Absorption cross section is a measure for the probability of an absorption process. More generally, the term cross section is used in physics to quantify the probability of a certain particle-particle interaction, e.g.,

scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...

, electromagnetic absorption, etc. (Note that light in this context is described as consisting of

particle In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, fro ...

s, i.e.,

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...

s.) In honor of the fundamental contribution of Maria Goeppert Mayer to this area, the unit for the two-photon absorption cross section is named the "GM". One GM is 10⁻⁵⁰ cm⁴⋅s⋅photon⁻¹. In the context of

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

shielding of

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...

, absorption cross section is the ability of a

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

to absorb a photon of a particular

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

and

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

. Analogously, in the context of

nuclear engineering Nuclear engineering is the branch of engineering concerned with the application of breaking down atomic nuclei (fission) or of combining atomic nuclei ( fusion), or with the application of other sub-atomic processes based on the principles of nu ...

it refers to the probability of a particle (usually a

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

) being absorbed by a nucleus. Although the units are given as an area, it does not refer to an actual size area, at least partially because the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

or state of the target molecule will affect the

probability Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...

of absorption. Quantitatively, the number

dN

of photons absorbed, between the points

x

and

x + dx

along the path of a beam is the product of the number

N

of photons penetrating to depth

x

times the number

n

of absorbing molecules per unit volume times the absorption cross section

\sigma

: :

\frac= -N n \sigma

. The absorption cross-section is closely related to

molar absorptivity Molar may refer to: *Molar (tooth), a kind of tooth found in mammals *Molar (grape), another name for the Spanish wine grape Listan Negro *Molar (unit), a unit of concentration equal to 1 mole per litre *Molar mass * Molar volume *El Molar, Tarrago ...

\varepsilon

and mass absorption coefficient.

\sigma= \log(10)10^3/N_\text*\varepsilon

For a given particle and its energy, the absorption cross-section of the target material can be calculated from mass absorption coefficient using: :

\sigma= (\mu/\rho) m_\text/N_\text

where: *

\mu/\rho

is the mass absorption coefficient *

m_\text

is the

molar mass In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecula ...

in g/mol *

N_\text

Avogadro constant The Avogadro constant, commonly denoted or , is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. It is an SI defining con ...

This is also commonly expressed as: :

\sigma= \alpha/n

where: *

\alpha

is the

absorption coefficient The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient valu ...

n

is the atomic number density

References

{{reflist Electromagnetism Nuclear physics Scattering, absorption and radiative transfer (optics)

See also

References