A collision can transfer kinetic energy to an atom or ion and excite a bound electron. Subsequent radiative de-excitation removes that energy from an optically thin gas. The thermal excitation rate is strongly reduced when the transition energy exceeds , giving the threshold behavior of atomic line cooling.
A collision transfers an atom or ion from an excited level to a lower level while increasing the collider's kinetic energy. It competes with spontaneous emission and quenches line radiation when its rate is comparable to or larger than the radiative rate. The collision itself does not emit the line photon.
The dimensionless collision strength describes electron-impact transitions between two atomic levels before thermal averaging. It can vary rapidly with collision energy, especially near excitation thresholds. The quantity entering a thermal collision rate coefficient for an atomic transition is its effective collision strength, usually denoted , rather than one value of .[ref]
The CHIANTI upsilon files (ups and scups) by Peter Young. PDF p. 2, introductory definition: Maxwellian-integrated strengths depend on temperature
For levels , write and measure the scattering variable above the upper-level threshold. With a Maxwellian electron population,Let be the statistical weights of atomic levels, and set . The excitation and de-excitation rate coefficients areTheir ratio obeys detailed balance. The numerical coefficient is when the rate coefficient is expressed in .[ref]
Fine-structure collision strengths and line ratios for Ne V in infrared and optical sources by Michael Dance and others. Section 2.3, equations (3)–(4): Maxwellian averaging and excitation coefficient
If is constant over the sampled energy range, the normalized integral gives . Otherwise low-energy resonances can strongly alter the thermal result. For a non-Maxwellian velocity distribution, compute the appropriate cross-section–speed average; a tabulated Maxwellian effective collision strength is not automatically transferable. Multiplying by electron number density gives the excitation rate per target chemical atom.
The effective collision strength is the dimensionless thermal average of an electron-impact collision strength. For a Maxwellian electron population, with collision energy measured above the upper-level threshold,It is a function of temperature, rather than a cross section evaluated at one representative energy.
If is constant, the normalized exponential weight gives . Near-threshold resonances can instead strongly change the average, particularly at low temperature. Atomic excitation and de-excitation rate coefficients use this average together with statistical weights and, for excitation, the appropriate Boltzmann threshold factor.[ref]
For a normalized electron speed distribution , the atomic transition rate coefficient is . Its dimensions are volume per time, so the rate per target atom is . Maxwellian detailed balance gives for .
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Collisional excitation is a process in which an atom or molecule absorbs energy during a collision with a particle, such as another atom, molecule, or electron. This energy transfer can promote an electron within the atom or molecule to a higher energy state, or excited state. Here's how it works: 1. **Encounter**: During a collision, kinetic energy from the colliding particle (which can be a gas particle or an electron) is transferred to the target atom or molecule.