Infrared interferogram

Figure 4.16. Experimental setting of the combined femtosecond pulsed laser and step scan IR spectrometer (left) and modifications of the infrared interferogram after the laser pulse (right) [187].

Figure 3.16 (a) Infrared interferogram of the absorption spectrum of air in the 400 3400 cm region and (b) the Fourier transformed spectrum... 

Collette, T.W. (1990) Ester hydrolysis rate constant prediction from infrared interferograms. Environ. Sci. Technol. 24, 1671-1676. 

The data acquisition rate (sampling frequency) for mid-IR interferograms is usually equal to the frequency, Hz, of the interferogram that is generated by a laser (usually a helium-neon laser at 632.8nm) simultaneously with the infrared interferogram. f is equal to the product of the wavenumber of the laser and the... 

Small, G. W., Harms, A. C., Kroutil, R. T., Ditillo, J. T. Loerop, W. R. (1990) Design of optimized finite impulse-response digital-filters for use with passive Fourier-transform infrared interferograms. Ana/. Chem. 62, 1768-1777. 

Mattu, M. J. Small, G. W, (1995) Quantitative analysis of bandpass-filtered Fourier-transform infrared interferograms. Ana/yfica/ Chemistry 67,2269-2278. 

Shaffer, R. E., Small, G. W., Combs, R. J., Knapp, R. B. Kroutil, R. T. (1995) Experimental-Design Protocol For the Pattern-Recognition Analysis of Bandpass Filtered Fourier-Transform Infrared Interferograms. Chemometrics and Intelligent Laboratory Systems 29,89-108. 

Figure 3.16(a) shows an interferogram resulting from the infrared absorption spectmm of air in the 400-3400 cm region. The Fourier transformed spectmm in Figure 3.16(b)... 

The growth and decay of all other species (including O3) were monitored by Fourier transform infrared (FT-IR) spectroscopy at a total pathlength of 460 meters and a spectral resolution of 1 cm". At this pathlength, the intense absorptions of H2O and CO limit the usable IR spectral windows to the approximate regions 750-1300, 2000-2300, and 2400-3000 cm". Each spectrum (700-3000 cm" ) was adequately covered by the response of the Cu Ge detector. Approximately 40 seconds were required to collect the 32 interferograms co-added for each spectrum. 

Transmission infrared spectra of species adsorbed on the catalyst were taken with a Digilab FTS-10M Fourier-transform infrared spectrometer, using a resolution of 4 cm-l. To improve the signal-to-noise ratio, between 10 and 100 interferograms were co-added. Spectra of the catalyst taken following reduction in H2 were subtracted from spectra taken in the presence of NO to eliminate the spectrum of the support. Because of the very short optical path through the gas in the reactor and the low NO partial pressures used in these studies, the spectrum of gas-phase NO was extremely weak and did not interfere with the observation of the spectrum of adsorbed species. 

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. 

Howard has applied his method to high-resolution infrared spectroscopic data obtained by dispersive techniques and to both experimental and simulated Fourier interferograms. The method in the latter application explicitly renders the data as they would be observed by an interferometer having a path difference exceeding the mechanical limits of the instrument used for the observation. Details on both the method and its application constitute Chapters 9 and 10. 

It is found that multiplication of the Fourier transform of the data by a carefully chosen window function is very effective in removing the artifacts around peaked functions. This process is called apodization. Apodization with the triangular window function is often applied to Fourier transform spectroscopy interferograms to remove the ringing around the infrared... 

In the operating mode customarily used, which is to determine the existence, location, and intensity of the spectral lines, the interferometer produces an interferogram that is symmetric about the zero displacement position. If the zero displacement position (the maximum point on the central fringe ) is taken as the origin of the interferogram function, the Fourier transform of this will produce an infrared spectrum that is real and symmetric about... 

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