多国科学家合作首次创建氢分子波函数平方图像Ψ²(H₂)

诸平

据物理学家组织网2018年1月9日报道，德国、美国、西班牙、俄罗斯以及澳大利亚的科学家合作首次创建了氢分子波函数平方图像Ψ²(H₂)，相关研究结果已经于2017年12月22日在《自然通讯》（Nature Communications）网站发表——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. Schöffler, V. V. Serov, A. S. Kheifets, L. Argenti, A. Palacios, F. Martín, T. Jahnke, R. Dörner. 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. H_2 two-electron wave function.pdf

参与此项研究的有来自德国歌德大学（J. W. GoetheUniversität）、德国卡塞尔大学（Universität Kassel）、德国汉堡大学（Universität Hamburg）、德国电子同步加速器中心（Deutsches Elektronen-SynchrotronDESY）、德国FS-FLASH-D；西班牙马德里自治大学（Universidad Autónoma de Madrid）、Instituto Madrileo de EstudiosAvanzados en Nanociencia；美国劳伦斯伯克利国家实验室（Lawrence BerkeleyNational Laboratory）、美国雷诺内华达大学（University of NevadaReno）、美国中佛罗里达大学（University of CentralFlorida）；俄罗斯萨拉托夫州立大学（Saratov StateUniversity）；澳大利亚国立大学（The AustralianNational University）的科研人员。审稿人对此评价是具有里程碑意义的研究成果。它不仅是最前沿的实验结果和非常全面的理论分析的完美结合，而且是氢分子作为一种基本的双电子体系，在相关双电子波函数成像方面的一项原始的、非常有趣的研究，该研究成果结构清晰，同时便于非专业读者阅读。在物理学和化学的广泛领域中，所提出的工作具有很高的影响和激发新思维的潜力。因此，审稿人非常热情地推荐这一具有里程碑意义的作品，在没有任何重大变化的情况下给予发表（

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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.

Explore further:Aspace-time sensor for light-matter interactions

H_2 two-electron wave function.pdf