The Moving Mirror

The moving mirror is the most crucial component of the interferometetf It has to be accurately aligned and must be capable of scanning two distances so that the path difference corresponds to a known value. A number of factors associated with the moving mirror need to be considered when calculating an infrared spectrum. 

The interferogram is in the form of an analog signal at the detector, which then has to be digitised in order that Fourier transformation into a conventional spectrum can be carried out. There are two particular sources of error in transforming the digitised information on the interferogram into a spectrum. 

Another source of error arises if the sample intervals are not exactly the same on each side of the maxima, corresponding to zero path difference. Phase correction is then required, and this correction procedure ensures that the sample intervals are the same on each side of the first interval and should thus correspond to a path difference of zero. 

The resolution is limited by the maximum path difference between the two beams. The limiting resolution in wavelength (cm ) is the reciprocal of the pathlength difference (cm). For example, a 

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The position of the moving mirror (Mt) determines the phase 8 between the intensities Jj and U as followsr ... 

In an industrial-design FTIR spectrometer, a modified form of the G enzel interferometer is utilized.A geometric displacement of the moving mirrors by one unit produces four units of optical path difference (compared with two units of optical difference for a Michelson type interferometer). The modified Genzel design reduces the time required to scan a spectrum and further reduces the noise effects asstxiated with the longer mirror translation of most interferometers. 

The final step in obtaining the spectrum by the FTIR method is turning back the data obtained as a result of the repetitive interference action of the moving mirror into an intensity wavelength line. It is here that Fourier Transform mathematics is utilized. It is the signal intensity that is stored in a digital representation of the Interferogram. This information is then Fourier transformed by the computer into the frequency spectrum. 

There are two light sources involved, a white light and a laser source. The white light uses the same moving mirror and therefore makes up a second interferometric system within the spectrometer. When the moving mirror and the fixed mirror of this secondary interferometer are equidistant, a centerburst is produced which is... 

No beam chopping device is shown in Figure 5. Motion of the moving mirror in the Michel son interferometer is equivalent to beam chopping and the frequency f is given by... 

Figure 3.29 The intensity of the second harmonic wave generated in an autocorrelator as a function of the displacement of the moving mirrors system. The insets show the intensity versus time curves for pulses A and B of Figure 3.28 (solid and dashed lines, respectively).

Figure 10.11—Optical arrangement of a Fourier transform IR spectrometer, a) A 90c Michelson interferometer including the details of the beam splitter (expanded view) b) optical diagram of a single beam spectrometer (based on a Nicolet model). A weak intensity HeNe laser (632.8 nm) is used as an internal standard to measure precisely the position of the moving mirror using an interference method (a simple sinusoidal interferogram caused by the laser is produced within the device). According to the Nyquist theorem, at least two points per period are needed to calculate the wavelength within the given spectrum.

To locate the position of the moving mirror with great precision, we superimpose in the apparatus a laser source of monochromatic radiation (v = 15 800 cm-1), which permits a computer to pick up a point of the interferogram each time the laser light is extinguished. 

Figure 10. Schematic diagram for the synchronization of the laser pulse to the mirror sweep in a CS FTIR instrument. The pulse repetition rate of the photolysis laser is determined by the velocity of the moving mirror, and the interferogram is composed of data points which have all been collected at the same delay time (=tj —t2) after the laser pulse. Reproduced with permission from Ref. 48.

Figure 6. The ratio of NEP of TGS and TC detector as a function of wavenumber (9). The chopping frequency for the TC detector is 15 Hz while that for the TGS detector is 2 X 0.3164 v Hz, where 0.3164 cm s 1 is the velocity of the moving mirror on the Digilab spectrometer.

The rate at which data are collected at an interferometer is dependent on the distance the moving mirror travels, which determines the optical resolution, and how fast the mirror travels (i.e., the scanner velocity). Modern... 

In the most common commercial implementation of interferometric measurements, the moving mirror is scanned at a constant velocity v. Consequently, the mirror... 

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