【作者】 宋远红；

【导师】 王友年；

【作者基本信息】 大连理工大学 ， 等离子体物理， 2004， 博士

【摘要】 载能离子与固体材料相互作用一直是非常重要的研究领域。特别是,关于离子在固体表面散射及相关问题的研究已成为这一领域新的热点,它不仅揭示了一些重要的科学问题,同时也与离子束表面改性、表面结构分析、原子溅射沉积薄膜等技术的应用紧密相关。近些年,加速器技术的发展,产生分子离子和大离子团已成为可能,为研究不同种类的离子及离子团与固体材料的相互作用过程提供了有利条件,有力地促进了离子束技术在材料科学和惯性约束聚变等领域中的应用。本文所要研究的重点是载能离子在固体表面掠角散射的过程,即离子的入射角度很小,只能在固体表面层电子气中散射。当离子以掠角接近固体表面时,表面电子气将受到激发而产生感应电势(电场),离子被这种长程感应电场所吸引。同时,它又要受到表面第一层原子的散射。研究离子的散射轨迹和能量损失,可以同时获得离子和固体表面的相关信息。本文根据介电响应理论以及镜反射模型,建立了离子在固体表面掠角散射的理论模型,并首先对氢离子与固体表面的相互作用过程进行了研究。在低速及高速近似下,分别采用局域场修正(Local-field corrections,LFC)介电函数及仅与频率有关的局域介电函数,数值计算得到了氢离子在固体表面掠角散射轨迹及能量损失。将计算结果与相关的实验结果作了比较,发现入射速度很低时,离子的能量损失随入射角度的变化略有下降,而当速度很高时能量损失随入射角度的变大而有缓慢增加。基于氢离子的研究结果,本文进一步研究了重离子与固体表面的相互作用过程。为了考虑重离子核外电荷态分布,首次提出了面电荷分布的概念,对Brandt-Kitagawa的有效电荷理论模型进行了修正,保证了离子在固体表面产生电势的连续性。考虑到低速和高速离子在表面的电荷交换过程不同,基于具体的实验结果和与速度有关的电子剥离模型,提出了双指数模型和线性插值模型分别表示低速和高速离子的电离度演化过程,并分别采用了LFC和等离子极点近似(plasmon-pole approximation,PLA)介电函数,得到了与位置有关的表面自能和阻止本领的解析表示式,对重离子在固体表面的掠角散射轨迹和能量损失进行了计算,并和相应的实验结果进行了比较。计算结果表明:低速重离子的能量损失随初始电荷态的升高而缓慢上升,却随入射角度的增加而缓慢下降,离子的电荷态和入射角度对离子的掠角散射轨迹和能量损失都有影响。另一方面,对于高速离子,由于入射角变大而使它进入到表面电子气较深,比低速离子更接近于表面第一层原子核,内壳层电子激发对阻止本领产生贡献,导致入射角度接近临界角度时,能量损失有缓慢增加。特别是,LFC介电函数考虑了固体表面电子气的关联交换相互作用,突破了随机相位近似(random phase approximation,RPA)介电响应理论的极限,对低速离子在固体表面电子气中的扰动进行了很好的描述。大连理工大学博士学位论文 利用重离子在表面掠角散射时产生的动力学势,并采用量子力学的一阶含时微扰理论,首次建立了研究重离子掠角散射过程中表面电子发射的理论模型,其中固体中的电子状态由有限方势垒下的薛定愕方程获得。计算结果表明,表面等离子激发和衰变是高速离子掠角散射所引发的电子激发和发射的主要原因。而对低速离子,单电子激发是电子发射的主要机制。本文所给出的动力学电子发射的理论模型中,确实存在一个动力学阐值,当离子速度小于临界速度时,等离子激发不会发生。 最后,本文首次建立了重分子离子在固体表面掠角散射的物理模型,给出了分子离子中单个离子之间的动力学相互作用势、相互作用力和屏蔽库仑力以及分子离子的阻止本领的解析表示式;模拟了分子离子的散射轨迹和库仑爆炸过程,并计算了散射过程中的能量损失,研究了散射过程中离子电荷态的变化和离子之间的邻近效应对它们的影响;考虑了分子轴在散射过程中的不同取向对分子离子掠角散射的影响。结果发现,与入射角度和初始电荷态分布的影响相比,分子离子的入射速度对其掠角散射过程的影响较大。当离子速度较高时,分子离子的能量损失大于两个单个离子的能量损失之和;而当速度较低时,出现了负分子效应。而且还发现,分子轴的取向与离子速度方向一致时也会出现负分子效应。由此验证了,与分子离子在固体中的运动类似,分子离子在固体表面掠角散射时也有负的邻近效应。而且,这种负分子效应使电子发射能谱谱线向低能区移动。 关键词:掠角散射,能量损失,电子发射,介电响应理论,镜反射模型,邻近效应,库仑爆炸更多还原

【Abstract】 The interaction of energetic ions with solid material has been a very important research field in the past few dedades. Especially, the studies of the ion-surface scattering and the aspects related to it have received much attention. Research in this field not only focuses on some fundamental problems, but also has resulted in astonishing progress in the applications, such as surface modification, surface structure analysis and the thin solid film deposited by the atom sputtering. In the recent few years, the development of accelerator technology has opened up the possibility of producing molecular ions and ion clusters in laboratories, has allowed us to probe the different dynamic of particle-matter interaction, and has promoted the utility of ion beam technology in the applications of material science and the inertial confined fusion. In this work, the grazing scattering process of the energetic ions on solid surface has been investigated. In this extreme geometry, as an energetic ion approaches solid surface under a grazing angle, it will be first attracted by a long-distance induced electric field due to the excitations of the electron gas and subsequently reflected by a short-ranged repulsive force of topmost atoms located in the first surface layer. By studying the scattering trajectory and the energy loss of the ion, one can obtain the corresponding information both about the ions and the solid surface.Based on the dielectric response theory and the specular reflection model, a theoretical model is given to describe the grazing scattering of ions incident on a solid surface, and the interaction of H+ ions and the surface is calculated. A local-field correction (LFC) dielectric function and a local frequency-dependent dielectric function with a damping factor have been adopted respectively, in low- and high-velocity regimes. The scattering trajectories and the energy losses are obtained and compared well with the experimental results. It is observed that the energy losses decrease slowly with the increasing incident angles for low velocity ions, whereas swift ions lose more energy with the increase of the incident angles.After the simulation of H+ ions, a further investigation has been taken for heavy ions glancing scattering from solid surface. Considering the distribution of the electron bound to the projectiles, as a correction of the Brandt-Kitagawa model, a planar charge distribution is first put forward which is a guarantee of the continuity of the potential induced by the ion. As for the different charge exchange processes near the surface for slow and swift projectiles, a double exponent model and a linear interpolation combined with a velocity-dependent electron-stripping model are adopted in the simulations, together with the plasmon pole approximation (PLA) and LFC dielectric functions are used. Thus, the analytical expressions for the position-dependent stopping power and the surface image potential have been obtained. The projectile trajectories and the energy losses, dependent on the charge state of the ions and the incident angle, are calculated and compared with the corresponding experimental results. The calculated results demonstrate that: in the low velocity regime, the energy loss exhibits a monotonic increase with the increasing charge state, and shows a slight decrease with the increasing incident angles; on the other hand, high velocity ions mayapproach the topmost layer much closer than slow ions as the incident angles increase so that the contribution of the inner-shell electrons to the stopping power should be included giving rise to a small increase of the energy loss for the incident angles which approach to the critical angle. Moreover, including the exchange-correlation interaction among the electrons, the adoption of the LFC dielectric function has done a good job in describing the perturbation given by the slow projectiles and breaks through the limitation of RPA (random phase approximation) dielectric theory.Using the dynamic interaction potential given by the study of heavy ion更多还原