Conventional interferogram

In the usual manifestation of this experiment, the velocity of the interferometer mirror must be low enough that the conventional interferogram can be filtered out before the signal is input to the lock-in amplifier. In addition, the amplitude of the PEM must be set at a certain value, so that for only one wavelength in the spectrum is the polarization of the output radiation at the extreme of the cycle rotated by exactly 90°. A method of performing polarization-modulation IRRAS... 

The method is proposed by Hartl et al. [19-21]. The colorimetric interferometry technique, in which him thickness is obtained by color matching between the interferogram and color/fllm thickness dependence obtained from Newton rings for static contact, represents an improvement of conventional chromatic interferometry. 

All aspects of interferogram and experimental data acquisition and optical test rig control are provided by a computer program that also performs film thickness evaluation. It is believed that the film thickness resolution of the colorimetric interferometry measurement technique is about 1 nm. The lateral resolution of a microscope imaging system used is 1.2 /u,m. Figure 10 shows a perspective view of the measurement system configuration. This is an even conventional optical test rig equipped with a microscope imaging system and a control unit. 

In mathematical terms, the relationship between the interferogram in FTIR and the absorbance spectrum obtained by conventional IR is that the interferogram is the Fourier transform of the absorbance spectrum - hence the term FTIR. Application of an inverse Fourier transform to the interferogram, therefore, converts the output of the FTIR into a conventional display. 

The newer instruments (Figure 2.4c) utilize a radiofrequency pulse in place of the scan. The pulse brings all of the cycloidal frequencies into resonance simultaneously to yield a signal as an interferogram (a time-domain spectrum). This is converted by Fourier Transform to a frequency-domain spectrum, which then yields the conventional m/z spectrum. Pulsed Fourier transform spectrometry applied to nuclear magnetic resonance spectrometry is explained in Chapters 4 and 5. 

The results may be displayed either as an isometric 3-D plot of intensity vs temperature and wavelength or as a contour map. Individual interferograms can also be summed to display a conventional 2-D glow-curve over the full temperature range. 

The two main differences between MCFT and conventional FT-Raman are both derived from the characteristics of the CCD, and both are fundamental. First, the resolution depends on the number of CCD elements along the interferogram axis. Since one cannot arbitrarily vary the size of the CCD or the pixel spacing, there is less flexibility than with a Michelson system, where mirror travel and sampling rate are variable. For the configuration shown in Figure 9.17, Eq. (9.8) applies (23) ... 

Another purpose of the measurement of the pseudosample is that it provides a phase correction needed for Fourier transform of the AC interferogram. The selfcorrection conventionally used in Fourier transformation for one-sign absorption... 

To obtain IR spectra on a time scale of nanoseconds, the sample cell in conventional spectrometers is usually excited by an Nd YAG laser. Flow cells with a pathlength of at least 0.1 mm must be used for photoreactive samples and the pulse repetition frequency is then limited to 1 Hz. In step scan FTIR spectroscopy,211 the time evolution is collected at single points of the interferogram, which is then reconstructed point-by-point and subsequently transformed to time-resolved IR spectra. Alternatively, dispersive instruments equipped with a strong IR source can be used.212 The time resolution of both methods is about 50 ns. FTIR instruments provide a triggerable fast-scan mode to collect a complete spectrum within a few milliseconds.213... 

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