Molecular Dynamics Study on Microstructure of Potassium Dihydrogen Phosphates Solution

用分子动力学模拟方法研究了不同温度下磷酸二氢钾(KDP)水溶液的内能和微结构. 水分子被看作简单点电荷模型,磷酸二氢根被看作七节点模型. 另外系统地研究了溶液的内能和径向分布函数. 在饱和温度附近,局域粒子数密度有很大的波动;饱和溶液的比热要比不饱和溶液的比热高表明溶液在低于饱和温度的过程中经历了结晶过程;水中的氧原子跟磷酸二氢根中的氢原子的径向分布函数表明二者之间形成很强的氢键结构;在KDP水溶液中,钾阳离子跟磷酸二氢根中的氧原子有很强的相互作用,并且在饱和溶液中形成的离子对数量更多.     

硝酸钾电解质溶液水化结构的分子动力学模拟

应用分子动力学方法对硝酸钾溶液中离子团簇的结构和离子的水化性质进行模拟研究．水分子采用简单点电荷模型，钾离子被看作带电硬球，硝酸根离子采用刚性四节点模型，同时考虑了节点间的库仑长程作用和L-J相互作用，库仑长程作用采用EWALD求和方法处理，得到了溶液中各种离子对的微观构型和径向分布函数，考察了溶液浓度对离子水化性质的影响．研究表明, 在KNO3溶液中存在一定程度的离子缔合，在高离子浓度情形，可以观察到二聚体、三聚体、溶剂分离阴阳离子对以及其它更复杂的离子团簇构型；离子水化数随离子浓度的升高而降低，对不同浓度的溶液得到的K+的水化数为5?7，NO3-的水化数为3.5?4.7，与蒙特卡罗模拟结果和飞行时间中子衍射实验的测量结果一致．     

Hα线型分析

利用高斯分布拟合HT-6M托卡马克Hα线型，得出了由反射进入等离子体中的氢原子、氢分子离解后产生的氢原子，以及电荷交换产生的氢原子辐射Ｈα谱线的份额，由Doppler频移和展宽分别得出它们的入射速度和离子温度．在简化模型下讨论了氢原子的输运行为，得出了氢原子的密度分布和体发射系数，以及入射速度大小对粒子约束时间的影响，并与实验数据进行比较．由中性氢原子能量分布得出发生在边界的分子过程是氢分子的离解激发和电离离解 关键词：     

采用分子动力学方法研究氯化钠水溶液的微观结构

溶液的微观结构对溶液的宏观性质具有决定性影响,团簇的存在和溶液不均性的认知是该领域研究的重要进展之一,也是关注的热点.为了考察溶液的微观结构,本文采用分子动力学方法对浓度适中的氯化钠水溶液进行了模拟,获得了各原子(离子)间的径向分布函数和溶液的瞬态图像.通过对比分析Na~+-Cl~-和Cl~--Na~+的径向分布函数,并结合已有研究结果,表明该径向分布函数上第三峰也来源于离子对,发现氯化钠水溶液存在着第三种离子对;上述径向分布函数上的三个峰分别与直接接触离子对、部分间隔离子对和完全间隔离子对相对应,并给出了这三种离子对的瞬态图像.三个时刻下系统瞬态图像的分析结果表明,溶液中存在水分子连续分布的区域和由离子与水分子共同构成的离子团簇.各瞬态图像中水分子连续分布的最大区域的平均尺寸至少为1.43 nm,溶液中部分间隔离子对的比例最大,约为0.68.在同时考虑直接接触离子对和部分间隔离子对时,三个时刻中最大团簇尺度的平均值为1.44 nm,该值约为部分间隔离子对最概然尺寸的3倍;团簇尺寸随团簇中离子数的增加而增大,在团簇中离子数较大时,满足无规行走(自由连接链)模型的结果.这些说明氯化钠水溶...     

非晶态贮氢材料锆镍合金的结构研究

本文报道了非晶态贮氢合金在吸氢前、后两种情况下作X射线散射分析得到的结果。根据对实验得到的径向分布函数RDF(r),简约径向分布函数G(r)的分析,表明吸氢后氢原子主要进入单一由锆原子组成的四面体间隙中,少部分进入由一个镍原子、三个锆原子组成的四面体间隙中;非晶态Ni_0.243Zr_0.757合金的平均配位数为10.6,内部原子配位情况在吸氢前后没有改变;样品在约20?范围内具有短程有序结构。 关键词：     

温稠密物质中不同价态离子分布对X-射线弹性散射光谱计算的影响

在天体物理和惯性约束聚变研究中涉及到的温稠密物质通常包含多种元素的混合,并且每种元素还被电离成多种离子价态,不同价态离子结构及其丰度将直接影响温稠密物质的诊断及其物理性质.同时,从电子结构计算出发来研究宏观物理性质时,还需要考虑温度、密度效应对离子结构的影响.本文从不同价态离子的电子结构计算出发,采用考虑了离子间相互作用的Saha方程获得了稠密环境下的离子丰度,并使用超网链(hypernetted-chain)近似对铝、金以及碳-氢混合物的径向分布函数进行了计算,结合离子周围电子的密度分布,最后获得X-射线汤姆逊散射的弹性散射谱.在X-射线散射谱计算中,计算了温稠密物质中同时存在不同离子价态时的电子结构和径向分布函数,发现在相同的等离子体环境下不同价态离子的径向分布函数和电子结构差别较大.这将对依赖于微观统计过程的物理性质,比如散射光谱,将产生较大的影响.     

非晶态硅氢结构及其缺陷态的蒙特卡罗模拟

本文在考虑了悬挂键的情况下,用蒙特卡罗方法模拟了非晶态硅氢结构,径向分布函数等统计量和实验基本相符。氢原子的结合形式也和实验基本一致。     

基于变分法求解氢原子的径向波函数和能级

基于变分法求解量子力学中氢原子模型并给出相应的推导,运用Mathematica软件辅助计算最优的试探波函数和相应的能级,并与由合流超几何函数(Kummer函数)所给出的氢原子能级分布与径向波函数进行比较.     

N-丁基吡啶四氟硼酸盐/水二元体系的分子动力学模拟研究

利用分子动力学模拟方法研究了离子液体N-丁基吡啶四氟硼酸盐([BPy]BF₄)与Tip4p模型水分子的二元体系的微观结构.比较了各组分间的径向分布函数,结果表明随着离子液体比例的增加,水与阴离子、水与阳离子头部吡啶环、阳离子头部与阴离子上相关原子间的径向分布函数峰值都呈现递增的趋势,而阳离子上丁基链末端碳原子间的径向分布函数没有明显变化;空间分布函数则直观地反映出阴离子主要分布在阳离子的吡啶环周围,水分子在阴离子周围近似呈均等分布,且几率随离子液体比例增大而增加;另外还探讨了不同离子液体比例下的二元体系中氢键的数目和寿命,结果均呈现一定规律的变化.     

铵水间质子交换的再研究:不受扩散约束的化学交换

对铵盐水溶液中铵离子与水分子间的质子交换进行了再研究. 以核磁共振(NMR) 波谱学方法测定了不同pH值, 不同温度和含不同共存盐浓度的氯化铵水溶液中质子交换速率, 组分自扩散系数及NMR自旋晶格弛豫时间. 结果显示铵水间的质子交换并不受交换主体分子或离子自扩散的约束. 交换机理包括化学键和氢键的交换. 在溶液中, 水分子以氢键为桥 梁连接铵和其因水解而产生的碱性部分--氨. 在氢交换的过程中, 氨从水分子接受一个氢 成为铵, 同时另外一端的铵提供一个氢给水分子而变成氨, 或者一个氢从水分子转移到氨而同 时另外一个氢从铵转移到水. 在铵水质子交换的过程中没有净的电荷转移. 在低pH条件下, 由水解产生的氨的浓度降低, 亦即铵-水-氨复合物减少, 结果氢交换的可能性降低. 这解释了生物氢交换体系中的一种常见的在低pH值条件下氢交换变慢的现象.     

Al原子在Pb基底上的沉积过程研究

利用分子动力学方法模拟了Al原子在Pb基底上的沉积过程. 对Al原子在Pb基底(001)面上沉积的形态与Pb原子在Al(001)基底上沉积的形态做了比较. 由于界面间势垒的不同, 两个体系界面间的形态有明显的差异. 分析了基底温度、基底晶面指向、沉积原子的入射动能对界面间原子混合的影响. 模拟结果显示: 随着基底温度升高, 基底原子的可移动性大大增加, 与沉积原子发生较大程度的混合; 入射能的改变对界面间原子的混合影响很小; 基底表面取不同的晶格指向时, 基底与沉积原子间的混合行为也有明显的不同. 利用径向分布函数分析了沉积原子的入射能对薄膜中原子排列有序性的影响. 较高入射能对应更有序的薄膜结构; 由径向分布函数的结构可以推测Al原子在Pb(001)基底表面沉积时界面间可能有金属间化合物生成. 关键词： Pb/Al体系 沉积过程 分子动力学 入射能     

Energy exchange between discrete breathers in graphane in thermal equilibrium

Graphane is a fully hydrogenated graphene which is practically interesting for application in electronics, hydrogen storage and transportation, in nanoscale devices. As it was previously shown, the energy of a discrete breather (nonlinear localized mode) in graphane close to the value of the energy barrier at which the dehydrogenation of graphene occurs. In the present work, molecular dynamics simulation is used to investigate the possibility of energy exchange between discrete breathers in graphane in thermal equilibrium at 400 K and 600 K. In thermally equilibrated graphane, hydrogen atoms are spontaneously excited and can be considered as discrete breathers. Comparison of the kinetic energy per atom as the function of time for the selected hydrogen atoms with their displacements along the z axis showed that there is an energy exchange between the discrete breathers at evaluated temperatures. Hydrogen atom, transmitting its energy to the neighboring atom no longer exists as discrete breather. At high temperatures (600 K) the energy exchange between closely located discrete breathers also take place but strong thermo-oscillations of atoms at high temperatures (above 400 K) considerably affect the process.     

Ripening of subsurface amorphous C clusters formed by low energy He ion bombardment of graphite

Surfaces of highly oriented pyrolytic graphite (HOPG) were bombarded with 100 eV and 500 eV He ions at ion doses of a few 10¹⁵ cm² and temperatures ranging from 300 K to 800 K. AFM images were recorded to investigate the topography of the surfaces after ion bombardment. Supplementary electron energy loss (EEL) and thermal desorption (TD) spectra were measured to determine the C sp² fraction of the bombarded surfaces and the amount of trapped He. The temperature at which He ion bombardment was performed had a drastic effect on the surface structure and topography of the targets on the angstrom-scale and micrometer-scale as well. At 300 K, limited defect atom transport revealed an amorphous but relatively flat HOPG surface. Bombardment at 400 K leads to a granular structure of small protrusions in micrometer-scale AFM images, however, without crystalline order on the surface. The protrusions are due to the formation of subsurface clusters of carbon formed by atoms displaced by ion irradiation. Towards higher temperatures during bombardment the clusters agglomerate and cause the surface layers to bend upwards in dome-like shapes. Simultaneously, the microscopic order of the graphite lattice recovers. At 800 K large areas of the top layer retain their order during bombardment, however, a small number of domes indicate that there still exist some subsurface C clusters. The cluster–cluster distance deduced from the dome distribution indicates that the clusters grow through a ripening process. Annealing of graphite at high temperatures subsequent to ion bombardment at low temperatures is much less effective for recovering the surface crystallinity than ion bombardment at high temperature.     

Local structure changes of Cu55 cluster during heating

The structural relaxation of a cluster containing 55 atoms at elevated temperatures is simulated by molecular dynamics. The interatomic interactions are given by using the embedded atom method （EAM） potential. By decomposing the peaks of the radial distribution functions （RDFs） according to the pair analysis technique, the local structural patterns are identified for this cluster. During increasing temperature, structural changes of different shells determined by atom density profiles result in an abrupt increase in internal energy. The simulations show how local structural changes can strongly cause internal energy to change accordingly.     

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采用分子动力学方法模拟200eV的CH3粒子轰击到不同基底温度的钨样品上,分析了C、H原子在钨表面的沉积、散射及溅射情况,结果表明C、H原子的沉降量均随入射剂量的增加而增加。在基底温度为100K时,相同入射剂量下沉积的C原子最多,而当基底温度为1200K,在入射剂量大于1.5X1016cm-2时,C原子的沉降量小于其它基底温度下的C的沉降量。CH3在轰击样品时发生了分解,各种分解情况随基底温度变化较小,其中不同基底温度下一级分解率在40%上下波动,二级分解在23%左右,而完全分解的CH3在9%左右。C、H原子的散射角主要分布在5°~85°间,散射C原子分布的最大值分布在40%~50°或50°~60°间,散射C原子分布的最小值分布在0°~10°或80°~90°间;而不同基底温度下散射H原子分布的最大值均在40°~50°间,最小值均在0°~10°间。散射C原子的能量在0~140eV之间,散射能量为0~120eV的C原子占散射总量的98%以上,散射C原子平均能量随基底温度的增加而增加,其变化从65.5eV增加到68.5eV;散射H原子的能量也在0~140eV之间,但大约70%的散射H原子能量在40eV以内,散射平均能量随基底温度的增加而减小,其变化从13.92eV减小到13.05eV。     

The strong OHO hydrogen bond. How much covalency?

In the strong OHO hydrogen bond of the phosphoric acid–urea 1?:?1 complex the proton shifts gradually with temperature from the donor towards the acceptor atom, passing through the center of the hydrogen bond at around 315?K. The AIM parameters were evaluated for the published neutron structures at different temperatures. The values of the electron density, its Laplacian, and the energy densities at both the critical points between the proton and the oxygen atoms in the OHO hydrogen bond were correlated with the OH and HO distances. Changes in the AIM parameters of the strong hydrogen bond were compared with those of the weak NHO bond in this complex.     

Simulation of silicon nanoparticles stabilized by hydrogen at high temperatures

The stability of different silicon nanoparticles are investigated at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a “coat” from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen “coat” has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si₆₀ cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si₆₀ cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen “coat” and containing 60 hydrogen atoms in the interior space has a higher stability. Such cluster has smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270-280 K.     

低温下Cu13团簇负载于Cu(001)表面上结构变化的分子动力学研究

应用分子动力学方法研究温度为10和50 K时具有二十面体结构的Cu13团簇以不同接触条件与Cu(001)表面结合后的结构变化,原子间的相互作用势采用Johnson的嵌入原子方法模型.通过基于原子密度分布函数的分析表明,负载团簇与表面的结合能主要受团簇与载体相接触的最低层原子数及这些原子所具有的不同几何构型影响,同时更高层的原子呈现出不同的几何结构.温度为10 K时,负载团簇的初始位置对团簇几何结构和结合能影响较大.     

Photoionization microscopy of hydrogen atom near a metal surface

We have studied the ionization of Rydberg hydrogen atom near a metal surface with a semiclassical analysis of photoionization microscopy. Interference patterns of the electron radial distribution are calculated at different scaled energies above the classical saddle point and at various atom-surface distances. We find that different types of trajectories contribute predominantly to different manifolds in a certain interference pattern. As the scaled energy increases, the structure of the interference pattern evolves smoothly and more types of trajectories emerge. As the atom approaches the metal surface closer, there are more types of trajectories contributing to the interference pattern as well. When the Rydberg atom comes very close to the metal surface or the scaled energy approaches the zero field ionization energy, the potential induced by the metal surface will make atomic system chaotic. The results also show that atoms near a metal surface exhibit similar properties like the atoms in the parallel electric and magnetic fields.