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Fig. 2 Setups for IR absorption spectroscopy. (a) Scheme for a Fourier transform infrared
spectroscopy (FTIR) experiment. The schematized interferometer is a Michelson-Morley one,
the red curve schematizes the intensity on the detector of a monochromatic light (e.g., the red line
of a HeNe laser) as a function of the mobile mirror (MM) position. Purple arrows represent
propagation of IR light. FM fixed mirror, BS beamsplitter, FFT fast Fourier transform. (b) Possible
scheme for a setup for picoseconds time-resolved infrared spectroscopy (TRIR) reproduced from
[ 5 ],
2011 Science and Technology Facilities Council
signal is then demodulated to measure directly the difference between the two light
intensities. The comparison of the signals with and without the sample allows to
obtain the transmission (T) spectrum, which can be converted to an absorbance
spectrum ( A
log 10 ( T )). The main differences with a UV/Vis spectrometer
consist in the different light sources, detectors, and optic materials, which must
be IR transparent. It should also be noted that typical cross sections and molar
extinction coefficients in the IR range are usually much lower compared to those of
the excitation of electronic levels, responsible for the UV/Vis absorption and
fluorescence spectra of molecules.
In the FTIR configuration, the “dispersion” is obtained by using a Michelson-
Morley interferometer with a mobile mirror; in this way, the information on
wavelengths is mapped in the interferogram (i.e., the intensity of the signal as
a function of the mirror position), from which the spectrum can be reobtained with
a Fourier transform. This technique allows obtaining a better signal-to-noise (S/N)
ratio in the same time when compared to the dispersive configuration. This advan-
tage stems from the facts that information for all wavelengths are collected for each
point of the interferogram, that Fourier transformation can filter out some part of the
noise, and that there is no necessity to change filters or gratings during the scan and
the detectors can be used always in the same “optimized” configuration. Moreover,
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