled such that half of the beam is directed towards a fixed mirror (Mi) and half towards a moveable mirror (M2). On reflection the beam is recombined and directed through the sample towards the detector. M2 is moved in and out by fractions of a wavelength creating a phase difference between the two beam paths. This type of device is called a Michelson interferometer.

Fourier-transform infrared (FTIR) spectrometers encode infrared wavenumbers by moving a mirror in a Michelson interferometer which results in a unique, path-dependent pattern of interference for each light wavelength in the IR beam. FTIRs have come to totally dominate the IR market and are the means by which most of the work described in this review was accomplished. Only for some special applications (modulation spectra and time-dependence studies) are dispersive-based (scanning monochromator or tuned laser) spectrometers still used. The advantages of the FTIR approach are that the entire spectral region of interest can... 

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