View source for Equivalence principle (geometric)

'''Spectral line shape''' describes the form of a feature, observed in [[spectroscopy]], corresponding to an energy change in an [[atom]], [[molecule]] or [[ion]]. Ideal line shapes include [[Cauchy distribution|Lorentzian]], [[Normal distribution|Gaussian]] and [[Voigt function|Voigt]] functions, whose parameters are the line position, maximum height and half-width.<ref name=Hollas>{{cite book|last=Hollas|first=M.J.|title=Modern Spectroscopy|edition=3rd|year=1996|publisher=Wiley|isbn=0471965227 |pages=30&ndash;34}}</ref> Actual line shapes are determined principally by [[Doppler broadening|Doppler]], [[Spectral line#Broadening and shift|collision and proximity]] broadening. For each system the half-width of the shape function varies with temperature, pressure (or [[concentration (chemistry)|concentration]]) and phase. A knowledge of shape function is needed for spectroscopic [[curve fitting]] and [[deconvolution]].

== Origins ==
{{Main|Spectral line#Broadening and shift}}
[[File:Permanganate spectrum.png|thumb|300px|Absorption spectrum of an aqueous solution of [[potassium permanganate]]. The spectrum consists of a series of overlapping lines belonging to a [[vibronic spectroscopy|vibronic progression]]]]

A spectroscopic transition is associated with a specific amount of energy, ''E''. However, when this energy is measured by means of some spectroscopic technique, the spectroscopic line is not infinitely sharp, but has a particular shape. Numerous factors can contribute to the broadening of [[spectral line]]s. Broadening can only be mitigated by the use of specialized techniques, such as [[Lamb dip]] spectroscopy. The principal sources of broadening are:
* [[Spectral line|Lifetime broadening]]. According to the [[uncertainty principle]] the uncertainty in energy, Δ''E'' and the lifetime, Δ''t'', of the excited state are related by
:<math> \Delta E \Delta t \gtrapprox \frac{\hbar}{2}. </math>
:This determines the minimum possible line width. As the excited state decays exponentially in time this effect produces a line with Lorentzian shape in terms of frequency (or wavenumber).
* [[Doppler broadening]]. This is caused by the fact that the velocity of atoms or molecules relative to the observer follows a [[Maxwell distribution]], so the effect is dependent on temperature. If this were the only effect the line shape would be Gaussian.<ref name=Hollas/>
* [[Spectral line#Broadening and shift|Pressure broadening]] (Collision broadening). Collisions between atoms or molecules reduce the lifetime of the upper state, Δ''t'', increasing the uncertainty Δ''E''. This effect depends on both the density (that is, pressure for a gas) and the temperature, which affects the rate of collisions. The broadening effect is described by a Lorentzian profile in most cases.<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 | doi = 10.1080/00018738100101467}}</ref>

* [[Spectral line#Broadening and shift|Proximity broadening]]. The presence of other molecules close to the molecule involved affects both line width and line position. It is the dominant process for liquids and solids. An extreme example of this effect is the influence of [[hydrogen bonding]] on the spectra of [[protic]] liquids.

Observed spectral line shape and line width are also affected by instrumental factors. The observed line shape is a convolution of the intrinsic line shape with the instrument [[transfer function]].<ref>Gans, Section 9.3, ''Convolution and Cross-correlation''</ref>

Each of these mechanisms, [[Spectral line#Broadening and shift|and others]], can act in isolation or in combination. If each effect is independent of the other, the observed line profile is a [[convolution]] of the line profiles of each mechanism. Thus, a combination of Doppler and pressure broadening effects yields a Voigt profile.

== Line shape functions ==
[[File:Gauss and Lorentz lineshapes.png|thumb|300px|Standardized line shapes with HWHM = 1.<br> Blue, Lorentzian; red, Gaussian.]]
[[Image:Voigt distributionPDF.png|thumb|300px|Plot of the centered Voigt profile for four cases. Each case has a full width at half-maximum of very nearly 3.6. The black and red profiles are the limiting cases of the Gaussian (γ&nbsp;=0) and the Lorentzian (σ&nbsp;=0) profiles respectively.]]
A [[Cauchy distribution|Lorentzian]] line shape function can be represented as
:<math>L=\frac{1}{1+x^2},</math>
where ''L'' signifies a Lorentzian function standardized, for spectroscopic purposes, to a maximum value of 1;<ref group=note>In [[statistics]] Lorentzian (Cauchy) and Gaussian functions are [[Normalizing constant|normalized]] to unit area</ref> ''x'' is a subsidiary variable defined as
:<math>x=\frac{p^0-p}{w/2},</math>
where ''p''<sup>0</sup> is the position of the maximum (corresponding to the transition energy ''E''), ''p'' is a position, and ''w'' is the [[full width at half maximum]] (FWHM), the width of the curve when the intensity is half the maximum intensity (this occurs at the points ''p'' = ''p''<sup>0</sup>±w/2).  The unit of ''p''<sup>0</sup>, ''p'' and ''w'' is typically [[wavenumber]] or [[Electromagnetic spectrum|frequency]]. The variable ''x'' is [[dimensionless]] and is zero at ''p''=''p''<sup>0</sup>.

The [[Normal distribution|Gaussian]] line shape has the standardized form,
:<math>G= e^{-(log_e 2) x^2}.</math>
The subsidiary variable, ''x'', is defined in the same way as for a Lorentzian shape. Both this function and the Lorentzian have a maximum value of 1 at ''x'' = 0 and a value of 1/2 at ''x''=±1.

The third line shape that has a theoretical basis is the [[Voigt function]], a [[convolution]] of a Gaussian and a Lorentzian,
:<math>V(x;\sigma,\gamma)=\int_{-\infty}^\infty G(x';\sigma)L(x-x';\gamma)\, dx',</math>
where σ and γ are half-widths. The computation of a Voigt function and its derivatives is more complicated than a Gaussian or Lorentzian.<ref>{{Cite journal| doi = 10.1016/0022-4073(77)90161-3|volume = 17| issue = 2| pages = 233–236| last = Olivero| first = J.J.| coauthors = R.L. Longbothum| title = Empirical fits to the Voigt line width: A brief review| journal = Journal of Quantitative Spectroscopy and Radiative Transfer}}</ref>

A spectroscopic peak may be fitted to multiples of the above functions or to sums or products of functions with variable parameters.<ref>{{cite journal|last=Pitha|first=J.|coauthors=Jones,R.N.|title=A Comparison of Optimization Methods for Fitting Curves to Infrared Band Envelopes|journal=Canad. J. Chem.|year=1966|volume=44|issue=24|pages=3031–3050|doi=10.1139/v66-445}}</ref> The above functions are all symmetrical about the position of their maximum.<ref group=note>Experimental profiles that are symmetrical when plotted on a scale proportional to energy (for example, frequency or wavenumber) will not be symmetrical when plotted on a wavelength scale.</ref> Asymmetric functions have also be used.<ref name=mad>{{cite journal|last=Maddams|first=W.F.|title=The Scope and Limitations of Curve Fitting|journal=Applied Spectroscopy|year=1980|volume=34|issue=3|pages=245–267}}</ref><ref group=note>In [[Electron paramagnetic resonance]], asymmetric lines are characterized by two half-widths, measured either side of the line centre.</ref>

== Instances ==
=== Atomic spectra ===
{{main|Atomic spectroscopy}}
For atoms in the gas phase the principal effects are Doppler and pressure broadening. Lines are relatively sharp on the scale of measurement so that applications such as [[atomic absorption spectroscopy]] (AAS) and [[Inductively coupled plasma atomic emission spectroscopy]] (ICP) are used for [[elemental analysis]]. Atoms also have distinct x-ray spectra that are attributable to the excitation of inner shell electrons to excited states. The lines are relatively sharp because the inner electron energies are not very sensitive to the atom's environment. This is applied to [[X-ray fluorescence spectroscopy]] of solid materials.

=== Molecular spectra ===
For molecules in the gas phase, the principal effects are Doppler and pressure broadening. This applies to [[rotational spectroscopy]],<ref>{{cite book|last=Kroto|first=H.W.|title=Molecular Rotation Spectra|year=1992|publisher=Wiley|isbn=0-486-49540-X}} Section 4.6, Line shapes and line widths</ref> [[rotational-vibrational spectroscopy]] and [[vibronic spectroscopy]].

For molecules in the liquid state or in solution, collision and proximity broadening predominate and lines are much broader than lines from the same molecule in the gas phase.<ref>Clarke, J.H.R, "Band Shapes and Molecular Dynamics in liquids", pp. 109-193, in ''Advances in Infrared and Raman Spectroscopy'', Volume 4 (1978), Editors Clark, R.J.H;  Hester, R.E.</ref><ref>{{cite journal|last=Bradley|first=Michael S.|coauthors=Bratu, Cheryl|title=Vibrational Line Profiles as a Probe of Molecular Interactions|journal=J. Chem. Educ.|year=1997|volume=74|issue=5|page=553|doi=10.1021/ed074p553}}</ref> Line maxima may also be shifted. Because there are many sources of broadening, the lines have a [[stable distribution]], tending towards a Gaussian shape.

=== Nuclear magnetic resonance ===
{{main|Nuclear magnetic resonance}}
The shape of lines in a nuclear magnetic resonance (NMR) spectrum is determined by the process of [[free induction decay]]. This decay is approximately [[exponential decay|exponential]], so the line shape is Lorentzian.<ref>{{cite journal|last=Petrakis|first=Leonidas|title=Spectral line shapes: Gaussian and Lorentzian functions in magnetic resonance|journal=J. Chem. Educ.|year=1967|volume=44|issue=8|page=432|doi=10.1021/ed044p432}}</ref>  This follows because the [[Fourier transform]] of an exponential function in the time domain is a Lorentzian in the frequency domain. In [[NMR spectroscopy]] the lifetime of the excited states is relatively long, so the lines are very sharp, producing high-resolution spectra.

=== Magnetic resonance imaging ===
{{main|Magnetic resonance imaging}}
[[File:Bluthirnschranke nach Infarkt nativ und KM.png|thumb|250px|Effect of contrast agent on images: Defect of the blood–brain barrier after stroke shown in MRI. T<sub>1</sub>-weighted images, left image without, right image with contrast medium administration.]]
[[MRI contrast agent#Gadolinium .28Gd.29: Paramagnetic|Gadolinium-based pharmaceuticals]] alter the relaxation time, and hence spectral line shape, of those protons that are in water molecules that are transiently attached to the [[paramagnetic]] atoms, resulting contrast enhancement of the MRI image.<ref name =Brown/> This allows better visualisation of some brain tumours.<ref name =Brown>{{cite book|last1=Brown  |first1=Mark A.|first2=Richard C.|last2= Semelka|title=MRI: Basic Principles and Applications|year=2010|publisher=Wiley-Blackwell||edition= 4th Edition |isbn=978-0470500989}} Chapter 3, ''Relaxation''</ref>

== Applications ==

=== Curve decomposition ===
{{See also|Chemometrics#Multivariate curve resolution}}
[[File:Curve decomposition.png|thumb|250px|The black curve is composed of a sum of two Lorentzians, each with HWHM=1, separated by one full half-width. The blue curve has ''h''<sub>1</sub> = 1 and the red curve has ''h''<sub>2</sub> = 0.5.]]
Some spectroscopic curves can be approximated by the sum of a set of component curves. For example, when [[Beer's law]]
:<math>I_\lambda=\sum_k \epsilon_{k,\lambda} c_k</math>
applies, the measured intensity, ''I'', at wavelength λ, is a [[linear combination]] of the intensity due to the individual components, ''k'', at [[concentration (chemistry)|concentration]], ''c''<sub>k</sub>. ε is an [[molar absorptivity|extinction coefficient]]. In such cases the curve of experimental data may be decomposed into sum of component curves in a process of [[curve fitting]]. This process is also widely called deconvolution. [[deconvolution|Curve deconvolution]] and [[curve fitting]] are a completely different mathematical procedures.<ref name=mad/><ref name=blass/>

Curve fitting can be used in two distinct ways.
# The line shapes and parameters ''p''<sup>0</sup> and HWHM of the individual component curves have been obtained experimentally. In this case the curve may be decomposed using a linear [[least squares]] process simply to determine the concentrations of the components. This process is used in [[analytical chemistry]] to determine the composition of a mixture of the components of known [[molar absorptivity]] spectra. For example, if the heights of two lines are found to be ''h''<sub>1</sub> and ''h''<sub>2</sub>, ''c''<sub>1</sub> = ''h''<sub>1</sub> / ε<sub>1</sub> and ''c''<sub>2</sub> = ''h''<sub>2</sub> / ε<sub>2</sub>.<ref>{{cite book|last=Skoog|first=D.A.|title= Fundamentals of Analytical Chemistry|year=2004|publisher=Brooks/Cole|isbn=0-03-035523-0|page=796|coauthors=West, D.H.; Holler, F.J.; Crouch, S.R.}}</ref>
# Parameters of the line shape are unknown. The intensity of each component is a function of at least 3 parameters, position, height and half-width. In addition one or both of the line shape function and baseline function may not be known with certainty. When two or more parameters of a fitting curve are not known the method of non-linear [[least squares]] must be used.<ref>Gans, Section 8.3, ''Gaussian, Lorentzian and related functions''</ref><ref>{{cite journal|last=Sundius|first=T|title=Computer fitting of Voigt profiles to Raman lines|journal=J. Raman Spectrosc.|year=1973|volume=1|issue=5|pages=457–488|doi=10.1002/jrs.1250010506}}</ref> The reliability of curve fitting in this case is dependent on the separation between the components, their shape functions and relative heights, and the [[signal-to-noise]] ratio in the data.<ref name=mad/><ref>{{cite journal|last=Gans|first=P|coauthors=Gill, J.B.|title=Comments on the critical evaluation of curve fitting in infrared spectrometry|journal=Anal. Chem.|year=1980|volume=52|issue=2|pages=351–352|doi=10.1021/ac50052a035}}</ref>

=== Derivative spectroscopy ===
Spectroscopic curves can be subjected to [[Numerical smoothing and differentiation|numerical differentiation]].<ref>{{cite journal|last=Bridge|first=T P|coauthors=Fell. A.F;  Wardman, R.H.|title=Perspectives in derivative spectroscopy Part 1-Theoretical principles|journal=Journal of the Society of Dyers and Colourists|year=1987|volume=103|issue=1|pages=17–27|doi=10.1111/j.1478-4408.1987.tb01081.x}}</ref>
[[File:Derivative sum Lorentzians.png|thumb|250 px| 2nd. derivative of a sum of Lorentzians, each with HWHM=1, separated by one full half-width. The two Lorentzians have heights 1 and 0.5]]
When the data points in a curve are equidistant from each other the [[Savitzky–Golay smoothing filter|Savitzky–Golay]] convolution method may be used.<ref>{{cite journal |first1=A. |last1=Savitzky |first2=M.J.E. |last2=Golay |year=1964 |title=Smoothing and Differentiation of data by Simplified Least Squares Procedures |journal=[[Analytical Chemistry (journal)|Analytical Chemistry]] |volume=36 |issue=8 |pages=1627–1639 |doi=10.1021/ac60214a047}}</ref> The best convolution function to use depends primarily on the [[signal-to-noise-ratio]] of the data.<ref>{{cite journal|last=Rzhevskii|first=Alexander M.|coauthors=Mardilovich, Peter P.|title=Generalized GansGill Method for Smoothing and Differentiation of Composite Profiles in Practice|journal=Applied Spectroscopy|year=1994|volume=48|issue=1|pages=13–20|doi=10.1366/0003702944027714}}</ref> The first derivative (slope, <math>\frac{dy}{dx}</math>) of all single line shapes is zero at the position of maximum height. This is also true of the third derivative; odd derivatives can be used to locate the position of a peak maximum.<ref>Gans, p. 158</ref>

The second derivatives, <math>\frac{d^2y}{dx^2}</math>, of both Gaussian and Lorentzian functions have a reduced half-width. This can be used to apparently improve [[spectral resolution]]. The diagram shows the second derivative of the black curve in the diagram above it. Whereas the smaller component produces a shoulder in the spectrum, it appears as a separate peak in the 2nd. derivative.<ref group=note>Component peak maxima in the spectrum are minima in the 2nd. derivative spectrum and maxima in the 4th. derivative spectrum</ref> Fourth derivatives, <math>\frac{d^4y}{dx^4}</math>, can also be used, when the signal-to-noise-ratio in the spectrum is sufficiently high.<ref>{{cite journal| last=Antonov| first=Liudmil| title=Fourth derivative spectroscopy — a critical view|journal=Analytica Chimica Acta|year=1997|volume=349|issue=1-3|pages=295–301|doi=10.1016/S0003-2670(97)00210-9}}</ref>

=== Deconvolution ===
{{main|Deconvolution}}
Deconvolution can be used to apparently improve [[spectral resolution]]. In the case of NMR spectra, the process is relatively straight forward, because the line shapes are Lorentzian, and the convolution of a Lorentzian with another Lorentzian is also Lorentzian. The [[Fourier transform]] of a Lorentzian is an exponential. In the co-domain (time) of the spectroscopic domain (frequency) convolution becomes multiplication. Therefore a convolution of the sum of two Lorentzians becomes a multiplication of two exponentials in the co-domain. Since, in FT-NMR, the measurements are made in the time domain division of the data by an exponential is equivalent to deconvolution in the frequency domain. A suitable choice of exponential results in a reduction of the half-width of a line in the frequency domain. This technique has been rendered all but obsolete by advances in NMR technology.<ref>{{cite book |last1=Banwell |first1=Colin N. |last2=McCash |first2=Elaine M. |title=Fundamentals of molecular spectroscopy |edition= 4th |year=1994 |publisher=McGraw-Hill|isbn=0-07-707976-0 |page=40 }}Section 7.2.6, ''Simplification of Complex Spectra.''</ref> A similar process has been applied for resolution enhancement of other types of spectra, with the disadvantage that the spectrum must be first Fourier transformed and then transformed back after the deconvoluting function has been applied in the spectrum's co-domain.<ref name=blass>{{cite book| last=Blass| first=W.E.| title=Deconvolution of Absorption Spectra|year=1981|publisher=Academic Press|isbn=0121046508|coauthors=Halsey, G.W.}}</ref>

== See also ==
*[[Fano resonance]]
*[[Holtsmark distribution]]
*[[Zero-phonon line and phonon sideband]]

== Notes ==
{{reflist|group=note}}

== References ==
{{reflist}}

== Further reading ==
{{Refbegin}}
*{{cite book|last=Atkins|first=P.W.|title=Physical Chemistry|year=2006|publisher=Oxford University Press|edition= 8th|isbn=0198700725|pages=436–438|coauthors=de Paula, J. |chapter= 13.3: Linewidths}}
*{{cite book|last=Davies|first=Christopher C.|title=Lasers and electro-optics : fundamemtals and engineering|year=2000|publisher=Cambridge University Press|location=New York|isbn=9780521484039|pages=27&ndash;30|edition=Reprint}}
*{{cite book|last=Demtröder|first=Wolfgang|title=Laser spectroscopy. basic principles|year=2008|publisher=Springer-Verlag|location=Drodrecht|isbn=9783540734185|pages=88&ndash;89|edition=4th}}
*{{cite book|last=De Natale|first=Paolo|coauthors=Pasquale Maddaloni, Marco Bellini |title=Laser-based Measurements for Time and Frequency Domain Applications.|year=2013|publisher=Taylor & Francis|isbn=9781439841518|pages=181&ndash;183}}
*{{cite book|last=Gans|first=P.|title=Data Fitting in the Chemical Sciences|year=1992|publisher=Wiley|isbn=0 471 93412 7}}
*{{cite newspaper|last=Hewitt |first=John |title=Manipulating Lorentz and Fano spectral line shapes |newspaper=phys.org |date=May 13, 2013 |url=http://phys.org/news/2013-05-lorentz-fano-spectral-line.html |accessdate=May 25, 2013}}
*{{cite book|last=Linne|first=Mark A.|chapter=11. Line broadening |title=Spectroscopic measurement an introduction to the fundamentals|year=2002|publisher=Academic Press|location=Amsterdam|isbn=9780080517537|pages=283&ndash;303}}
*{{cite book|last=Pelikan|first=Peter|title=Applications of Numerical Methods in Molecular Spectroscopy|year=1994|publisher=CRC Press|isbn=9780849373220|coauthors=Ceppan, Michal; Liska, Marek}}
*{{cite encyclopedia|last=Poole, Jr.|first=Charles P. |title=Line shape |encyclopedia=Encyclopedic Dictionary of Condensed Matter Physics, Volume 1 |pages=718&ndash;719 |publisher=Academic Press |year=2004 |isbn=9780080545233}}
*{{cite book|last=Simmons|first=Joseph H.|title=Optical materials|year=2000|publisher=Academic|location=San Diego, Calif.|isbn=9780126441406|pages=274&ndash;277|edition=[Online-Ausg.]|coauthors=Potter, Kelly S.}}
*{{cite book|last=Telle|first=Helmut H |coauthors=Angel González Ureña, Robert J. Donovan|title=Laser chemistry spectroscopy, dynamics and applications|year=2007|publisher=John Wiley & Sons|location=Chichester, West Sussex, England|isbn=9780470059401|pages=24&ndash;25}}
{{Refend}}

== External links ==
*[https://www.thermo.com/eThermo/CMA/PDFs/Product/productPDF_57565.PDF Curve Fitting in Raman and IR Spectroscopy: Basic Theory of Line Shapes and Applications]
*[http://icsls21.spbu.ru 21st International Conference on Spectral Line Shapes, St. Petersburg (2012)]

{{BranchesofSpectroscopy}}

[[Category:Spectroscopy]]