Infrared signals wavenumber

Between the source and the detector is put either monochromators used in dispersive instruments or interferometers used in Fourier transform infrared (FT-IR) instruments. In a dispersive instrument the intensity at each wavenumber is measured one by one in sequence and only a small spectral range falls on the detector at any one time. In a FT-IR instrument the intensities of all the wavenumbers are measured simultaneously by the detector. Fourier transform infrared spectroscopy offers some advantages compared to dispersive instruments, namely (i) higher signal-to-noise ratios for spectra obtained under conditions of equal measurement time, and (ii) higher accuracy in frequency for spectra recorded over a wide range of frequencies. Therefore we will give below a brief picture of the principle of FT-IR spectroscopy, based on a Michelson interferometer (Fig. 2). 

Fourier transform infrared spectroscopy (FTIR) is the most widely used vibrational spectroscopic technique. FTIR is an infrared spectroscopy in which the Fourier transform method is used to obtain an infrared spectrum in a whole range of wavenumbers simultaneously. It differs from the dispersive method, which entails creating a spectrum by collecting signals at each wavenumber separately. Currently, FTIR has almost totally replaced the dispersive method because FTIR has a much higher signal-to-noise ratio than that of dispersive method. 

The Raman spectrum, represented as the dependence of the relative intensity of the detected signal on wavenumber (Fig. 13.5) provides information on the sample s composition, structure, and vibrational modes that is complementary to the infrared spectrum. 

A single-mode He Ne laser has an emission at 632.8 nm (visible, red) which corresponds to a frequency of 15,800 wavenumbers. The reference He Ne laser beam is coupled to the moving mirror of the interferometer by either its own beamsplitter in parallel with the infrared beamsplitter, or directly through the infrared optics. The laser radiation is detected separately from the infrared radiation and is recorded as a cosine wave, as indicated by Equation 6. According to the Nyquist sampling criterion, to correctly sample the He Ne signal data would have to be collected at twice its frequency. 

Not all molecular vibrations can be detected. For a dbration to be infrared active, it must produce a net change in the dipole moment of the molecule, which means that symmetrical vibrations are either weak or in dsible in the IR region of the electromagnetic spectrum. For example, ethylene shows no IR signal for the carbon-carbon double-bond stretch. However, Raman spectroscopy, which is related to IR spectroscopy, does detect symmetrical dbrations, and shows a signal at 1623 cm for ethylene. Signals in IR and Raman are usually broad and actually extend over several wavenumbers. As a result the signal is often referred to as a band. 

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