多國科學家合作首次創建氫分子波函數平方圖像

　　據物理學家組織網2018年1月9日報道，德國、美國、西班牙、俄羅斯以及澳大利亞的科學家合作首次創建了氫分子波函數平方圖像Ψ2(H2)，相關研究結果已經于2017年12月22日在《自然通訊》（Nature Communications）網站發表——M. Waitz, R. Y. Bello, D. Metz, J. Lower, F. Trinter, C.Schober, M. Keiling, U. Lenz, M. Pitzer, K. Mertens, M. Martins, J. Viefhaus,S. Klumpp, T. Weber, L. Ph. H. Schmidt, J. B. Williams, M. S. Sch?ffler, V. V. Serov, A. S. Kheifets, L. Argenti, A. Palacios,F. Martín, T. Jahnke, R. D?rner. Imaging thesquare of the correlated two-electron wave function of a hydrogen molecule.Nature Communications, 2018, 8: 2266. DOI: 10.1038/s41467-017-02437-9.

　　參與此項研究的有來自德國歌德大學（J. W. GoetheUniversit?t）、德國卡塞爾大學（Universit?t Kassel）、德國漢堡大學（Universit?t Hamburg）、德國電子同步加速器中心（Deutsches Elektronen-SynchrotronDESY）、德國FS-FLASH-D；西班牙馬德里自治大學（Universidad Autónoma de Madrid）、Instituto Madrileo de EstudiosAvanzados en Nanociencia；美國勞倫斯伯克利國家實驗室（Lawrence BerkeleyNational Laboratory）、美國雷諾內華達大學（University of NevadaReno）、美國中佛羅里達大學（University of CentralFlorida）；俄羅斯薩拉托夫州立大學（Saratov StateUniversity）；澳大利亞國立大學（The AustralianNational University）的科研人員。審稿人對此評價是具有里程碑意義的研究成果。它不僅是最前沿的實驗結果和非常全面的理論分析的完美結合，而且是氫分子作為一種基本的雙電子體系，在相關雙電子波函數成像方面的一項原始的、非常有趣的研究，該研究成果結構清晰，同時便于非專業讀者閱讀。在物理學和化學的廣泛領域中，所提出的工作具有很高的影響和激發新思維的潛力。因此，審稿人非常熱情地推薦這一具有里程碑意義的作品，在沒有任何重大變化的情況下給予發表

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　　Physicistscreate first direct images of the square of the wave function of a hydrogenmolecule

　　January9, 2018 by Lisa Zyga

　　Image of the square of thewave function of a hydrogen molecule with two electrons. Credit: Waitz et al.Published in Nature Communications

　　For the first time, physicistshave developed a method to visually image the entanglement between electrons.As these correlations play a prominent role in determining a molecule's wavefunction—which describes the molecule's quantum state—the researchers then usedthe new method to produce the first images of the square of the two-electronwave function of a hydrogen (H2) molecule.

　　Although numerous techniquesalready exist for imaging the individual electrons of atoms and molecules, this is the first method that can directly image thecorrelations between electrons and allow researchers to explore how theproperties of electrons depend on one another.

　　The researchers, M. Waitz etal., from various institutes in Germany, Spain, the US, Russia, and Australia,have published a paper on the new imaging method in a recent issue of NatureCommunications.

　　"There are other methodsthat allow one to reconstruct correlations from different observations；however, to my knowledge, this is the first time that one gets a directimage of correlations by just looking at a spectrum," coauthor FernandoMartín at the Universidad Autónoma de Madrid told Phys.org. "Therecorded spectra are identical to the Fourier transforms of the differentpieces of the square of the wave function (or equivalently, to therepresentation of the different pieces of the wave function in momentum space).No reconstruction or filtering or transformation is needed: the spectrumdirectly reflects pieces of the wave function in momentum space."

　　The new method involvescombining two imaging methods that are already widely used: photoelectronimaging and the coincident detection of reaction fragments. The researcherssimultaneously employed both methods by using the first method on one electronto project that electron onto a detector, and using the second method on theother electron to determine how its properties change in response.

　　The simultaneous use of bothmethods reveals how the two electrons are correlated and produces an image ofthe square of the H2 correlated two-electron wave function. Thephysicists emphasize one important point: that these are images of the squareof the wave function, and not the wave function itself.

　　"The wave function is notan observable in quantum physics, so it cannot be observed," Martín said."Only the square of the wave function is an observable (if you have thetools to do it). This is one of the basic principles of quantum physics. Thosewho claim that they are able to observe the wave function are not using theproper language because this is not possible: what they do is to reconstruct itfrom some measured spectra by making some approximations. It can never be adirect observation."

　　The researchers expect thatthe new approach can be used to image molecules with more than two electrons aswell, by detecting the reaction fragments of multiple electrons. The methodcould also lead to the ability to image correlations between the wave functionsof multiple molecules.

　　"Obviously, the naturalstep to follow is to try a similar method in more complicated molecules,"Martín said. "Most likely, the method will work for small molecules, butit is not clear if it will work in very complex molecules. Not because oflimitations in the basic idea, but mainly because of experimental limitations,since coincidence experiments in complex molecules are much more difficult toanalyze due to the many nuclear degrees of freedom."

　　The ability to visualizeelectron-electron correlations and the corresponding molecular wave functions hasfar-reaching implications for understanding the basic properties of matter. Forinstance, one of the most commonly used methods for approximating a wavefunction, called the Hartree-Fock method, does not account forelectron-electron correlations and, as a result, often disagrees withobservations.

　　In addition, electron-electroncorrelations lie at the heart of fascinating quantum effects, such assuperconductivity (when electrical resistance drops to zero at very coldtemperatures) and giant magnetoresistance (when electrical resistance greatlydecreases due to the parallel alignment of the magnetization of nearby magneticlayers). Electron correlations also play a role in the simultaneous emission oftwo electrons from a molecule that has absorbed a single photon, a phenomenoncalled "single-photon double ionization."

　　And finally, the results mayalso lead to practical applications, such as the ability to realize correlation imaging withfield-electron lasers and with laser-based X-ray sources.

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