Editing Stark effect

{{Short description|Spectral line splitting in electrical field}}
[[Image:hfspec1.svg|300px|thumb|Computed energy level spectrum of hydrogen as a function of the electric field near ''n'' = 15 for [[magnetic quantum number]] ''m'' = 0.  Each [[Principal quantum number|''n'' level]] consists of ''n'' − 1 [[Degenerate energy level|degenerate sublevels]]; application of an [[electric field]] breaks the degeneracy. Energy levels can cross due to [[Laplace–Runge–Lenz vector|underlying symmetries]] of motion in the [[Coulomb potential]].]]

The '''Stark effect''' is the shifting and splitting of [[spectral line]]s of atoms and molecules due to the presence of an external [[electric field]]. It is the electric-field analogue of the [[Zeeman effect]], where a spectral line is split into several components due to the presence of the [[magnetic field]]. Although initially coined for the static case, it is also used in the wider context to describe the effect of time-dependent electric fields. In particular, the Stark effect is responsible for the [[Spectral_line#Pressure_broadening|pressure broadening]] (Stark broadening) of spectral lines by charged particles in [[Plasma_(physics)|plasma]]s. For most spectral lines, the Stark effect is either linear (proportional to the applied electric field) or quadratic with a high accuracy.

The Stark effect can be observed both for emission and absorption lines. The latter is sometimes called the '''inverse Stark effect''', but this term is no longer used in the modern literature.

[[Image:lfspec1.jpg| 400px | right | thumb | Lithium [[Rydberg atom|Rydberg]]-level spectrum as a function of the electric field near ''n'' = 15 for ''m'' = 0. Note how a complicated pattern of the energy levels emerges as the electric field increases, not unlike 
[[bifurcation theory|bifurcations]] of [[elliptical orbit|closed orbits]] in classical [[dynamical system]]s leading to [[chaos theory|chaos]]. <ref name=Courtney1995>{{cite journal |last=Courtney |first=Michael |author2=Neal Spellmeyer |author3=Hong Jiao |author4=Daniel Kleppner |title=Classical, semiclassical, and quantum dynamics of lithium in an electric field |journal=Physical Review A |date=1995 |volume=51 |issue=5 |pages=3604–3620 |doi=10.1103/PhysRevA.51.3604|bibcode = 1995PhRvA..51.3604C |pmid=9912027}}</ref>]]

==History==
The effect is named after the German physicist [[Johannes Stark]], who discovered it in 1913. It was independently discovered in the same year by the Italian physicist [[Antonino Lo Surdo]]. The discovery of this effect contributed importantly to the development of quantum theory and Stark was awarded with the [[Nobel Prize in Physics]] in the year 1919.

Inspired by the magnetic [[Zeeman effect]], and especially by [[Hendrik Lorentz]]'s explanation of it, [[Woldemar Voigt]]<ref>W. Voigt, ''Ueber das Elektrische Analogon des Zeemaneffectes'' (On the electric analogue of the Zeeman effect), Annalen der Physik,  vol. '''309''',
pp. 197–208 (1901).</ref> performed classical mechanical calculations of quasi-elastically bound electrons in an electric field. By using experimental indices of refraction he gave an estimate of the Stark splittings. This estimate was a few orders of magnitude too low. Not deterred by this prediction, Stark undertook measurements<ref>J. Stark, ''Beobachtungen über den Effekt des elektrischen Feldes auf Spektrallinien I. Quereffekt'' (Observations of the effect of the electric field on spectral lines I. Transverse effect), Annalen der Physik,  vol. '''43''', pp. 965–983 (1914). Published earlier (1913) in Sitzungsberichten der Kgl. Preuss. Akad. d. Wiss.</ref> on excited states of the hydrogen atom and succeeded in observing splittings.

By the use of the Bohr–Sommerfeld [[old quantum theory|("old") quantum theory]], [[Paul Sophus Epstein|Paul Epstein]]<ref>P. S. Epstein, ''Zur Theorie des Starkeffektes'', Annalen der Physik, vol. '''50''', pp. 489–520 (1916)</ref> and [[Karl Schwarzschild]]<ref>K. Schwarzschild, Sitzungsberichten der Kgl. Preuss. Akad. d. Wiss. April 1916, p. 548</ref> were independently able to derive equations for the linear and quadratic Stark effect in [[hydrogen]]. Four years later, [[Hendrik Anthony Kramers|Hendrik Kramers]]<ref>H. A. Kramers, Roy. Danish Academy, ''Intensities of Spectral Lines. On the Application of the Quantum Theory to the Problem of Relative Intensities of the Components of the Fine Structure and of the Stark Effect of the Lines of the Hydrogen Spectrum'', p. 287 (1919);''Über den Einfluß eines elektrischen Feldes auf die Feinstruktur der Wasserstofflinien'' (On the influence of an electric field on the fine structure of hydrogen lines), Zeitschrift für Physik, vol. '''3''', pp. 199–223  (1920)</ref> derived formulas for intensities of spectral transitions. Kramers also included the effect of [[fine structure]], with corrections for relativistic kinetic energy and coupling between electron spin and orbital motion. The first quantum mechanical treatment (in the framework of [[Werner Heisenberg]]'s [[matrix mechanics]]) was by [[Wolfgang Pauli]].<ref>W. Pauli, ''Über dass Wasserstoffspektrum vom Standpunkt der neuen Quantenmechanik'' (On the hydrogen spectrum from the point of view of the new quantum mechanics). Zeitschrift für Physik, vol. '''36''' p. 336 (1926)</ref> [[Erwin Schrödinger]] discussed at length the Stark effect in his  third paper<ref>E. Schrödinger, ''Quantisierung als Eigenwertproblem'', Annalen der Physik, vol. '''385''' Issue 13, 437–490 (1926)</ref> on quantum theory (in which he introduced his perturbation theory), once in the manner of the 1916 work of Epstein (but generalized from the old to the new quantum theory) and once by his (first-order) perturbation approach.
Finally, Epstein reconsidered<ref name="epstein:1926a">P. S. Epstein, ''The Stark Effect from the Point of View of Schroedinger's Quantum Theory'', Physical Review, vol '''28''', pp. 695–710 (1926)</ref> the linear and quadratic Stark effect from the point of view of the new quantum theory. He derived equations for the line intensities which were a decided improvement over Kramers's results obtained by the old quantum theory.

While the first-order-perturbation (linear) Stark effect in hydrogen is in agreement with both the old Bohr–Sommerfeld model and the [[quantum mechanics|quantum-mechanical]] theory of the atom, higher-order corrections are not.<ref name="epstein:1926a" /> Measurements of the Stark effect under high field strengths confirmed the correctness of the new quantum theory.

==Mechanism==

===Overview===
Imagine an atom with occupied 2s and 2p [[Electron configuration|electron states]]. In the [[Bohr model]], these states are [[Degenerate energy levels|degenerate]]. However, in the presence of an external electric field, these electron orbitals will [[Orbital hybridisation|hybridize]] into eigenstates of the [[Perturbation theory (quantum mechanics)|perturbed Hamiltonian]] (where each perturbed hybrid state can be written as a superpositon of unperturbed states). Since the 2s and 2p states have opposite [[Parity (physics)|parity]], these hybrid states will lack inversion symmetry and will possess a time-averaged electric dipole moment. If this dipole moment is aligned with the electric field, the energy of the state will shift down; if this dipole moment is anti-aligned with the electric field, the energy of the state will shift up. Thus, the Stark effect causes a splitting of the original degeneracy. 

Other things being equal, the effect of the electric field is greater for outer [[electron shell]]s because the electron is more distant from the nucleus, resulting in a larger electric dipole moment upon hybridization. 

===Multipole expansion===
{{Main article|Multipole expansion}}

The Stark effect originates from the interaction between a [[electric charge|charge]] distribution (atom or molecule) and an external [[electric field]].
The interaction energy of a continuous charge distribution <math>\rho(\mathbf{r})</math>, confined within a finite volume <math>\mathcal{V}</math>, with an external [[Electrostatic#Electrostatic potential|electrostatic potential]] <math>\phi(\mathbf{r})</math> is 
<math display="block"> V_{\mathrm{int}} = \int_\mathcal{V} \rho(\mathbf{r}) \phi(\mathbf{r}) \, d^3 \mathbf r.</math>
This expression is valid [[classical physics|classically]] and quantum-mechanically alike.
If the potential varies weakly over the charge distribution, the [[multipole expansion]] converges fast, so only a few first terms give an accurate approximation. Namely, keeping only the zero- and first-order terms,
<math display="block">\phi(\mathbf{r}) \approx \phi(\mathbf{0}) - \sum_{i=1}^3 r_i F_i,</math>
where we introduced the electric field <math display="inline">F_i \equiv - \left. \left(\frac{\partial \phi}{\partial r_i} \right)\right|_{\mathbf{0}}</math> and assumed the origin '''0''' to be somewhere within <math>\mathcal{V}</math>.
Therefore, the interaction becomes
<math display="block">V_{\mathrm{int}} \approx \phi(\mathbf{0}) \int_\mathcal{V} \rho(\mathbf{r}) d^3r - \sum_{i=1}^3 F_i  \int_\mathcal{V} \rho(\mathbf{r}) r_i d^3r \equiv q \phi(\mathbf{0}) - \sum_{i=1}^3 \mu_i F_i = q \phi(\mathbf{0}) - \boldsymbol{\mu} \cdot \mathbf{F} , </math>
where <math>q</math> and <math>\mathbf{\mu}</math> are, respectively, the total charge (zero [[moment (physics)|moment]]) and the [[dipole|dipole moment]] of the charge distribution.

Classical macroscopic objects are usually neutral or quasi-neutral (<math>q = 0</math>), so the first, monopole, term in the expression above is identically zero. This is also the case for a neutral atom or molecule. However, for an [[ion]] this is no longer true. Nevertheless, it is often justified to omit it in this case, too. Indeed, the Stark effect is observed in spectral lines, which are emitted when an electron "jumps" between two [[bound state]]s. Since such a transition only alters the internal [[degree of freedom|degrees of freedom]] of the radiator but not its charge, the effects of the monopole interaction on the initial and final states exactly cancel each other.

===Perturbation theory===
Turning now to quantum mechanics an atom or a molecule can be thought of as a collection of point charges (electrons and nuclei), so that the second definition of the dipole applies. The interaction of atom or molecule with a uniform external field is described by the operator
<math display="block"> V_{\mathrm{int}} = - \mathbf{F}\cdot \boldsymbol{\mu}.</math>
This operator is used as a perturbation in first- and second-order [[perturbation theory]] to account for the first- and second-order Stark effect.

====First order====
Let the unperturbed atom or molecule be in a ''g''-fold degenerate state with orthonormal zeroth-order state functions <math> \psi^0_1, \ldots, \psi^0_g </math>. (Non-degeneracy is the special case ''g'' = 1). According to perturbation theory the first-order energies are the eigenvalues of the ''g'' × ''g''  matrix with general element 
<math display="block">
(\mathbf{V}_{\mathrm{int}})_{kl} = \langle \psi^0_k |  V_{\mathrm{int}} | \psi^0_l \rangle =
-\mathbf{F}\cdot \langle \psi^0_k | \boldsymbol{\mu} | \psi^0_l \rangle,
\qquad k,l=1,\ldots, g.
</math>
If ''g'' = 1 (as is often the case for electronic states of molecules)  the first-order energy becomes proportional to the expectation (average) value of the dipole operator <math>\boldsymbol{\mu}</math>,
<math display="block">
E^{(1)} = -\mathbf{F}\cdot \langle \psi^0_1 | \boldsymbol{\mu} | \psi^0_1 \rangle =
-\mathbf{F}\cdot \langle  \boldsymbol{\mu} \rangle.
</math>Since the electric dipole moment is a vector ([[tensor]] of the first rank), the diagonal elements of the perturbation matrix '''V'''<sub>int</sub> vanish between states that have a definite [[parity (physics)|parity]]. Atoms and molecules possessing inversion symmetry do not have a (permanent) dipole moment and hence do not show a linear Stark effect.

In order to obtain a non-zero matrix '''V'''<sub>int</sub> for systems with an inversion center it is necessary that some of the unperturbed functions <math> \psi^0_i</math> have opposite parity (obtain plus and minus under inversion), because only functions of opposite parity give non-vanishing matrix elements. Degenerate zeroth-order states of opposite parity occur for excited hydrogen-like (one-electron) atoms or Rydberg states. Neglecting [[fine structure|fine-structure]] effects, such a state with the principal quantum number ''n'' is ''n''<sup>2</sup>-fold degenerate and
<math display="block">n^2 = \sum_{\ell=0}^{n-1} (2 \ell + 1),</math>
where <math>\ell </math> is the azimuthal (angular momentum) quantum number. For instance, the excited ''n'' = 4 state contains the following <math>\ell</math> states,
<math display="block">16 = 1 + 3 + 5 +7 \;\; \Longrightarrow\;\;  n=4\;\text{contains}\; s\oplus p\oplus d\oplus f.</math>
The one-electron states with even <math>\ell</math> are even under parity, while those with odd <math>\ell</math> are odd under parity. Hence hydrogen-like atoms with ''n''>1 show first-order Stark effect.

The first-order Stark effect occurs in rotational transitions of [[Rotational spectroscopy#Classification of molecular rotors|symmetric top molecules]] (but not for linear and asymmetric molecules). In first approximation a molecule may be seen as a rigid rotor. A symmetric top [[rigid rotor]] has the unperturbed eigenstates
<math display="block">|JKM \rangle = (D^J_{MK})^* \quad\text{with}\quad M,K= -J,-J+1,\dots,J</math>
with 2(2''J''+1)-fold degenerate energy for |K| > 0 and (2''J''+1)-fold degenerate energy for K=0.
Here ''D''<sup>''J''</sup><sub>''MK''</sub> is an element of the [[Wigner D-matrix]]. The first-order perturbation matrix  on basis of the unperturbed rigid rotor function is non-zero and can be diagonalized. This gives shifts and splittings
in  the rotational spectrum. Quantitative analysis of these Stark shift yields the permanent [[electric dipole moment]] of the symmetric top molecule.

====Second order====
As stated, the quadratic Stark effect is described by second-order perturbation theory. The zeroth-order [[Eigenvalues and eigenvectors|eigenproblem]]
<math display="block">H^{(0)} \psi^0_k = E^{(0)}_k \psi^0_k, \quad k=0,1, \ldots, \quad E^{(0)}_0 < E^{(0)}_1 \le E^{(0)}_2, \dots </math>
is assumed to be solved. The perturbation theory gives
<math display="block">
E^{(2)}_k = \sum_{k' \neq k} \frac{\langle \psi^0_k | V_\mathrm{int} | \psi^0_{k^\prime} \rangle \langle \psi^0_{k'} | V_\mathrm{int} | \psi^0_k \rangle}{E^{(0)}_k - E^{(0)}_{k'}} \equiv -\frac{1}{2} \sum_{i,j=1}^3 \alpha_{ij} F_i F_j
</math>
with the components of the [[polarizability|polarizability tensor]] α defined by
<math display="block">
\alpha_{ij} = -2\sum_{k' \neq k} \frac{\langle \psi^0_k | \mu_i | \psi^0_{k'} \rangle \langle \psi^0_{k'} | \mu_j | \psi^0_k \rangle}{E^{(0)}_k - E^{(0)}_{k'}}.
</math>
The energy ''E''<sup>(2)</sup> gives the quadratic Stark effect.

Neglecting the [[hyperfine structure]] (which is often justified — unless extremely weak electric fields are considered), the polarizability tensor of atoms is isotropic,
<math display="block">\alpha_{ij} \equiv \alpha_0 \delta_{ij} \Longrightarrow E^{(2)} = -\frac{1}{2} \alpha_0 F^2.</math>
For some molecules this expression is a reasonable approximation, too.

For the ground state <math>\alpha_0</math> is ''always'' positive, i.e., the quadratic Stark shift is always negative.

====Problems====
The perturbative treatment of the Stark effect has some problems. In the presence of an electric field, states of atoms and molecules that were previously bound ([[square-integrable]]), become formally (non-square-integrable) [[resonance]]s of finite width. These resonances may decay in finite time via field ionization. For low lying states and not too strong fields the decay times are so long, however, that for all practical purposes the system can be regarded as bound. For highly excited states and/or very strong fields ionization may have to be accounted for. (See also the article on the [[Rydberg atom]]).{{cn|date=November 2024}}

==Applications==
The Stark effect is at the basis of the spectral shift measured for [[Voltage-sensitive_dye|voltage-sensitive dyes]] used for imaging of the firing activity of neurons.<ref>{{Cite journal|last1=Sirbu|first1=Dumitru|last2=Butcher|first2=John B.|last3=Waddell|first3=Paul G.|last4=Andras|first4=Peter|last5=Benniston|first5=Andrew C.|date=2017-09-18|title=Locally Excited State-Charge Transfer State Coupled Dyes as Optically Responsive Neuron Firing Probes|journal=Chemistry - A European Journal|volume=23|issue=58|pages=14639–14649|doi=10.1002/chem.201703366|pmid=28833695|issn=0947-6539|url=https://publications.aston.ac.uk/id/eprint/40362/1/Locally_Excited_State_Charge_Transfer_State.pdf}}</ref>

==See also==
*[[Zeeman effect]]
*[[Autler–Townes effect]]
*[[Quantum-confined Stark effect]]
*[[Stark spectroscopy]]
*[[Inglis–Teller equation]]
*[[Electric field NMR]]
*[[Coherent effects in semiconductor optics#The excitonic optical Stark effect|Stark effect in semiconductor optics]]

== References ==
{{reflist}}

==Further reading==
* {{cite book |author= Edmond Taylor Whittaker|author-link=E. T. Whittaker |title=[[A History of the Theories of Aether and Electricity]]. II. The Modern Theories (1800-1950) |publisher=American Institute of Physics |date=1987 |isbn=978-0-88318-523-0}} ''(Early history of the Stark effect)''
* {{cite book | author=E. U. Condon | author2=G. H. Shortley | name-list-style=amp|title=The Theory of Atomic Spectra | url=https://archive.org/details/in.ernet.dli.2015.212979 | publisher=Cambridge University Press |date=1935 |isbn=978-0-521-09209-8 }} ''(Chapter 17 provides a comprehensive treatment, as of 1935.)''
* {{cite book | author=H. Friedrich|title=Theoretical Atomic Physics| url=https://archive.org/details/theoreticalatomi0000frie| url-access=registration| publisher=Springer-Verlag, Berlin |date=1990 |isbn=978-0-387-54179-2 }} ''(Stark effect for atoms)''
* {{cite book | author=H. W. Kroto|title=Molecular Rotation Spectra| publisher=Dover, New York |date=1992 |isbn=978-0-486-67259-5 }} ''(Stark effect for rotating molecules)''

{{DEFAULTSORT:Stark Effect}}
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