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金属原子束流源的研制及Li和CO的快电子能量损失谱研究
【作者】 姜维春；
【作者基本信息】 中国科学技术大学 ， 原子分子物理， 2008， 博士
【摘要】 本论文研制了一套金属原子束流源装置,将其成功地应用于快电子能量损失谱仪进行碱金属和碱土金属的快电子碰撞研究。重新设计改造了快电子能量损失谱仪的多道数据采集系统,并利用多线程技术解决了原有采集系统在高计数率条件下会丢失数据的问题。利用快电子能量损失谱方法研究了锂的内壳层激发自电离态和一氧化碳价壳层跃迁的振动态。在锂的自电离态的研究中,观测到两个光学禁戒跃迁,并研究了1s(2s2p~3P)~2P~0与1s(2s2p~1P)~2P~0的广义振子强度比的动量转移依赖特性。在一氧化碳的的研究中,测量了若干价壳层跃迁的Franck-Condon因子的动量转移依赖行为,确定了相关振动态的广义振子强度、光学振子强度和积分截面,并结合已有结果进行了分析比较。在第一章中,介绍了快电子能量损失谱方法及其理论基础。重点介绍了微分截面、积分截面、光学振子强度和广义振子强度的物理概念,以及利用快电子能量损失谱方法测量广义振子强度的理论基础。另外,简单介绍了混合气体法测量广义振子强度的理论依据和具体过程。在第二章中,介绍新研制的一套工作温度可达719℃的金属原子束流源装置。该装置可以获得较高的靶密度,并且得到的锂原子束流纯度高,没有发现可觉察的分子成份。该装置的收集效率达90%以上,污染小,可以连续工作一个月以上不需要中途清洗。同时该装置还具有加热效率高、对周围真空环境的热传导和热辐射小及引线方便等特点。在第三章中,结合快电子能量损失谱仪的需要,将原来技术上比较陈旧的数据获取系统进行了改造,并将原本不能用于扫描测量的商用采集卡DAQ2010,结合定时卡PCI8554,通过硬件和软件方面的设计改造,使其可以满足快电子能损谱对扫描测量的要求。通过采用多线程编程技术对控制软件算法的改进,解决了原有数据获取系统在高计数率条件下会丢计数的问题,提高了采集系统的容错能力。在第四章中,利用新研制的金属原子束流源结合快电子能量损失谱仪,在入射能量为2500 eV的条件下测量了锂原子在散射角度为0°、2°、4°和6°的电子能量损失谱,观测到两个光学禁戒跃迁(1s2s~2)。S和(1s2s~3S)3s~2S。同时还测量了光学允许跃迁1s(2s2p~3P)~2P~0与1s(2s2p~1P)~2P~0的广义振子强度比,并发现该比值不随动量转移的变化而变化。最后,在中间耦合框架下,解释了所观察到的现象。在第五章中,利用角分辨的快电子能量损失谱仪,在入射电子能量为2500 eV的条件下测量了散射角度范围为0.5°-7°的一氧化碳的振动分辨的电子能量损失谱,研究了一氧化碳A~1∏、D~1△、B~1∑~+、C~1∑~+和E~1∏电子态的振动能级的相对强度和广义振子强度。分析了一氧化碳A~1∏、B~1∑~+、C~1∑~+和E~1∏电子态中各振动能级的Franck-Condon因子的动量转移依赖特性。同时首次获得了D~1△的ν′=9-25和A~1∏的ν′=9-11的广义振子强度。而且在确定其它电子态的相对强度和广义振子强度的时候,充分考虑了D~1△的贡献。最后,利用测量的广义振子强度数据得到了A~1∏、B~1∑~+、C~1∑~+和E~1∏电子态的振动能级的光学振子强度和积分截面。作者还将本次实验结果与以前发表的实验结果和理论计算结果进行了比较,并解释了其中的异同。
【Abstract】 In the present work,a resistively heated atomic beam source has been developed to provide vapor target for the fast electron energy loss spectrometer.A data acquisition system for the same spectrometer has been developed,where the multithreading technology is used to improve the accommodating-err ability.Two optically forbidden autoionizing states of(1s2s~2) ~2S and (1s2s ~3S)3s ~2S were observed in the electron energy loss spectra of lithium,and the momentum transfer dependence behaviors of the generalized oscillator strength ratios for 1s(2s2p ~3P)~2P~0 to 1s(2s2p ~1P)~2P~0 were investigated.The relative intensity distributions and generalized oscillator strengths for the valence shell vibronic bands of carbon monoxide were also determined.Then the momentum transfer dependence behaviors of Franck-Condon factors for these vibronic bands were investigated,and the optical oscillator strengths,as well as the integral cross sections of these transitions,were determined.All the present results were compared with the previous ones.In chapter 1,the method of angle-resolved fast-electron-energy-loss spectroscopy were described. Then the basic conception of the optical and generalized oscillator strengths,the differential and integral cross section,as well as the elements for the measurements of the generalized oscillator strengths utilizing electron energy loss spectrometer,were introduced.In addition,the gas mixture method of measuring the generalized oscillator strengths were introduced.In chapter 2,A resistively heated atomic beam source has been developed to provide vapor target of the alkali and alkaline-earth metals.The operating temperature of the oven is as high as 719℃,and the target intensity is enough high for the investigation of the autoionizing states.The contamination of the vacuum system and electron optical system by metal vapor deposition has also been minimized by a black body trap,whose collection efficiency is more than 90%.So the beam source can work continually for more than one month without cleaning.The heat transfer to vacuum chamber,as well as electric and magnetic fields created by the heating currents,were minimized by some special design.In chapter 3,the new data acquisition system for the one dimensional multichannel fast electron energy loss spectrometer,which can work in scan acquisition mode and static acquisition mode,was described.The timing precision of the scan mode is less than 4μs by utilizing the gated signal generated by data acquisition card DAQ2010 and an AND logic circuit.A timer card PCI8554 was used to synchronize the data acquisition card and the personal computer.The scan voltage supply was controlled by the personal computer through the RS232 interface.The multi-threading technology was used in the acquisition software in order to improve the accommodatingerr ability of the acquisition system. In chapter 4,the electron energy loss spectra for the inner shell excitations of atomic lithium were measured at an incident electron energy of 2500 eV and scattering angles of 0°,2°,4°and 6°.Two optically forbidden transitions of(1s2s~2) ~2S and(1s2s ~3S)3s ~2S were observed.The generalized oscillator strength ratios for 1s(2s2p ~3P)~2P~0 to 1s(2s2p ~1P) ~2P~0 were determined, and they are independent of the momentum transfer.The phenomena was elucidated based on the framework of the intermediate coupling.In chapter 5,the relative intensity distributions and generalized oscillator strengths for the vibronic bands of A~1Π,D~1Δ,B~1Σ~+,C~1Σ~+ and E~1Πof carbon monoxide were determined by an angle-resolved electron energy loss spectroscopy at an incident electron energy of 2500 eV and in an angular range of 0°-7°.The momentum transfer dependence behaviors of Franck-Condon factors for the vibronic bands of A~1Π,B~1Σ~+,C~1Σ~+ and E~1Πwere investigated.For the fist time, generalized oscillator strengths for v=9-11 of A~1Πand v=9-25 of D~1Δwere reported and the contribution from D~1Δwere taken into account when determining the relative intensities and generalized oscillator strengths for other transitions.The optical oscillator strengths,as well as the integral cross sections,of the vibronic bands of A~1Π,B~1Σ~+,C~1Σ~+ and E~1Πwere determined from the generalized oscillator strength data.The present experimental results were compared with previous works,and some differences between them have been explained.