Absorption spectroscopy

Exploiting the interaction between radiation and matter

When electromagnetic radiation interacts with solids, liquids or gases, various effects such as absorption, reflection or scattering occur. The general interaction of radiation with matter is analyzed. When electromagnetic radiation is absorbed by atoms or molecules, they are transferred from a ground state to an energetically excited state. In the process, energy of a certain frequency is absorbed, resulting in molecular states with altered energy levels that are characteristic for the respective substance.

Absorption spectrum for identification and analysis of substances

In the infrared light range, rotation and oscillation of a molecule can be excited. In the visible and ultraviolet light range, we can observe the excitation of valence electrons caused by the absorption of defined energy packets (quanta). If a light beam with the intensity I0 penetrates a medium with the layer thickness d, the light beam is weakened by specific absorption properties of the sample, apart from reflection and scattering losses. The outgoing light beam (transmission) now has the intensity I. With some limitations, the relationship between absorption and concentration is described by the Beer-Lambert law. In addition to classical transmission, absorption can be verified indirectly via subsequent processes such as fluorescence, photoacoustics or the change in optical diffraction of the medium. Molecules of a specific substance carry the absorption depending on the wavelength of the electromagnetic radiation (absorption spectrum) like an identity card, so that, in turn, absorption spectra can be used by spectroscopy to identify and analyze substances. Various spectroscopic methods have become established on the market and must be adapted from case to case for process or laboratory analysis. Fraunhofer IPM specializes in laser spectroscopic methods, but FTIR, Raman and ATR spectroscopy are also routinely used. In practice, sample preparation and the associated sampling technique are a particular challenge, strongly influencing both the specific sensitivity and the reproducibility of the analysis.

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