Editing Spectral line (section)

==== Pressure broadening ====
The presence of nearby particles will affect the radiation emitted by an individual particle. There are two limiting cases by which this occurs:

* ''Impact pressure broadening'' or ''collisional broadening'': The collision of other particles with the light emitting particle interrupts the emission process, and by shortening the characteristic time for the process, increases the uncertainty in the energy emitted (as occurs in natural broadening).<ref>{{cite web|url=http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html |title=Collisional Broadening |publisher=Fas.harvard.edu |archive-url=https://web.archive.org/web/20150924042746/http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html |access-date=2015-09-24|archive-date=2015-09-24 }}</ref><!--<ref>{{cite web|url=http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html |title=Collisional Broadening |publisher=Fas.harvard.edu |access-date=2015-05-02}}</ref>--> The duration of the collision is much shorter than the lifetime of the emission process. This effect depends on both the [[density]] and the [[temperature]] of the gas. The broadening effect is described by a [[Lorentzian function|Lorentzian profile]] and there may be an associated shift.
* ''Quasistatic pressure broadening'': The presence of other particles shifts the energy levels in the emitting particle (see [[spectral band]]), thereby altering the frequency of the emitted radiation. The duration of the influence is much longer than the lifetime of the emission process. This effect depends on the [[density]] of the gas, but is rather insensitive to [[temperature]]. The form of the line profile is determined by the functional form of the perturbing force with respect to distance from the perturbing particle. There may also be a shift in the line center. The general expression for the lineshape resulting from quasistatic pressure broadening is a 4-parameter generalization of the Gaussian distribution known as a [[stable distribution]].<ref>{{cite journal | first = G. | last = Peach | year = 1981 | title = Theory of the pressure broadening and shift of spectral lines | journal = Advances in Physics | volume = 30 | issue = 3 | pages = 367–474 | url = http://journalsonline.tandf.co.uk/openurl.asp?genre=article&eissn=1460-6976&volume=30&issue=3&spage=367 | archive-url = https://archive.today/20130114060003/http://journalsonline.tandf.co.uk/openurl.asp?genre=article&eissn=1460-6976&volume=30&issue=3&spage=367 | url-status = dead | archive-date = 2013-01-14 | doi = 10.1080/00018738100101467 | bibcode = 1981AdPhy..30..367P | access-date = 2005-12-09 | url-access = subscription }}</ref>

Pressure broadening may also be classified by the nature of the perturbing force as follows:

* ''Linear Stark broadening'' occurs via the [[linear Stark effect]], which results from the interaction of an emitter with an electric field of a charged particle at a distance <math>r</math>, causing a shift in energy that is linear in the field strength. <math>(\Delta E \sim 1/r^2)</math>
* ''Resonance broadening'' occurs when the perturbing particle is of the same type as the emitting particle, which introduces the possibility of an energy exchange process. <math>(\Delta E \sim 1/r^3)</math>
* ''Quadratic Stark broadening'' occurs via the [[quadratic Stark effect]], which results from the interaction of an emitter with an electric field, causing a shift in energy that is quadratic in the field strength. <math>(\Delta E \sim 1/r^4)</math>
* ''Van der Waals broadening'' occurs when the emitting particle is being perturbed by [[Van der Waals force]]s. For the quasistatic case, a [[Lévy distribution|Van der Waals profile]]<ref group="note">"Van der Waals profile" appears as lowercase in almost all sources, such as: [https://books.google.com/books?id=Wve2AAAAIAAJ&q=%22Van+der+Waals+profile%22&dq=%22Van+der+Waals+profile%22&hl=en Statistical mechanics of the liquid surface] by Clive Anthony Croxton, 1980, A Wiley-Interscience publication, {{ISBN|0-471-27663-4}}, {{ISBN|978-0-471-27663-0}}; and in [https://books.google.com/books?id=2XpVAAAAMAAJ&q=%22Van+der+Waals+profile%22&dq=%22Van+der+Waals+profile%22&hl=en Journal of technical physics], Volume 36, by Instytut Podstawowych Problemów Techniki (Polska Akademia Nauk), publisher: Państwowe Wydawn. Naukowe., 1995,<!-- and many more --></ref> is often useful in describing the profile. The energy shift as a function of distance between the interacting particles is given in the wings by e.g. the [[Lennard-Jones potential]]. <math>(\Delta E \sim 1/r^6)</math>