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In some cases, overtone bands are observed. Such a band appears at approximately twice the energy of the fundamental band for the same normal mode. Some excitations, so-called combination modes, involve simultaneous excitation of more than one normal mode.

The phenomenon of Fermi resonance can arise when two modes are similar in energy; Fermi resonance results in an unexpected shift in energy and intensity of the bands etc. When the frequency of the IR is the same as the vibrational frequency of a bond or collection of bonds, absorption occurs.

Examination of the transmitted light reveals how much energy was absorbed at each frequency or wavelength. This measurement can be achieved by scanning the wavelength range using a monochromator. Alternatively, the entire wavelength range is measured using a Fourier transform instrument and then a transmittance or absorbance spectrum is generated using a dedicated procedure.

This technique is commonly used for analyzing samples with covalent bonds. Simple spectra are obtained from samples with few IR active bonds and high levels of purity.

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More complex molecular structures lead to more absorption bands and more complex spectra. Typical IR solution cell.

The windows are CaF2. Gas samples[ edit ] Gaseous samples require a sample cell with a long pathlength to compensate for the diluteness. The pathlength of the sample cell depends on the concentration of the compound of interest.

A simple glass tube with length of 5 to 10 cm equipped with infrared-transparent windows at the both ends of the tube can be used for concentrations down to several hundred ppm.]

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