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Science publishes a new finding on quantum effects of water
中心江颖研究组、王恩哥研究组等在《科学》发文首次揭示水的量子效应

The mystery of water mainly arises from the intermolecular hydrogen-bonding interaction. It is well known that hydrogen bonds have a strong classic component coming from electrostatics. However, its quantum component can be exceptionally prominent due to the zero-point motion of light hydrogen nuclei (proton), which is a natural result of the Heisenberg uncertainty principle. Therefore, the assessment of nuclear quantum effects has been a key issue for understanding the structure, dynamics, and macroscopic properties of water. Despite enormous scientific efforts in past decades, it still remains an open question to what extent the quantum motion of the hydrogen nuclei can affect the hydrogen bond.

Now, the teams led by Prof. Ying Jiang and Prof. Enge Wang of International Center for Quantum Materials (ICQM) of Peking University provide a smoking gun for this important question. As published in Science on Apr. 15, 2016 (Science DOI: 10.1126/science.aaf2042), the researchers unravel quantitatively, for the first time, the quantum component of a single hydrogen bond at a water-solid interface, through a combined study using a scanning tunneling microscope (STM) and density functional theory (DFT) calculations.

'The main difficulty of extracting the quantum component of hydrogen bond lies in that the quantum states of hydrogen nuclei are extremely sensitive to the coupling with local environments, leading to significant broadening and averaging effects when conventional spectroscopic or diffraction techniques are used.' says Jiang. Therefore, the ability to probe water with single bond precision is crucial.

To this end, the researchers succeeded to push the limit of vibrational spectroscopy of water down to the single-bond level using a novel technique called tip-enhanced inelastic electron tunneling spectroscopy (IETS) based on STM, which combines sub-ångström spatial resolution and single-bond vibrational sensitivity. The signal-to-noise ratios of the tip-enhanced IETS are enhanced by orders of magnitude over the conventional STM-IETS, which was pioneered by Prof. Wilson Ho's group of UC Irvine 18 years ago.

'The conventional IETS signals of water are extraordinarily weak since the frontier orbitals of water are located far away from the Fermi level. The key to defeat this limitation is gating the frontier orbitals of water towards the Fermi level with a chlorine-terminated STM tip to resonantly enhance the electron-vibration coupling.' explains Jiang. With such a tip-enhanced IETS, the hydrogen-bonding strength can be determined with unprecedentedly high accuracy from the redshift in the O-H stretching frequency of water.

By conducting isotopic substitution experiments (replacing hydrogen atom with heavier deuterium atom), the researchers could extract the quantum component of the hydrogen bond, which accounts for up to 14% of the bond strength. Surprisingly, the quantum contribution is much greater than the thermal energy contribution, even at room temperature. In-depth investigation combined with ab initio path integral molecular dynamics (PIMD) simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the strong ones. However, this trend can be completely reversed when the hydrogen bond is strongly coupled to the polar atomic sites of the surface.

'This joint experimental and theoretical work yields a cohesive picture for the nuclear quantum effects of hydrogen bonds.' adds Wang. 'Those findings may completely renovate our understanding of water and provide answers to many weirdness of water from a quantum mechanical view. It would be very interesting to further explore the quantum effects on the cooperativity of correlated H-bonds beyond the single hydrogen bond.' 

This work received supports from Ministry of Science and Technology of China, National Natural Science Foundation of China, Ministry of Education of China, National Program for Support of Eminent Professionals, and Collaborative Innovation Center of Quantum Matter, China.

Article link: http://science.sciencemag.org/content/352/6283/321

D:WorkProjectsWaterpaperIETSsubmitted to ScienceReportsiguresexperimental_00658.png

Figure caption: Left is the schematic of STM experimental setup. The hydrogen atoms of water show prominent zero-point motion thanks to the Heisenberg uncertainty principle. Right is the tip-enhanced IETS of a single water molecule, in which stretching, bending and rotational modes are identified. Those vibrational modes can be used as sensitive probes to sense the influence of quantum motion of hydrogen nuclei on the hydrogen bond. (Design: Mingcheng Liang)

    最近,中心江颖研究员课题组和王恩哥院士课题组以及物理学院李新征研究员、华中科技大学吕京涛研究员合作,国际上率先测定了氢键的量子成分,揭示了水的核量子效应,从全新的角度诠释了水的奥秘。相关研究成果于2016年4月15日刊发在国际顶级学术期刊《科学》(Science DOI: 10.1126/science.aaf2042)上。

     “水的结构是什么”这是《科学》杂志在创刊125周年的特刊中提出的125个最具挑战性的科学问题之一。水的结构之所以如此复杂,其中一个很重要的原因就是源于水分子之间的氢键相互作用。人们通常认为氢键的本质为经典的静电相互作用,然而由于氢原子核质量很小,其量子特性(量子隧穿和量子涨落)往往不可忽视,因此氢键同时也包含一定的量子成分。氢核的量子效应对氢键相互作用到底有多大影响?或者说氢键的量子成分究竟有多大?这个问题对于理解水/冰的微观结构和反常物性至关重要。但是,氢核的量子化研究无论对于实验还是理论都非常具有挑战性。

    为实现对氢核量子特性的精确探测和描述,江颖课题组和王恩哥课题组近年来在相关实验技术和理论方法上分别取得突破。他们成功发展了对于氢核敏感的超高分辨扫描探针显微术,开发了基于第一性原理的路径积分分子动力学方法(全量子化计算),实现了单个水分子内部自由度的成像和水的氢键网络构型的直接识别[Nature Materials 13, 184 (2014)和Nature Communications 5, 4056 (2014],并在此基础上探测到氢核的动态转移过程[Nature Physics 11, 235 (2015)]。

    最近,他们又基于扫描隧道显微镜研发了一套“针尖增强的非弹性电子隧穿谱”技术,突破了传统非弹性电子隧穿谱技术在信噪比和分辨率方面的限制,国际上首次获得了单个水分子的高分辨振动谱,并由此测得了单个氢键的强度。通过可控的同位素替换实验,并结合全量子化计算模拟,他们发现氢键的量子成分可远大于室温下的热能,表明氢核的量子效应不只是对经典相互作用的简单修正,其足以对水的结构和性质产生显著的影响。进一步深入分析表明,氢核的非简谐零点运动会弱化弱氢键,强化强氢键,这个物理图像对于各种氢键体系具有相当的普适性,澄清了学术界长期争论的氢键的量子本质。

    《科学》杂志的审稿人盛赞该工作是“实验的杰作(tour de force experiments)”、“一定会引起谱学界的广泛兴趣(they are certainly of interest to the spectroscopy community)”、“为研究氢核量子效应提供了一个绝佳的平台(this measurement is unique and provides a fantastic opportunity to examine the contribution of quantum motion of the proton)”。

    江颖研究员和王恩哥院士分别负责该工作的实验和理论部分,北大直博生郭静、博士后冯页新和华中科技大学吕京涛教授是文章的共同第一作者,北大江颖研究员、王恩哥院士和李新征研究员为文章共同通讯作者。这项工作得到了国家自然科学基金委、科技部国家重点基础研究发展计划、“万人计划”青年拔尖人才支持计划和量子物质科学协同创新中心的经费支持。

  文章链接:http://science.sciencemag.org/content/352/6283/321

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   图:左图为利用扫描隧道显微镜测量水的量子效应的示意图,由于量子力学的不确定性原理,水分子的氢原子表现出显著的零点运动。右图为单个水分子的非弹性电子隧穿谱,从中可分辨水分子的拉伸、弯曲和转动等振动模式,这些振动可以作为灵敏的探针来探测氢核的量子运动对氢键的影响。(图片设计:梁明诚)