Atomic Processes Induced by Electronic Excitation in Solids

这项研究项目的目的是了解电子激发在几种类型的非金属固体中诱导的基本原子过程。主要的研究方向有两个：一是对激子自陷和激子弛豫引起的缺陷形成过程有一个完整的认识，并将在碱卤化物中得到的概念应用于其他材料，包括无定形二氧化硅，以及由密集电子激发引起的体相和表面过程。研制了飞秒级联激发光谱、激发态喇曼光谱和激光辐射中性粒子共振电离光谱等新的实验装置。主要研究结果如下：1)利用飞秒时间分辨光谱研究了NaC l、KI和KbR最低激发态绝热势能面上的动态去激发过程。对于Na…更多的飞秒激光脉冲被用来将自陷激发的最低态激发到下一个更高的激发态。这种级联激发技术可用于研究碱金属卤化物中自陷态激子在最低绝热势能面处的退激过程。新发现了双势能面中的特定相干振荡。最低的绝热势能面具有声子频率较小的多势垒结构。2)共振拉曼光谱的实验结果表明，自陷态激子是由一个F中心和一个H中心组成的复合体。使用了Kan‘no等人目前的观察结果。由于碱卤化物中的激子可以弛豫成几种结构，我们提出了碱卤化物中几种类型激子弛豫组态的新模型。3)检验了碱卤化物的概念是否适用于其他固体，如氟化物和二氧化硅，发现这些材料不仅在激子弛豫过程中表现出相似的行为，而且在激子弛豫过程中缺陷的演化也是相似的。特别是，对于非晶态二氧化硅的结果清楚地表明，Mott和Street提出的发光模型在这种材料中不成立。4)在自陷激子附近创建另一个激子可以在单一激发不导致缺陷形成的情况下导致缺陷的形成。这种高密度效应发生在NaC l，MgF_2，CaF_2中，并通过两个空穴的局域化形成可变形晶格来解释。5)利用激光辐照GaP和GaAs(110)表面发射的高灵敏度Ga原子的测量，发现了Ga原子的缺陷激发发射。某些类型的缺陷的发射对于表面态的激发是最有效的，并且产额与注量的关系可以用幂函数来拟合，根据缺陷的类型，幂函数的范围从1到6。较高的功率归因于多空穴局域化发射的必要性。总之，本研究的结果揭示了碱金属卤化物中激子弛豫的静态和动态性质，以及包括无定形SiO_2在内的几种固体中激子弛豫过程的相似性。此外，多空穴局域化可以导致系统结构的剧烈变化，其中空穴局域化不会造成任何影响。较少

The purpose of this research project is to understand basic atomic processes induced by electronic excitation in several types of non-metallic solids. Two major directions of research are taken : one to reach complete understanding of the processes of the self-trapping of excitons and of defect formation arising from exciton relaxation and to apply the concept obtained in alkali halides to other materials including amorphous silica and to the processes in the bulk and on surfaces induced by dense electronic excitation. Several new experimental apparatus are developed : femtosecond cascade excitation spectroscopy, Raman spectroscopy for excited states and resonance ionization spectroscopy of neutral particles emitted by laser irradiation.Major results of the studies are summarized as follows.1) The dynamical de-excitation processes on the adiabatic potential energy surfaces of the lowest excited state of NaCl, KI and KBr were examined using femtosecond time-resolved spectroscopy. For Na … More Cl, femtosecond laser pulses were used to excite the lowest state of the self-trapped excited to next higher excited states. This cascade excitation technique turns out to be useful to examine the de-excitation processes at the lowest adiabatic potential energy surface of the self-trapped excitons in alkali halides. Specific coherent oscillation in a double-well potential energy surface is newly found. It is found also that the lowest adiabatic potential energy surface has a multi-well structure with small phonon frequencies.2) The experimental results of resonant Raman spectra revealed that the self-trapped excitons is a composite comprising an F center and an H center. Using the results of current observation by Kan'no et al. that the excitons in alkali halides can relax into several structures, we suggested a new model of several types of relaxed configurations of excitons in alkali halides.3) It has been examined whether the concept obtained for alkali halides can be used in other solids, such as fluorides and silicon dioxide and these materials are found to behave similarly not only for relaxation of exciton but also for evolution of defects in the process of exciton relaxation. In particular the results for amorphous silicon dioxide indicate clearly that the model for the luminescence suggested by Mott and Street does not hold in this material.4) Creation of another exciton in the proximity of a self-trapped exciton was shown to result in formation of defects in a condition where single excitation does not lead to defect formation. This high density effects are found to occur in NaCl, MgF_2, CaF_2 and are interpreted in terms of formation of deformable lattice by localization of two holes.5) Using high-sensitivity measurements of Ga atoms emitted from GaP and GaAs (110) surfaces by laser irradiation, defectinitiated emission of Ga atoms is found. The emission from some types of defects is found to be most efficient for excitation at the surface states and that the yield vs. fluence relations can be fitted to power functions with powers ranging from 1 to 6, depending, on the type of defects. The higher power is attributed to the necessity of multi-hole localization for emission to be induced.In conclusion the results of the present investigation have revealed details of static and dynamic properties of exciton relaxation in alkali halides and similarity of the processes in several solids including amorphous SiO_2. Furthermore localization of multi-holes can cause drastic structural changes in the system where localization of a hole cannot cause any effects. Less