“看见”一种新型的二维冰结构

 固体表面与水(冰)形成的界面是自然界中最常见的界面之一。它直接或者间接地影响到我们的日常生活以及工业技术的方方面面,对于该问题的科学研究涉及表面物理、大气化学、环境科学等多个领域。其中,水在固体表面发生的多相结晶过程是人们研究水和固体界面的一个重要方面。在地上,这种结冰过程能形成美丽的冰川,也会导致路面和铁轨结冰而阻碍交通;在空中,这种结冰过程是大气层循环中的一个重要组成部分,同时也是很多起飞机事故的元凶。     

Ab initio path-integral molecular dynamics and the quantum nature of hydrogen bonds

The hydrogen bond(HB) is an important type of intermolecular interaction, which is generally weak, ubiquitous,and essential to life on earth. The small mass of hydrogen means that many properties of HBs are quantum mechanical in nature. In recent years, because of the development of computer simulation methods and computational power, the influence of nuclear quantum effects(NQEs) on the structural and energetic properties of some hydrogen bonded systems has been intensively studied. Here, we present a review of these studies by focussing on the explanation of the principles underlying the simulation methods, i.e., the ab initio path-integral molecular dynamics. Its extension in combination with the thermodynamic integration method for the calculation of free energies will also be introduced. We use two examples to show how this influence of NQEs in realistic systems is simulated in practice.     

Frequencies-Selected Enhancement of the Extended High-Order Harmonic Generation Plateau from a United Two-Atom System Irradiated by a Combined Pulse

We present a high-order harmonic generation (HHG) spectrum from a united two-atom system exposed to a combined laser pulse, by numerically solving a one-dimensional time-dependent Schr6dinger equation. The combined laser pulse is composed of a low-frequency femtosecond pulse and a high-frequency attosecond one with respective appropriate amplitudes. For a suitable inter-nuclear separation, the harmonic emission efficiencies near the second cutoff of the extended plateau can be effectively enhanced by more than four orders of magnitude compared with the case of the low-frequency pulse alone. Such a combined pulse irradiating on the united twoatom system ionizes each atom, in a large rate (but not to a too large ionization yield), mainly at a particular time-interval. When the ionized electron from an atom reaches at the vicinity of the other atom and recombines with it, significant HHG enhancement is achieved for particular harmonics. This result, to our knowledge, is one of the best up to now in the endeavor for dramatically extending the width and simultaneously enhancing the height of the plateau.     

一维纳米限域物质的结构

低维材料的结构探索是我们全面认识元素物态的关键.近年来,研究方法的发展使包括一维原子链在内的各种低维结构逐渐被报道.根据一维限域材料的发展现状,本文重点对直径1 nm以下单质材料的结构和物理性质进行了综述,并简要总结了此类研究中常用的实验技术和理论方法.希望通过材料结构特性的解读和研究方法的讨论,说明目前理论计算层面存在的困难及需要面临的挑战,并以此对一维限域材料的研究前景进行展望.     

基于路径积分分子动力学与热力学积分方法的高压氢自由能计算

在相图研究中, 严格计算一个真实系统在特定温度、压强下的自由能是近年来该领域理论方法发展的前沿. 自Mermin提出有限温度密度泛函理论后, 在电子结构层面, 弱关联系统中人们就其在对自由能贡献的描述已相对完善, 但在原子核运动的描述上, 热运动与量子运动的非简谐项却总被忽视. 本文将路径积分分子动力学与热力学积分结合, 对300 GPa下氢晶体Cmca 结构中原子核热涨落与量子涨落对自由能的影响进行了分析. 发现在100 K核量子涨落非简谐项的贡献约为15 meV每原子, 远大于不同结构间静态焓的差别. 该研究提醒人们简谐近似在核量子效应描述中可能存在的不准确性(即使在低温下). 同时, 我们采取的方法 也为人们进行自由能的准确计算提供了一个简单有效的手段.     

Ultrahigh Harmonic Generation from an Atom with Superposition of Ground State and Highly Excited States

We investigate the high-order harmonic generation from an atom prepared in a superposition of ground state and highly excited state. When the atom is irradiated by an ultrashort pulse, the cutoff position of the plateau in the harmonic spectrum is largely extended compared with the case that the atom is initially in the ground state. The physics of the extension of the high-order harmonic plateau can be interpreted by the spatial structure of the atomic initial wave packet. We can optimize the generation of high-order harmonics by substituting the excited state for a particular coherent superposition of some highly excited states to form a spatially localized excited wave packet.     

吲哚类菁染料Cy3嵌入的核壳荧光纳米颗粒的制备及其性质研究

以蛋白质或多肽修饰的吲哚类菁染料Cy3为内核, 采用实验条件简单的油包水反相微乳液方法成核, 通过正硅酸乙酯水解形成的网状二氧化硅包壳的方法制备吲哚类菁染料Cy3嵌入的核壳荧光纳米颗粒. 考察了以不同等电点的蛋白质和多肽修饰的Cy3为内核材料对吲哚类菁染料Cy3嵌入的核壳荧光纳米颗粒制备的影响. 结果表明, 分别采用人免疫球蛋白(IgG)或多聚赖氨酸修饰的Cy3为内核材料, 都能制备荧光强度高、荧光稳定性强和染料泄漏极少的Cy3嵌入的核壳荧光纳米颗粒. 进一步对Cy3嵌入的核壳荧光纳米颗粒进行了表征, 并将基于这一新型的荧光纳米颗粒建立起来的生物标记方法初步应用于流感病毒DNA的检测, 其检测线性范围为3.18×10-10～1.27×10-9 mol/L, 检测下限为3.51×10-10 mol/L, 相关系数r为0.986 5.     

物理学中的全量子化效应与新的探索

在一般的第一性原理计算中,原子核总是被近似成经典粒子.然而,在一些特殊的体系中,原子核的量子效应对体系的物理性质和物理过程有着至关重要的影响.在相关问题的模拟中,考虑了原子核量子效应的全量子化计算,展示了其独有的准确性.目前,路径积分分子动力学是被广泛采用的全量子化计算方法.而第一性原理的路径积分分子动力学不仅保留了第一性原理计算中电子结构和电子基态能量计算的方法,同时还应用费恩曼(Feynman)路径积分原理,得到了包含原子核量子信息的运动方程.张千帆等人应用第一性原理路径积分分子动力学,计算了BaZrO3中氢核的输运情况.结果表明,原子核的量子化对输运中两个不同的子过程有不同程度的影响,它使得有氢氧键断裂的T过程的势垒下降更多,使T过程成为快过程,从而验证了红外光谱实验的结果,同时否定了传统计算给出的T过程是慢过程的结论.     

A Time-Distinguished Analysis of the Harmonic Structure from a Model Molecular Ion

We present high-order harmonic generation spectra resulted from a single-electron model molecular ion exposed to intense laser fields by numerically solving a one-dimensional time-dependent Schrodinger equation. There are three plateaus in the spectra and the maximal cutoff energy is Ip 8.5Up, when the inter-nuclear distance R     

Harmonic Enhancement Mechanism of a Superposition State Atom Irradiated by Short Pulses

We investigate the high-order harmonic generation （HHG） of a model atom whose initial state is prepared in a superposition of its ground state and an excited state irradiated by different duration laser pulses. Compared to the HHG generated from an atom whose initial state is in its ground state, its conversion efficiency obtains some enhancement. The enhancement originates from the higher ionization rate （rather than the ionization yield） of the atom with superposition initial state.