【作者】 周晓林；

【导师】 杨向东；

【作者基本信息】 四川大学 ， 原子与分子物理， 2004， 博士

【摘要】 本文由两部分组成:第一部分是关于惰性气体原子与氢分子碰撞转动激发同位素效应的理论研究和计算;第二部分采用密度泛函理论研究惰性气体原子吸附在石墨表面上的电子结构和特征。以下就两部分内容分别进行摘要。原子和双原子分子的转动激发碰撞在理论研究和实际应用中都具有重要意义,它在一些实过程,如星际特质的冷却、气体激光行为预测、气体碰撞的共振荧光、气象弛豫等过程中起着重要作用。而且,它还为通常分子间转动激发碰撞过程提供了一个简单模型。惰性气体原子与氢分子碰撞,作为最简单的原子与双原子分子碰撞系统之一而倍受人们的重视。由于氢有三种同位素,因此在实际问题中就必须考虑同位素替代的影响。然而,关于这方面的研究及数据至今报道很少。本文第一部分将对惰性气体原子与氢的非对称同位素替代分子碰撞体系的转动激发截面进行系统的理论计算和分析研究。在计算中,对氢分子及其同们素替代分子的内部相互作用用,我们采用了半解析形式的Murrell-sorbie势函数,并由此势函数计算了同位素替代分子HD、HT、DT的振转能级,对惰性气体原子与氢分子之间的相互作势,本文采用Tang-Toennides等人提出的原子与双原子分子碰撞相互作用势模型。对惰性气体原子与氢的非对称同位素替代分子碰撞系统,在Born-Oppenhdimer近似下,同位素替代不影响体系的相互作用势,但它又能导致双原子分子的质心产生一个位移δ。对于对称同位素替代体系,不仅势能面不变,双原子分子的质心也没有因为同位素替代而发生移动,故其在质心坐标系中的势能函数保持不变。而对非对称同位素替代体系,虽然其势能面未变,但双原子<WP=3>分子的质心已发生了偏移量为δ的位移,故不能直接运用T-T势函数表达式。对此,我们通过适当的坐标变换,重新构造了非对称同位素替代体系在其质心坐标中的势能函数。在此基础上,本文采用公认精确度较高的密耦(Close-Coupling)近似方法,计算了上述碰撞体系的转动激发截面。对每一个碰撞体系,我们都分别计算了入射原子能量为0.05eV、0.10eV、0.15eV、0.20eV、0.25eV时的分波散射截面和微分散射截面。通过分析和讨论以上各截面值,我们归纳和总结出He、Ne、Ar、Kr、Xe-HD、HT、DT碰撞体系的散射截面的变化规律,并详细讨论了非对称同位素替代碰撞体系中体系的约化质量和入射原子能量变化对散射截面的影响,以及氢的非对称同位素替代分子的质心偏移对散射截面的影响。最后,找出这些影响的规律性。在论文的第二部分,我们用密度泛函理论研究惰性气体原子吸附在石墨单层表面的电子结构和特征。自1981年G.Binnig和H. Rohrer发明了表面测试分析仪器——扫描遂道显微镜(Scanning tunneling microscopy简称STM)后,人类才第一次能够实时地观察单个原子或分子在物质表面的排列状态。这种分析仪器在研究与材料表面的电子行为有关的物理和化学性质方面起着非常重要的作用,在原子与分子物理、表面科学、材料科学、生命科学等领域中有着重大的意义和广阔的应用前景。近十多年来,吸附现象引起了人们极大的兴趣,研究碳素材料及其吸附气相小分子(包括惰性气体原子,H2,O2,NH3,CH4等)后所形成系统的电子结构和特性更具重要意义。本文仅对惰性气体原子(Ar、Kr、Ke)吸附在石墨表面上的情形进行理论上的探索。众所周知,惰性气体原子具有简单的闭壳层结构,与被吸附表面的相互作用也较弱。因此,惰性气体原子作为被吸附物具有很长的历史。目前,描述材料表面系统的STM图像较为成功的理论是由J.Tersoff和D.R.Hamann提出的(1983年)。该理论认为,对于实验采用的小偏电压和小电流,研究系统在探针位置形成的STM图像与隧道电流密度,亦即与费米(Fermi)能级处电子的局域状态密度(Local Density of State,简称LDOS)有密切关系。根据Tersoff-Hamann定理可知,在STM实验中能否观察到被吸附的原子或分子,依赖于其改善STM探针位置吸附系统的Fermi能级附近的局域态密度的程度。然而,惰性气体原子吸附在金属或石墨表面上时,不可能改善STM探针位置Fermi能级附近的局域态密度,其原因是惰性气体原子的电子充满轨道的最<WP=4>高能级远低于Fermi能级,而空轨道的最低能级远高于Fermi能级。因此,从吸附系统的电子能带结构来看,很奇怪能在其STM实验中观察到惰性气体原子的STM图像。Eigler(1991年)、Matsui(1996年)等人用不同的方法解释了惰性气体原子吸附在金属表面的STM图像,认为惰性气体原子的空态电子轨道的共振是Fermi能级附近的电子局域态密度产生的主要原因。本文首次利用密度泛函理论框架下的局域密度近似对惰性气体原子吸附在石墨表面的STM图像作详细的研究。在计算过程中,我们采用层模型来模拟石墨单层表面。在层模型中,我们使石墨单层表面之间相距为1.8nm的间隔,以避免各石墨表面之间的相互作用。碳原子电子之间的相互作用采用由N.Troulier和J.L.Martins提出的模守恒赝势来描述,电子之间的交换关联能使用J.P.Perdew和A.Zunger提出的用于拟合均匀电子气的Monte Carlo结果的一个函数形式。在构造Troulier-Martins模守恒赝势时,碳原子2s,2p和3d电子态的芯半径分别取1.50a.u.,1.54a.u.和1.54a.u.,Ar原子3s,3p,3d电子态的芯半径分别取1.64a.u.,1.68a.u.更多还原

【Abstract】 There are two parts in this dissertation. In part one, theoretical calculations and systematical studies on effect of isotopic substitution for rotational excitation in He、Ne、Ar、Kr、Xe—HD、HT、DT collisions are presented; In part two, The Scanning-tunnelling-microscopy(STM)images of inertia gas atoms adsorbed on a manslayer graphite sheet are calculated using the first-principle total-energy clectronic structure calculations.Rotationally excitational scattering of an atom by a diatomic molecule is of both theoretical and practical importance. It plays a very important role in practical processes such as the cooling of interstellar matter, predicating of laser action, resonance fluorescence and gas phase relaxation ets..Moreover, it can serve as a prototype for general molecular collisions. Collisions between inertia atoms and a hydrogen molecule is one of the simplest molecular scattering systems and have received much attention. Since hydrogen has three isotopes. Effect of isotopic substitution must be considered in practical problems. However, little has been reported in this field few data are available. In the paper, theoretical calculations and systematical studies of the cross sections for rotational excitation in He-H2,D2,T2,HD,HT,DT collisions are presented. In calculation, we employ the Murrell-Sorbie potential to calculate the intermolecular potential of hydrogen molecule and isotopic substitution systems. Moreover and calculate the vibration and rotational energy. For inertia atoin and hydrogen molecular scattering system. We employ Tang-Toennies model potential. Within the Born-Oppenheimer approximation, isotopic substitution does not change the intermolecular potential However it may cause the c.m. of the diatom to shift a distance δ. For systems of symmetric isotopic substitution, the c.m. of hydrogen molecule remains unshifted, and so the symmetric isotopic substitution does not change the intermolecular potential. For asymmetric isotopic substitution systems, the potential surface is till the same as that of He-H2, however, the c.m. of the hydrogen molecule shifts a distance ofδ, <WP=7>and since it is more convenient to treat this kind of scattering problem in a c.m. coordinate system, the potential must be reconstructed in the new coordinate. Furthermore, the differential cross sections(DCS), the partial wave cross sections(PCS)and the rotational excitation cross sections(RECS)for the collision of inertia gas atoms He、Ne、Ar、Kr、Xe with asymmetric isotopic substitution molecule HD、HT、DT have been calculated by using the close- coupling approximation method at incidence energy of 0.05eVto0.25eV,respectively. By analyzing the differentiae of the total cross sections(TCS), the differential cross sections(DCS), the partial wave cross sections(PCS), the change patterns of the cross sections and the influence on the cross sections(PCS), the change patterns of the cross sections and the influence on the cross sections because of the variations in the mass of systems and the relative kinetic energy of incoming atoms for symmetric isotopically substituted systems He、Ne、Ar、Kr、Xe—HD、HT、DT have been obtained.In part two of this paper, the scanning-tunnelling-microscopy (STM)images of inertia atoms adsorbed on a monolayer graphite sheet are calculated using the first-principle calculations.Scanning tunneling microscopes(STM) have become a powerful tool in atomic and molecular physics, surface science et al to imaging of various adsorbate atoms and molecules on atomically flast surfaces since the invention(1981). In the constant tunneling current mode, the STM image is given by recording the dip-to-surfact distance changes. In the current-imaging mode, however, an STM image is given by recording the tunnelling current as the tip rapidly scans the surface at a constant distance. In most cases, STM images can be associated with the topography of the electronic states of materials, such as Si(111)-7×7. In some cases, STM images show the electronic structure efforts and form different patterns更多还原