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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Absorption spectroscopy | 2/3 | https://en.wikipedia.org/wiki/Absorption_spectroscopy | reference | science, encyclopedia | 2026-05-05T10:03:35.666525+00:00 | kb-cron |
=== Relation to scattering and reflection spectra === The scattering and reflection spectra of a material are influenced by both its refractive index and its absorption spectrum. In an optical context, the absorption spectrum is typically quantified by the extinction coefficient, and the extinction and index coefficients are quantitatively related through the Kramers–Kronig relations. Therefore, the absorption spectrum can be derived from a scattering or reflection spectrum. This typically requires simplifying assumptions or models, and so the derived absorption spectrum is an approximation.
== Applications ==
Absorption spectroscopy is useful in chemical analysis because of its specificity and its quantitative nature. The specificity of absorption spectra allows compounds to be distinguished from one another in a mixture, making absorption spectroscopy useful in wide variety of applications. For instance, Infrared gas analyzers can be used to detect pollutants in the air, distinguishing them from nitrogen, oxygen, water, and other expected constituents. The specificity also allows unknown samples to be identified by comparing a measured spectrum with a library of reference spectra. In many cases, qualitative information about a sample can be determined even if it is not in a library. Infrared spectra, for instance, have characteristic absorption bands that indicate if carbon-hydrogen or carbon-oxygen bonds are present. An absorption spectrum can be quantitatively related to the amount of material present using the Beer–Lambert law . Determining the absolute concentration of a compound requires knowledge of the compound's absorption coefficient. The absorption coefficient for some compounds is available in reference sources, and it can also be determined by measuring the spectrum of a calibration standard with a known concentration of the target compound.
=== Remote sensing === One of the unique advantages of spectroscopy as an analytical technique is that measurements can be made without bringing the instrument and sample into contact. Radiation that travels between a sample and an instrument will contain the spectral information, so the measurement can be made remotely. Remote spectral sensing is valuable in many situations. For example, measurements can be made in toxic or hazardous environments without placing an operator or instrument at risk. Also, sample material does not have to be brought into contact with the instrument—preventing possible cross contamination. Remote spectral measurements present several challenges compared to laboratory measurements. The space in between the sample of interest and the instrument may also have spectral absorptions. These absorptions can mask or confound the absorption spectrum of the sample. These background interferences may also vary over time. The source of radiation in remote measurements is often an environmental source, such as sunlight or the thermal radiation from a warm object, and this makes it necessary to distinguish spectral absorption from changes in the source spectrum. To simplify these challenges, differential optical absorption spectroscopy has gained some popularity, as it focusses on differential absorption features and omits broad-band absorption such as aerosol extinction and extinction due to rayleigh scattering. This method is applied to ground-based, airborne, and satellite-based measurements. Some ground-based methods provide the possibility to retrieve tropospheric and stratospheric trace gas profiles.
=== Astronomy ===
Astronomical spectroscopy is a particularly significant type of remote spectral sensing. In this case, the objects and samples of interest are so distant from earth that electromagnetic radiation is the only means available to measure them. Astronomical spectra contain both absorption and emission spectral information. Absorption spectroscopy has been particularly important for understanding interstellar clouds and determining that some of them contain molecules. Absorption spectroscopy is also employed in the study of extrasolar planets. Detection of extrasolar planets by transit photometry also measures their absorption spectrum and allows for the determination of the planet's atmospheric composition, temperature, pressure, and scale height, and hence allows also for the determination of the planet's mass.
=== Atomic and molecular physics === Theoretical models, principally quantum mechanical models, allow for the absorption spectra of atoms and molecules to be related to other physical properties such as electronic structure, atomic or molecular mass, and molecular geometry. Therefore, measurements of the absorption spectrum are used to determine these other properties. Microwave spectroscopy, for example, allows for the determination of bond lengths and angles with high precision. In addition, spectral measurements can be used to determine the accuracy of theoretical predictions. For example, the Lamb shift measured in the hydrogen atomic absorption spectrum was not expected to exist at the time it was measured. Its discovery spurred and guided the development of quantum electrodynamics, and measurements of the Lamb shift are now used to determine the fine-structure constant.
== Experimental methods ==