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<h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Instanton</span></h1>
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<div id="mw-content-text" class="mw-body-content mw-content-ltr" lang="de" dir="ltr"><div class="mw-content-ltr mw-parser-output" lang="de" dir="ltr"><p><b>Instantone</b> sind in Raum und Zeit lokalisierte <a href="Soliton" title="Soliton">Solitonlösungen</a> „euklidifizierter“ <a href="Quantenfeldtheorie" title="Quantenfeldtheorie">Quantenfeldtheorien</a>, speziell der <a href="Yang-Mills-Gleichungen" title="Yang-Mills-Gleichungen">Yang-Mills-Gleichungen</a> in der <a href="Quantenchromodynamik" title="Quantenchromodynamik">Quantenchromodynamik</a> nach einer <a href="Wick-Rotation" title="Wick-Rotation">Wick-Rotation</a> vom <a href="Minkowski-Raum" title="Minkowski-Raum">Minkowski-Raum</a> zum vierdimensionalen <a href="Euklidischer_Raum" title="Euklidischer Raum">Euklidischen Raum</a>. Instantone beschreiben in allen diesen Theorien den <a href="Quantenmechanik" title="Quantenmechanik">quantenmechanischen</a> Übergang (<a href="Tunneleffekt" title="Tunneleffekt">Tunnel-Übergang</a>) zwischen den verschiedenen Klassen des <a href="Vakuum" title="Vakuum">Vakuumzustandes</a> des betrachteten Feldes und sollen speziell für die <a href="Starke_Wechselwirkung" title="Starke Wechselwirkung">starke Wechselwirkung</a> im niederenergetischen Bereich große Bedeutung haben.
</p><p>Das Instanton (ebenso wie das Antiinstanton) vermittelt zusammen mit weitgetrennten Instanton-Antiinstanton-Paaren den Tunnelprozess für den quantenmechanischen Grundzustand im Doppelmuldenpotential (Euklidische Zeit gegen unendlich). Alle angeregten Zustände werden von periodischen Instantonen vermittelt (Euklidische Zeit endlich) — wie alle Zustände im Fall des invertierten Doppelmuldenpotential<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>.
</p><p>Instantone liefern auch eine Erklärung für eine wichtige <a href="Symmetriebrechung" title="Symmetriebrechung">Symmetriebrechung</a>: sie können die <a href="Chiralit%C3%A4t_(Physik)" title="Chiralität (Physik)">Händigkeit</a> von <a href="Elementarteilchen" title="Elementarteilchen">Elementarteilchen</a> im Quantenchromodynamik-Vakuum verändern. Die dazugehörige <a href="Chirale_Symmetrie" title="Chirale Symmetrie">chirale Symmetrie</a> spielt eine zentrale Rolle in der Physik der <a href="Hadron" title="Hadron">Hadronen</a>.
Eine weitere Anwendung finden Instantone bei dem „<a href="Inflaton" class="mw-redirect" title="Inflaton">Inflaton-Feld</a>“ der <a href="Kosmologie" title="Kosmologie">Kosmologie</a> zur Erklärung des <a href="Inflation_(Kosmologie)" title="Inflation (Kosmologie)">inflationären Phasenüberganges</a> in der Frühzeit des Universums.
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<div class="mw-heading mw-heading2"><h2 id="Instantontheorie_in_der_chemischen_Ratentheorie">Instantontheorie in der chemischen Ratentheorie</h2></div>
<p>Im Zusammenhang mit der <a href="Kinetik_(Chemie)" title="Kinetik (Chemie)">Theorie der Reaktionsgeschwindigkeit</a> werden periodische Instantone verwendet, um die Tunnelgeschwindigkeit von Atomen in chemischen Reaktionen zu berechnen. Der Verlauf einer chemischen Reaktion kann als Bewegung eines Pseudoteilchens auf einer hochdimensionalen <a href="Potentialhyperfl%C3%A4che" title="Potentialhyperfläche">Potentialenergieoberfläche</a> (PES) beschrieben werden. Die thermische <a href="Geschwindigkeitskonstante" title="Geschwindigkeitskonstante">Geschwindigkeitskonstante</a> <i>k</i> kann dann aus dem Imaginärteil der freien Energie <i>F</i> berechnet werden:<sup id="cite_ref-:inst_chapter_2-0" class="reference"><a href="#cite_note-:inst_chapter-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
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<dl><dd><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle k(\beta )=-{\frac {2}{\hbar }}\operatorname {Im} \mathrm {F} ={\frac {2}{\beta \hbar }}\operatorname {Im} \ln(Z_{k})\approx {\frac {2}{\hbar \beta }}{\frac {\operatorname {Im} Z_{k}}{\operatorname {Re} Z_{k}}},\quad \operatorname {Re} Z_{k}\gg \operatorname {Im} Z_{k}}">
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<annotation encoding="application/x-tex">{\displaystyle k(\beta )=-{\frac {2}{\hbar }}\operatorname {Im} \mathrm {F} ={\frac {2}{\beta \hbar }}\operatorname {Im} \ln(Z_{k})\approx {\frac {2}{\hbar \beta }}{\frac {\operatorname {Im} Z_{k}}{\operatorname {Re} Z_{k}}},\quad \operatorname {Re} Z_{k}\gg \operatorname {Im} Z_{k}}</annotation>
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</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/3f2e9539196aa6ce3feb6a95be1a38b6d4321119.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.338ex; width:64.881ex; height:5.676ex;" alt="{\displaystyle k(\beta )=-{\frac {2}{\hbar }}\operatorname {Im} \mathrm {F} ={\frac {2}{\beta \hbar }}\operatorname {Im} \ln(Z_{k})\approx {\frac {2}{\hbar \beta }}{\frac {\operatorname {Im} Z_{k}}{\operatorname {Re} Z_{k}}},\quad \operatorname {Re} Z_{k}\gg \operatorname {Im} Z_{k}}" loading="lazy"></span></dd></dl>
<p>wobei <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Z_{k}}">
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</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/29a05237076c50ce9cf9a75c02ff57abefac0de4.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:2.676ex; height:2.509ex;" alt="{\displaystyle Z_{k}}" loading="lazy"></span> die <a href="Kanonische_Zustandssumme" class="mw-redirect" title="Kanonische Zustandssumme">kanonische Zustandssumme</a> ist, die aus der Spur des Boltzmann-Operators in der Ortsdarstellung berechnet wird.
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<dl><dd><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Z_{k}=\operatorname {Tr} \left(e^{-\beta {\hat {H}}}\right)=\int d\mathbf {x} \left\langle \mathbf {x} \left|e^{-\beta {\hat {H}}}\right|\mathbf {x} \right\rangle }">
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<annotation encoding="application/x-tex">{\displaystyle Z_{k}=\operatorname {Tr} \left(e^{-\beta {\hat {H}}}\right)=\int d\mathbf {x} \left\langle \mathbf {x} \left|e^{-\beta {\hat {H}}}\right|\mathbf {x} \right\rangle }</annotation>
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</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/edefacab48a05af813afbb71206f78d8790cf9ad.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.338ex; width:37.554ex; height:5.676ex;" alt="{\displaystyle Z_{k}=\operatorname {Tr} \left(e^{-\beta {\hat {H}}}\right)=\int d\mathbf {x} \left\langle \mathbf {x} \left|e^{-\beta {\hat {H}}}\right|\mathbf {x} \right\rangle }" loading="lazy"></span></dd></dl>
<p>Unter Verwendung der Wick-Rotation und der Identifizierung der euklidischen Zeit mit <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \hbar \beta =1/(k_{b}T)}">
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<annotation encoding="application/x-tex">{\displaystyle \hbar \beta =1/(k_{b}T)}</annotation>
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</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/619aaf5c3e50ffe3354cf7c7498a860c3aec4580.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:13.656ex; height:2.843ex;" alt="{\displaystyle \hbar \beta =1/(k_{b}T)}" loading="lazy"></span> erhält man eine Pfadintegraldarstellung der Zustandssumme in massengewichteten Koordinaten:<sup id="cite_ref-:inst_review_4-0" class="reference"><a href="#cite_note-:inst_review-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>
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<dl><dd><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Z_{k}=\oint {\mathcal {D}}\mathbf {x} (\tau )e^{-S_{E}[\mathbf {x} (\tau )]/\hbar },\quad S_{E}=\int _{0}^{\beta \hbar }\left({\frac {\dot {\mathbf {x} }}{2}}^{2}+V(\mathbf {x} (\tau ))\right)d\tau }">
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<annotation encoding="application/x-tex">{\displaystyle Z_{k}=\oint {\mathcal {D}}\mathbf {x} (\tau )e^{-S_{E}[\mathbf {x} (\tau )]/\hbar },\quad S_{E}=\int _{0}^{\beta \hbar }\left({\frac {\dot {\mathbf {x} }}{2}}^{2}+V(\mathbf {x} (\tau ))\right)d\tau }</annotation>
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</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/0223e38e03d448bdcad2f486e292403ee2cb4ec0.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.505ex; width:59.846ex; height:6.509ex;" alt="{\displaystyle Z_{k}=\oint {\mathcal {D}}\mathbf {x} (\tau )e^{-S_{E}[\mathbf {x} (\tau )]/\hbar },\quad S_{E}=\int _{0}^{\beta \hbar }\left({\frac {\dot {\mathbf {x} }}{2}}^{2}+V(\mathbf {x} (\tau ))\right)d\tau }" loading="lazy"></span></dd></dl>
<p>Das Pfadintegral wird dann durch eine „steepest-descent“-Integration angenähert, die nur die Beiträge der klassischen Lösungen und der quadratischen Fluktuationen um sie herum berücksichtigt. Daraus ergibt sich für die Geschwindigkeitskonstante in massegewichteten Koordinaten
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<dl><dd><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle k(\beta )={\frac {2}{\beta \hbar }}\left({\frac {\operatorname {det} \left[-{\frac {\partial ^{2}}{\partial \tau ^{2}}}+\mathbf {V} ''(x_{\text{RS}}(\tau ))\right]}{\operatorname {det} \left[-{\frac {\partial ^{2}}{\partial \tau ^{2}}}+\mathbf {V} ''(x_{\text{Inst}}(\tau ))\right]}}\right)^{\frac {1}{2}}{\exp \left({\frac {-S_{E}[x_{\text{Inst}}(\tau )+S_{E}[x_{\text{RS}}(\tau )]}{\hbar }}\right)}}">
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<annotation encoding="application/x-tex">{\displaystyle k(\beta )={\frac {2}{\beta \hbar }}\left({\frac {\operatorname {det} \left[-{\frac {\partial ^{2}}{\partial \tau ^{2}}}+\mathbf {V} ''(x_{\text{RS}}(\tau ))\right]}{\operatorname {det} \left[-{\frac {\partial ^{2}}{\partial \tau ^{2}}}+\mathbf {V} ''(x_{\text{Inst}}(\tau ))\right]}}\right)^{\frac {1}{2}}{\exp \left({\frac {-S_{E}[x_{\text{Inst}}(\tau )+S_{E}[x_{\text{RS}}(\tau )]}{\hbar }}\right)}}</annotation>
</semantics>
</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/e5bb1380adb42dd74e204b92b68b9dcd135d3e23.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -4.505ex; width:77.75ex; height:11.343ex;" alt="{\displaystyle k(\beta )={\frac {2}{\beta \hbar }}\left({\frac {\operatorname {det} \left[-{\frac {\partial ^{2}}{\partial \tau ^{2}}}+\mathbf {V} ''(x_{\text{RS}}(\tau ))\right]}{\operatorname {det} \left[-{\frac {\partial ^{2}}{\partial \tau ^{2}}}+\mathbf {V} ''(x_{\text{Inst}}(\tau ))\right]}}\right)^{\frac {1}{2}}{\exp \left({\frac {-S_{E}[x_{\text{Inst}}(\tau )+S_{E}[x_{\text{RS}}(\tau )]}{\hbar }}\right)}}" loading="lazy"></span></dd></dl>
<p>dabei ist <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle x_{\text{Inst}}}">
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<annotation encoding="application/x-tex">{\displaystyle x_{\text{Inst}}}</annotation>
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</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/b941e5d35d0de5ff68cb3003e81990f7608c8863.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:4.357ex; height:2.009ex;" alt="{\displaystyle x_{\text{Inst}}}" loading="lazy"></span> ein periodisches Instanton und <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle x_{\text{RS}}}">
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</math></span><img src="./_assets_/eb734a37dd21ce173a46342d1cc64c92/bf59697f67095cf6797009851e881937bf422674.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.685ex; height:2.009ex;" alt="{\displaystyle x_{\text{RS}}}" loading="lazy"></span>ist die triviale Lösung des Pseudoteilchens in Ruhe, die den Reaktanden repräsentiert. Der Instantonpfad kann als wahrscheinlichster Tunnelpfad interpretiert werden.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>
</p>
<div class="mw-heading mw-heading2"><h2 id="Literatur">Literatur</h2></div>
<ul><li><a href="Marcus_Hutter" title="Marcus Hutter">Marcus Hutter</a>: <i>Instantonen in der QCD</i>, Dissertation, 1995.</li>
<li>R.Rajaraman: <i>Solitons and instantons - an introduction to solitons and instantons in quantum field theory.</i> Elsevier, Amsterdam 2005, ISBN 0-444-87047-4</li>
<li>Mikhail A. Shifman: <i>Instantons in gauge theories.</i> World Scientific, Singapore 1994, ISBN 981-02-1681-5</li>
<li><a href="Sidney_Coleman" title="Sidney Coleman">Sidney Coleman</a>: <i>Aspects of Symmetry</i>, Cambridge University Press, 1985, ISBN 0-521-31827-0</li></ul>
<div class="mw-heading mw-heading2"><h2 id="Einzelnachweise">Einzelnachweise</h2></div>
<ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><a href="#cite_ref-1">↑</a></span> <span class="reference-text">Harald J.W. Müller-Kirsten, Introduction to Quantum Mechanics: Schrödinger Equation and Path Integral, 2nd ed., World Scientific, 2012, ISBN 978-981-4397-73-5.</span>
</li>
<li id="cite_note-:inst_chapter-2"><span class="mw-cite-backlink"><a href="#cite_ref-:inst_chapter_2-0">↑</a></span> <span class="reference-text"><span class="book">Viktor Zaverkin, Johannes Kästner: <cite class="lang" lang="en" dir="auto" style="font-style:italic">Tunnelling in Molecules: Nuclear Quantum Effects from Bio to Physical Chemistry</cite>. Royal Society of Chemistry, London 2020, ISBN 978-1-83916-037-0, Instanton Theory to Calculate Tunnelling Rates and Tunnelling Splittings, <span style="white-space:nowrap">S.<span style="display:inline-block;width:.2em"> </span>245–260</span>, <a href="Digital_Object_Identifier" title="Digital Object Identifier">doi</a>:<span class="uri-handle" style="white-space:nowrap"><a rel="nofollow" class="external text" href="https://doi.org/10.1039/9781839160370">10.1039/9781839160370</a></span> (englisch).<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abookitem&rfr_id=info:sid/de.wikipedia.org:Instanton&rft.atitle=Instanton+Theory+to+Calculate+Tunnelling+Rates+and+Tunnelling+Splittings&rft.au=Viktor%26%2332%3BZaverkin%2C%26%2332%3BJohannes%26%2332%3BK%C3%A4stner&rft.btitle=Tunnelling+in+Molecules%3A+Nuclear+Quantum+Effects+from+Bio+to+Physical+Chemistry&rft.date=2020&rft.doi=10.1039%2F9781839160370&rft.genre=bookitem&rft.isbn=9781839160370&rft.pages=245-260&rft.place=London&rft.pub=Royal+Society+of+Chemistry" style="display:none"> </span></span></span>
</li>
<li id="cite_note-3"><span class="mw-cite-backlink"><a href="#cite_ref-3">↑</a></span> <span class="reference-text">Jeremy O. Richardson: <cite class="lang" lang="en" dir="auto" style="font-style:italic">Perspective: Ring-polymer instanton theory</cite>. In: <cite class="lang" lang="en" dir="auto" style="font-style:italic">J. Chem. Phys.</cite> <span style="white-space:nowrap">Band<span style="display:inline-block;width:.2em"> </span>148</span>, 2018, <span style="white-space:nowrap">S.<span style="display:inline-block;width:.2em"> </span>200901</span>, <a href="Digital_Object_Identifier" title="Digital Object Identifier">doi</a>:<span class="uri-handle" style="white-space:nowrap"><a rel="nofollow" class="external text" href="https://doi.org/10.1063/1.5028352">10.1063/1.5028352</a></span> (englisch).<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rfr_id=info:sid/de.wikipedia.org:Instanton&rft.atitle=Perspective%3A+Ring-polymer+instanton+theory&rft.au=Jeremy+O.+Richardson&rft.btitle=J.+Chem.+Phys.&rft.date=2018&rft.doi=10.1063%2F1.5028352&rft.genre=book&rft.pages=200901&rft.volume=148" style="display:none"> </span></span>
</li>
<li id="cite_note-:inst_review-4"><span class="mw-cite-backlink"><a href="#cite_ref-:inst_review_4-0">↑</a></span> <span class="reference-text"><a href="Johannes_K%C3%A4stner" title="Johannes Kästner">Johannes Kästner</a>: <cite class="lang" lang="en" dir="auto" style="font-style:italic">Theory and Simulation of Atom Tunneling in Chemical Reactions</cite>. In: <cite class="lang" lang="en" dir="auto" style="font-style:italic">WIREs Comput. Mol. Sci.</cite> 4. Jahrgang, 2014, <span style="white-space:nowrap">S.<span style="display:inline-block;width:.2em"> </span>158</span>, <a href="Digital_Object_Identifier" title="Digital Object Identifier">doi</a>:<span class="uri-handle" style="white-space:nowrap"><a rel="nofollow" class="external text" href="https://doi.org/10.1002/wcms.1165">10.1002/wcms.1165</a></span> (englisch).<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rfr_id=info:sid/de.wikipedia.org:Instanton&rft.atitle=Theory+and+Simulation+of+Atom+Tunneling+in+Chemical+Reactions&rft.au=Johannes%26%2332%3BK%C3%A4stner&rft.btitle=WIREs+Comput.+Mol.+Sci.&rft.date=2014&rft.doi=10.1002%2Fwcms.1165&rft.genre=book&rft.pages=158&rft.volume=4.+Jahrgang" style="display:none"> </span></span>
</li>
<li id="cite_note-5"><span class="mw-cite-backlink"><a href="#cite_ref-5">↑</a></span> <span class="reference-text">Jan Meisner, Johannes Kästner: <cite style="font-style:italic">Der Tunneleffekt von Atomen in der Chemie</cite>. In: <cite style="font-style:italic">Angewandte Chemie</cite>. <span style="white-space:nowrap">Band<span style="display:inline-block;width:.2em"> </span>128</span>, 2016, <span style="white-space:nowrap">S.<span style="display:inline-block;width:.2em"> </span>5488–5502</span>, <a href="Digital_Object_Identifier" title="Digital Object Identifier">doi</a>:<span class="uri-handle" style="white-space:nowrap"><a rel="nofollow" class="external text" href="https://doi.org/10.1002/ange.201511028">10.1002/ange.201511028</a></span>.<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rfr_id=info:sid/de.wikipedia.org:Instanton&rft.atitle=Der+Tunneleffekt+von+Atomen+in+der+Chemie&rft.au=Jan+Meisner%2C+Johannes+K%C3%A4stner&rft.btitle=Angewandte+Chemie&rft.date=2016&rft.doi=10.1002%2Fange.201511028&rft.genre=book&rft.pages=5488-5502&rft.volume=128" style="display:none"> </span></span>
</li>
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