Determination of first ionization potentials from spectra of electronic energy loss in the vapor of polyatomic organic compounds

It is shown that an analysis of energy loss spectra of electrons scattered at θ=90° in the vapor of polyatomic organic molecules allows determination of the first ionization potentials. Thus, for pyrene, perylene, and POPOP molecules they are 7.7, 7.2, and 8.7 eV, respectively. It is found that the first ionization potentials measured depend considerably on the thermal distribution of the free molecules. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 351–355, May–June, 1999.     

Calculation of absolute electron-impact ionization cross-sections of dimers and trimers

We report calculations of the electron-impact ionization cross-sections of selected dimers (homonuclear diatomic molecules) and trimers (homonuclear triatomic molecules) using a method which relies only on macroscopic quantities in conjunction with a “defect concept”. The empirically determined defect describes the deviation of the cluster (dimer, trimer) cross-sections from a simple linear dependence on the cluster size. We compare the calculated cross-sections to experimental data for the dimers S₂ and F₂ and the trimer O₃ and we present predictions for the ionization cross-sections of Br₂, I₂, C₂ and C₃ for which no experimental data are available. Lastly, we extend the method to the calculation of ionization cross-sections for the fullerenes C₆₀ and C₇₀. Received 6 December 1999 and Received in final form 10 April 2000     

电子与高电荷离子碰撞的细致能级电离截面的相对论计算

电子离子碰撞电离的细致能级截面在X射线激光等需要确定能级布居数的高技术研究中有非常重要的意义.本文给出了在全相对论框架下利用扭曲波波恩交换近似计算细致能级电离截面的方法.作为示例,计算了若干高电荷离子细致能级的电离截面,并分析了组态相互作用的影响.计算结果与其它理论值进行了比较,获得了较好的符合.     

Υ三胶子碎裂粒子产率的普适质量关系

发现Υ→3g三胶子碎裂的末态强子多重数满足与e⁺e^－→q₀q₀事例相同的普适质量关系,表明胶子、夸克碎裂成强子时有相同的规律性.不需引入任何其它假定,就自然解释了Υ→3g三胶子碎裂末态重子多重数增高的重要特征.     

高能电子碰撞电离对高空核爆炸辐射电离的影响

利用数值模拟程序模拟了不同高度核爆炸下距离爆点不同位置处大气的X射线电离及演化过程,分析了高能电子碰撞电离过程对高空核爆炸电离效应的影响.结果表明:高能电子碰撞电离过程对高空核爆炸的电离结果产生了重要影响,在80 km爆高1 kt当量高空核爆炸条件下,在距离爆点1.5 km处高能电子的碰撞电离效应使得电子的峰值数密度提高了约2个数量级;随着时间的增加,高能电子能谱逐渐向低能方向发展,在5μs后高能电子数密度随电子能量近似呈负指数分布;电子峰值时间与爆高有关,随着爆高的增加而增大,电离的影响区域也随着爆高的增加而增大.在80 km爆高1 kt当量情况下电离效应对近百千米范围内的微波通信具有严重影响.     

1，3-二甲基尿嘧啶二聚体的飞行时间质谱裂解规律研究

采用不同进样方式、不同电子轰击能量和不同反应气压力测定了1,3 二甲基尿嘧啶二聚体(DMUD) 的4个立体异构体A、B、C、D的化学电离(CI)和电子轰击电离(EI)飞行时间质谱.不同方式所测得的CI谱结 果一致,4个异构体出现了强度不同的准分子离子峰(m/z=281),由此推断它们结构之间的相对稳定性次序为: B(trans syn)>D(cis syn)>A(trans anti)>C(cis anti).这一结论被低能量(25eV)电子轰击的EI谱所证实, 并且与合成产物的比例相吻合.EI谱用任何方式测谱均不出现分子离子峰(m/z=280),而出现其单体离子(m/z =140)且为基峰。给出了DMUD的飞行时间质谱裂解途径,同时对CI谱的裂解碎片进行了细致的讨论.     

Electron impact ionization of large krypton clusters

We show that the detection of ionization of very large van der Waals clusters in a pulsed jet or a beam can be realized by using a fast ion gauge. Rapid positive feedback electron impact ionization and fragmentation processes, which are initially ignited by electron impact ionization of the krypton clusters with the electron current of the ion gauge, result in the appearance of a progressional oscillation-like ion spectrum, or just of a single fast event under critical conditions. Each line in the spectrum represents a correlated explosion or avalanche ionization of the clusters. The phenomena have been analysed qualitatively along with a Rayleigh scattering experiment of the corresponding cluster jet.     

近阈值下He原子的双电子电离实验中出射电子研究

利用适用于低能电子入射的反应显微成像谱仪，对电子入射He原子近阈值下的双电离过程进行了研究，实验测量了反应后3个粒子的全部动量，获得了出射电子间的关联信息.主要介绍近阈值下的双电离实验装置及实验技术，集中分析反应后出射电子的动量能量关系，对描述近阈值双电离的Wannier理论进行了检验，发现在入射电子能量为106eV时，实验结果具有Wannier理论预言的性质.     

能量范围从20keV到34keV的锆元素K壳层电离截面

用20~34keV能量的电子束轰击锆靶,从而测得锆元素的K壳层电离截面。这些数据是国际上首次报道。在实验中采用电子输运双群模型修正了由厚衬底产生的反射电子对计数的影响。同时用蒙特卡罗EGS4程序计算了电子在质量厚度为24.3mg/cm2的锆靶中的平均路径长度。     

能量范围从20keV到34keV的锆元素K壳层电离截面

 用20~34keV能量的电子束轰击锆靶,从而测得锆元素的K壳层电离截面。这些数据是国际上首次报道。在实验中采用电子输运双群模型修正了由厚衬底产生的反射电子对计数的影响。同时用蒙特卡罗EGS4程序计算了电子在质量厚度为24.3mg/cm²的锆靶中的平均路径长度。     

Mutual electron-ion interactions in electron impact ionization of a Ca beam

Summary We evaluate the role of electron-ion mutual interactions as a possible source of systematic errors in an experiment of electron impact ionization and excitation of an effusive Ca beam. By monitoring photons resulting from the impact excitation of a Ca resonant transition, the transmitted electron beam current by a sectored Faraday cup and total ion yields, we extract information about elementary processes responsible for ionization. We show that our diagnosis allows us to monitor the influence of the produced ionization on the elementary processes and on the density profile of the electro beam.     

纳秒激光电离分子团簇产生高价离子实验研究

利用飞行时间质谱仪,研究了功率密度为109~1011 W/cm2,波长为532 nm 的纳秒激光对苯、呋喃、甲醇及碘甲烷分子团簇的激光电离过程。实验观察到了高平动能的高价离子Cq+(q≤3),Oq+(q≤3)和Iq+(q≤4),该过程经历了以“初始的多光子电离引发-逆轫致吸收加热-电子碰撞电离模式”为主的激光团簇作用过程,后期经历了团簇的库仑爆炸过程。实验发现：即使激光能量变化一个量级以上时,主要高价离子的种类及占全部离子产物的比率也没有明显的变化,但是高价离子的初始平动能随激光强度的增大而增加;分子中含有较多个外壳层电子的氧、碘原子更容易电离产生高价离子,而碳离子的价态和强度相对较低。     

纳秒激光电离分子团簇产生高价离子实验研究

 利用飞行时间质谱仪，研究了功率密度为10⁹~10¹¹ W/cm²，波长为532 nm 的纳秒激光对苯、呋喃、甲醇及碘甲烷分子团簇的激光电离过程。实验观察到了高平动能的高价离子C^q+(q≤3)，O^q+(q≤3)和I^q+(q≤4)，该过程经历了以“初始的多光子电离引发-逆轫致吸收加热-电子碰撞电离模式”为主的激光团簇作用过程，后期经历了团簇的库仑爆炸过程。实验发现：即使激光能量变化一个量级以上时，主要高价离子的种类及占全部离子产物的比率也没有明显的变化，但是高价离子的初始平动能随激光强度的增大而增加；分子中含有较多个外壳层电子的氧、碘原子更容易电离产生高价离子，而碳离子的价态和强度相对较低。     

Double ionization of helium by electron-impact: complete pictures of the four-body breakup dynamics

The dynamics of He double ionization by 2 keV electron impact is studied experimentally for a momentum transfer of 0.6 a.u. at excess energies of 10 and 40 eV. Complete sets of fivefold differential cross sections are presented for all electron emission angles in coplanar geometry. Contributions beyond the first Born approximation are identified comparing experimental data with first order convergent close-coupling calculations which are in considerably better agreement with the present experiment than with the earlier measurement of Kheifets et al. [J. Phys. B 32, 5047 (1999)].     

Application of Dynamic Fragmentation Methods in Multimedia Databases: A Review

Fragmentation is a design technique widely used in multimedia databases, because it produces substantial benefits in reducing response times, causing lower execution costs in each operation performed. Multimedia databases include data whose main characteristic is their large size, therefore, database administrators face a challenge of great importance, since they must contemplate the different qualities of non-trivial data. These databases over time undergo changes in their access patterns. Different fragmentation techniques presented in related studies show adequate workflows, however, some do not contemplate changes in access patterns. This paper aims to provide an in-depth review of the literature related to dynamic fragmentation of multimedia databases, to identify the main challenges, technologies employed, types of fragmentation used, and characteristics of the cost model. This review provides valuable information for database administrators by showing essential characteristics to perform proper fragmentation and to improve the performance of fragmentation schemes. The reduction of costs in fragmentation methods is one of the most desired main properties. To fulfill this objective, the works include cost models, covering different qualities. In this analysis, a set of characteristics used in the cost models of each work is presented to facilitate the creation of a new cost model including the most used qualities. In addition, different data sets or reference points used in the testing stage of each work analyzed are presented.     

BEPC共振事例的碎裂特性

发现e⁺e^－→J/ψ→3g三胶子碎裂的末态强子多重数〈n〉满足与e⁺e^－→q₀q₀连续区事例相同的普适质量关系〈n〉=A2S+12I+1exp(–bm2).表明胶子、夸克碎裂成强子时有相同的规律性.同样可以把参数A解释为直生介子、重子多重数〈M〉+〈B〉.虽然对Υ→3g与连续区事例进行比较,末态重子多重数增高2.5倍,但是利用我们对J/ψ→3g共振区的计算研究,没有发现J/ψ→3g事例有类似的特征.     

Triple differential cross-sections for the fast electron/positron impact ionization of hydrogen and helium in the Glauber approximation

Triple differential cross-section for electron and positron impact ionization of hydrogen and helium are calculated by using the Glauber approximation along with post-collision interaction effects which are estimated classically. The present results are compared with the recent absolute data of Ehrhardt, Jung and coworkers for the electron impact case. The positron impact case is found to lead to a larger binary to recoil peak maxima ratio (compared to the electron impact case) which further increases when post-collision interaction effects are included.     

Distorted wave cross sections for electron-impact ionization of the lithium-like ions

Electron-impact ionization cross sections for lithium-like ions have been computed in three variants of the distorted wave approximation. It is found that the inclusion of the electron exchange in the standard form yields the best results, in very good agreement with the experiment and the elaborate calculations of Jakubowicz and Moores.     

Modeling the dissociation and ionization of a sputtered organic molecule

The evolution of an organic molecule after sputtering from a gold surface has been analyzed by classical molecular dynamics and ab initio calculations to gain insight into the ionization and fragmentation processes occurring in SIMS. The calculated ionization potential (6.2 eV) of the tetraphenylnaphthalene (TPN) molecule has been found to be close to the unimolecular dissociation energy (5.4 eV) of the most favorable reaction channel involving the loss of a phenyl ring. On the other hand, our calculations show that the internal energies of sputtered TPN molecules can be significantly larger than 5-6 eV. Therefore, it appears energetically possible to relax such excited molecules via both fragmentation and ionization.We propose to virtually decompose the TPN molecule into its basic fragments. The rationale is that, if the molecule is very excited, then separate parts (e.g. pendant phenyl rings) can interact with each other almost independently. The analysis of the molecular motion after emission shows that the oscillations along the phenyl-naphthalene bond direction, expected to induce the molecule fragmentation by the loss of a phenyl ring, are relatively small (they store only about 0.2 eV). On the other hand, the relative energy stored in the inter-phenyl interactions, modulated by their bending and responsible for ionization according to our hypothesis, oscillates over a range of 6-7 eV and favors ionization.