Infra-red Absorption Spectroscopy

As was discussed in the article Scattering and Absorption, a molecule will only absorb a photon and be promoted to an excited state if the energy of the incident radiation matches the difference in energy between the ground state and an excited state of the molecule, which can be an excited electronic, vibrational, or rotational state. The type of transition that occurs depends on the wavelength of the incident light (which is inversely proportional to its energy). Broadly speaking, ultraviolet and visible light excites electronic transitions, infrared light excites vibrational transitions, and microwaves excite rotational transitions.

The absorption of light by molecules forms the basis of the measurement technique known as molecular absorption spectroscopy, which is used to determine the presence of a given chemical compound in a sample, and in some cases, its prevalence. The technique measures the amount of radiation absorbed by the sample as a function of the wavelength of the incident light, yielding an absorption spectrum. The level of absorption is determined by measuring the fraction of the incident light that is transmitted through the sample. Absorption only occurs when the frequency of the incident radiation matches one of the electronic, rotational, or vibrational transitions of a molecule. Hence a dip in transmission at a given wavelength indicates the presence of a molecular transition at that wavelength. Because electronic, rotational, and vibrational transitions are particular to the chemical structure and symmetry of a molecule, the various dips in transmission recorded can be used to determine the presence of a given molecule.

The dips in transmission observed in absorption spectroscopy are analogous to the shifts in frequency observed in Raman spectroscopy, and they too can be seen as “optical fingerprints” by which molecules can be identified (in fact the absorption spectrum and the Raman spectrum of a given molecule often resemble each other quite closely). The technique is referred to as molecular absorption spectroscopy to distinguish it from atomic absorption spectroscopy, in which the transitions involved are purely electronic. In molecular absorption spectroscopy, the transitions can be electronic, vibrational, or rotational, and depend on the wavelength of the incident light. Approximately speaking, microwave wavelengths excite rotational transitions, infrared wavelengths excite vibrational transitions, and visible and UV wavelengths excite electronic transitions.

In infrared absorption spectroscopy, the light directed onto the sample covers a range of frequencies in the infrared region of the electromagnetic spectrum. This type of absorption spectroscopy is most useful for the identification and analysis of organic compounds.

Click here to read about scattering and absorption.


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