In chemistry, absorption (also called gas absorption, gas scrubbing, or gas washing), there is a transfer of one or more species from the gas phase to a liquid solvent. The species transferred to the liquid phase are referred to as solutes or absorbate. Absorption involves no change in the chemical species present in the system. Absorption is used to separate gas mixtures, remove impurities, or recover valuable chemicals. The operation of removing the absorbed solute from the solvent is called stripping. Absorbers are normally used with strippers to permit regeneration (or recovery) and recycling of the absorbent. Since stripping is not perfect, absorbent recycled to the absorber contains species present in the vapor entering the absorber. When water is used as the absorbent, it is normally separated from the solute by distillation rather than stripping.

In physics, absorption of electromagnetic radiation is how matter (typically electrons bound in atoms) takes up a photon’s energy — and so transforms electromagnetic energy into internal energy of the absorber (for example, thermal energy). Absorption of electromagnetic radiation is the combined result of the Compton effect), and photoelectric absorption. A notable effect (attenuation) is to gradually reduce the intensity of light waves as they propagate through a medium. Although the absorption of waves does not usually depend on their intensity (linear absorption), in certain conditions (optics) the medium’s transparency changes by a factor that varies as a function of wave intensity, and saturable absorption (or nonlinear absorption) occurs.

In acoustics, absorption refers to the process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. The energy transformed into heat is said to have been ‘lost’.

Absorption band

An absorption band is a range of wavelengths, frequencies, or energies (a series of very closely spaced absorption lines) in the electromagnetic spectrum resulting from the absorption by molecules, which are characteristic of a particular transition from initial to the final state in a substance.

According to quantum mechanics, atoms and molecules can only hold certain defined quantities of energy or exist in specific states. When electromagnetic radiation is absorbed by an atom or molecule, the energy of the radiation changes the state of the atom or molecule from an initial state to a final state. The number of states in a specific energy range is discrete for gaseous or diluted systems, with discrete energy levels.

Condensed systems, like liquids or solids, have a continuous density of states distribution and often possess continuous energy bands. In order for a substance to change its energy, it must do so in a series of “steps” by the absorption of a photon. This absorption process can move a particle, like an electron, from an occupied state to an empty or unoccupied state. It can also move a whole vibrating or rotating system, like a molecule, from one vibrational or rotational state to another or it can create a quasiparticle like a phonon or a plasmon in a solid.

Absorption spectroscopy

Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample.

Absorption spectrum

An absorption spectrum is a spectrum of absorption lines or bands, produced when light from a hot source, itself producing a continuous spectrum, passes through a cooler gas. A material’s absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. The absorption spectrum is primarily determined by the atomic and molecular composition of the material. An absorption spectrum is, in a sense, the opposite of an emission spectrum. Radiation is more likely to be absorbed at frequencies that match the energy difference between two quantum mechanical states of the molecules. The absorption that occurs due to a transition between two states is referred to as an absorption line and a spectrum is typically composed of many lines.

Every chemical element has absorption lines at several particular wavelengths corresponding to the differences between the energy levels of its orbitals. The frequencies where absorption lines occur, as well as their relative intensities, primarily depend on the electronic and molecular structure of the sample. The frequencies will also depend on the interactions between molecules in the sample, the crystal structure in solids, and on several environmental factors (e.g., temperature, pressure, electromagnetic field). The lines will also have a width and shape that are primarily determined by the spectral density or the density of states of the system. For example, an object that absorbs blue, green, and yellow light will appear red when viewed under white light. Absorption spectra can, therefore, be used to identify elements present in a gas or liquid. This method is used in deducing the presence of elements in stars and other gaseous objects which cannot be measured directly.

How an absorption spectrum is formed

Atoms and molecules may change states when they absorb specific amounts of energy. Atomic states are defined by the arrangement of electrons in atomic orbitals. An electron in some orbital may be excited to a more energetic orbital by absorbing exactly one photon which has an energy equal to the energy difference of the two orbitals.

Molecular states are defined by the molecule’s modes of vibration and rotation. These vibrational and rotational modes are quantized, similar to the atomic orbitals, and maybe excited by absorbing single photons.

In both the atomic and molecular cases, the excited states do not persist: after some random amount of time, the atoms and molecules revert back to their original, lower energy state. In atoms, the excited electron returns to a lower orbital, emitting a photon. In molecules, the vibrational or rotational mode decays, also emitting a photon.

When this decay occurs, the photon produced is not necessarily emitted in the same direction as the original photon. The most common angle of this has been shown to be about 45 degrees of the original photon. This applies to any situation where gases lie between a light source and an observer: the observer will see gaps in the spectrum of the light corresponding to the wavelengths of the photons which were absorbed. These gaps occur despite the re-emission of photons because the re-emitted photons are equally likely to travel in all directions, and it is statistically unlikely to travel along the original path to the observer. These gaps appear as black lines in an image of the spectrum.

Spectral line (absorption line)

Spectral lines are the result of interactions between a quantum system (usually atoms, but sometimes molecules or atomic nuclei) and single photons. All spectra and spectral lines arise from transitions between discrete energy states of matter, as a result of which photons of corresponding energy (and hence characteristic frequency or wavelength) are absorbed or emitted. From the energy levels thus determined, atomic and molecular may be studied. A photon is absorbed when it has the correct energy to allow a change in the energy state of the system (in the case of an atom it is usually an orbital jump of an electron); then it will be re-emitted spontaneously, or in the same frequency as the original or in cascade, where the sum of the energy of the emitted photons will be the same as the absorbed photon. The direction of the new photons will not be in relation to the direction of the original photon.

Depending on the geometry of the gas, the source of the photon and the observer, either an emission line or an absorption line will be produced: if the gas is between the photon source and the observer, a drop in light intensity is observed in the frequency of the incident photon, since the re-emitted photons will be in different directions than the original. This is an absorption line. If the observer sees the gas, but not the photon source, he will only see the photons re-emitted in a narrow frequency range. This is an issue line.

The absorption lines and the emission lines are extremely atom-specific and can be used to easily identify the chemical composition of all the means that light can pass through (usually gases). They also depend on the physical conditions of the gas and therefore are widely used to determine the chemical composition of stars and other celestial bodies that cannot be analyzed by other means, as well as their physical conditions. 

Other mechanisms, besides atom-photon interaction, can produce spectral lines. Depending on the physical interaction (with molecules, single particles, etc.) the frequency of the photons involved varies widely, and the lines can be observed along the entire electromagnetic spectrum, from radio waves to gamma rays.

An absorption line is a dark line in a continuous spectrum that corresponds to the absorption of light, or some other form of electromagnetic radiation, at a well-defined wavelength; the pattern of such lines is characteristic of specific atoms or molecules in the path of the radiation. If a light source with a continuous spectrum is viewed through a cool gas then dark lines appear in the spectrum. The wavelengths of the lines are identical to the wavelengths of emission lines from the same gas when heated. Spectral lines are often used to identify atoms and molecules.

The pattern of absorption lines in a spectrum is diagnostic of the types of atoms and molecules present, for example, in the surface layers of a star or the atmosphere of a planet. Absorption lines are seen in the spectra of the Sun and other stars. Most of them are Fraunhofer lines but some arise in the cool interstellar gas along the line of sight and give clues to the physics and chemistry of the interstellar medium. Absorption lines in quasars carry information about intergalactic space.

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