二氧化碳分子的电子贴附解离

本文综述了我们在二氧化碳分子电子贴附解离动力学实验研究方面的进展. 首先,介绍电子贴附解离基本概念并列举二氧化碳研究已有的结果. 其次,研制相关仪器,特别是近两年来发展的高分辨装置. 再次,利用上述装置,我们系统开展了二氧化碳电子贴附解离动力学实验研究,并与他人的研究进行了分析比较. 最后,对火星原地制氧和二氧化碳催化转化等方面的潜在应用进行讨论并展望.     

BrCN分子的电子碰撞离子对解离

离子对解离是一类重要的分子过程,常发生于分子被激发到超激发态. 与光激发的离子对解离实验研究不同,电子碰撞的相关过程研究尚存在实验挑战,特别是在测定其阈值方面. 本文报道了相关的利用单色化电子碰撞分子的实验研究进展. 以BrCN→Br^-+CN⁺离子对解离为例,根据CN⁺离子出现能测定其解离阈值为13.78 eV,同时在16.09 eV获得了CN⁺离子的时间切片速度影像且显示出动量分布的各向异性.     

低能电子直线加速器的应用

本文着重介绍了能量在数十ＭｅＶ 以下的低能电子直线加速器在核物理以外领域的应用，而加速器本身仅在论述应用特点所必须时作简要说明．     

低能电子径迹中能量沉积频率分布

根据我们建立的径迹结构Monte Carlo计算模型,计算了低能电子在液态水中径迹范围内DNA分子片段模型和染色质丝片段模型区域内的能量沉积频率分布,结果与OREC和CPA程序结果进行了比较,这些计算结果为解释低能重离子注入生物体造成深部生物效应而提出的一种物理机制──软X射线机制提供了理论依据.     

低能电子与氮分子碰撞振动激发动量迁移截面的研究

采用作者改进的振动密耦合方法和基于量子力学从头计算的静电势、交换势、相关极化势,研究了低能电子与N₂振动激发散射动量迁移截面. 计算结果与试验符合较好. 关键词： 动量迁移截面 低能电子 分子碰撞 振动激发     

D_2~+强场解离的电子局域化随激光波长的非线性变化

研究了周期量级激光脉冲的波长变化对氘分子离子D_2~+强场解离过程中电子局域化的影响.通过求解波恩,奥本海默近似下关于核波包演化的双能级含时薛定谔方程,发现电子局域化的不对称性对激光波长有反常的依赖关系.电子局域程度随着波长的增加呈现增强的趋势,但在某些波长范围内电子局域化出现了显著的衰减.导致电子局域化程度被削弱的直接原因是在某些波长下不同振动态的电子局域化对脉冲载波包络相位的响应出现了反相抵消.分析表明,当波长发生变化时,决定电子局域化的核运动和电子运动在外场作用下出现不一致的运动响应,最终导致了电子局域化的非线性变化.     

低能电子冲击下,混合有机分子薄膜中苯对正己烷离解的作用

本文报导了苯与正已烷二元有机混合吸附薄膜在低能电子(<50eV)轰击下正已烷的原位离解,研究了苯的加入对正已烷离解的影响及作用机制。发现苯增加了正已烷的离解,这种现象由苯与正已烷分子间的激发转移予以解释。     

低能电子在水介质中的输运过程

考虑到低能电子(能量下限为1eV)在水中输运时的电离,激发,俘获以及超激发引起的自电离等非弹性散射机制,并且考虑到OH⁺,H⁺等自由基的产生和分布,运用Monte Carlo方法模拟了电子在水中输运的径迹结构,揭示了电子在低能情况下输运时单条轨道的空间分布结构特点(云团,团点和短径迹等空间分布实体)和包含在大量轨道中的径迹结构的统计性质,并通过对比截止能量取30和1eV两种情况,分析了30eV以下的低能电子的作用.     

低能电子在水中的弹性散射

基于平均散射截面概念,结合Mott截面,计算E≤10 keV范围低能电子在水中弹性散射的微分散射截面、总散射截面及弹性散射角概率分布,与应用Rutherford模型的计算结果及实验结果符合较好.在此基础上,给出一个低能电子在水中弹性散射的模拟方法,既严格可靠,又能够方便地应用于低能电子在水中径迹结构的模拟.     

Effects of solvation on dissociative electron attachment to methyl iodide clusters

Using laser photoelectron attachment to methyl iodide clusters in a differentially-pumped seeded supersonic helium beam and mass spectrometric ion detection, we have measured the rate coefficients for formation of (q = 0-2) ions over the electron energy range 0-100 meV with an effective energy width of about 2.5 meV. Whereas a prominent vibrational Feshbach resonance just below the onset for the C-I stretch vibration ( ) is observed for dissociative attachment to monomers (yielding I^- ions), only weak and broad structure, shifted to lower energies, is detected for formation of ions and essentially no structure is left in the attachment spectrum for . These observations are interpreted by model R-matrix calculations which successfully describe the DA cross-section for the monomer and qualitatively recover the trend observed for cluster ion formation. For the clusters, the effects of increased electron-target long-range interaction and of solvation as well as coupling to soft vibrational modes lead to strong broadening and shifting of the vibrational Feshbach resonance and, ultimately, to its disappearance. Received 29 November 1999 and Received in final form 14 January 2000     

利用基于荧光屏探测器的质谱方法研究邻位二氯苯的电子贴附解离 (cited: 1)

在自制的负离子速度成像装置上发展了负离子质谱技术．测量获得了电子贴附解离邻位二氯苯产物Cl^-的0.2～8 eV效率谱,并且在两个峰位1.2和6.0 eV测量了其切片速度影像．     

Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters

Using a high resolution ( meV) laser photoelectron attachment method, we have studied the formation of (CO ₂) _q ⁻ ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO₂) _N () clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists of several narrow vibrational Feshbach resonances of the type [(CO ₂) _{N −1}CO which involve a vibrationally-excited molecular constituent ( denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at energies below those of the vibrational excitation energies of the neutral clusters [(CO ₂) _{N −1}CO ]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies. The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster. Received 11 January 2000 and Received in final form 10 April 2000     

An X-band time domain electron paramagnetic resonance spectrometer

A computer-controlled X-band time domain electron paramagnetic resonance (EPR) spectrometer, with a time resolution of the order of 0.5μsec, has been constructed with many of the crucial microwave components designed and fabricated by the Microwave Engineering Group of TIFR. The spectrometer operates either in a microwave power pulsed mode for determination of spin-lattice relaxation times by the saturation recovery technique or in the kinetic mode for determination of the time dependence of EPR signal after laser excitation. It has an automatic frequency control, an automatic phase control and, most importantly, a field-frequency lock which ensures good stability of the EPR line positions enabling signal averaging for extended periods. The constructional details of the spectrometer and its performance in both the modes are described here by reporting results on certain typical systems.     

Low energy electron attachment to formic acid

Using a high resolution electron energy monochromator low energy electron attachment to formic acid is studied for the first time by means of mass spectrometric detection of the product anions. The largest dissociative electron attachment (DA) cross-section produces HCOO ^-+H with weaker channels for OH^- and O^- becoming apparent at higher incident energies. Received 23 January 2002 and Received in final form 9 February 2002 Published online 13 September 2002     

Dissociative attachment of low-energy electrons to vibrationally excited molecules using a photoelectron source

The process of dissociative attachment (DA) of low-energy electrons ) to vibrationally excited sodium dimer molecules is studied with high electron energy resolution () in a supersonic molecular beam. A novel photoelectron source, based on two-step photoionization of the sodium atoms in the beam, may deliver a current of up to 1 nA and has been used with a current of typically 0.2 nA in this experiment. The energy dependence of the rate of sodium anion formation is determined by ion detection based on a time-of-flight analysis. The molecules are selectively excited to levels using the technique of coherent population transfer by delayed pulses (STIRAP). The comparison of the experimental data with recent resonance model calculations based on improved potential curves reveals generally good agreement for levels v ”>12. For some distinct differences between theoretical and experimental results persist. Received: 21 November 1998 / Received in final form: 7 April 1999     

The role of dissociative electron attachment to metal halides in a low-pressure glow discharge

The conditions are analyzed under which the dissociative attachment of electrons determines the properties of a nonequilibrium glow discharge plasma containing metal halides. The possibility of applying a simplified approach for the estimation of dissociative attachment and vibrational excitation cross sections of metal halides by combining quantum and semiclassical methods is discussed. By way of example, processes in a nonequilibrium Ar-SnI₂ plasma are analyzed in detail.     

Ionisation energy,electron affinity,and mass spectral decomposition mechanisms of RDX isomers upon electron attachment and electron ionisation

ABSTRACT

The structures, ionisation energies (IE), and electron affinities (EA) of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) isomers upon loss and gain of an electron were calculated using density functional theory (DFT) methods. The adiabatic electron affinities (EA_ad) range from 1 to 2 eV. The vertical detachment energies are between 1.3 and 4.0 eV. The adiabatic ionisation energies (IE_ad) are in the 9.9–10.2 eV range. The vertical ionisation energies are in the 10.4–10.9 eV range. It is shown that NO₂^?/NO₂ loss would be common in anions and cations, respectively. Isomerisation and N—N bond dissociation accompany cation and anion formation, respectively. The suggested mass spectral fragmentation products for the cations along the S₀ surface are 84, 130, and 176 amu, in agreement with earlier mass spectrometry studies.     

Probing the orientation of porphyrin oligomers in a liquid crystal solvent – a triplet state electron paramagnetic resonance study

ABSTRACT

Linear porphyrin oligomers have found various applications as synthetic molecular wires in the context of light harvesting, solar energy conversion and molecular electronics. In many of these applications a partial ordering of the molecules helps to improve the reaction efficiency or device performance. In this work we study the orientational properties of the building blocks of such porphyrin-based molecular wires, namely a porphyrin monomer and the corresponding butadiyne-bridged dimer. The porphyrins have been embedded in the nematic liquid crystal solvent 4-cyano-4'-pentylbiphenyl (5CB) and the anisotropic properties of their photogenerated triplet states were characterised by transient electron paramagnetic resonance (EPR) spectroscopy. When aligned in strong magnetic fields, the liquid crystal molecules impose their orientational anisotropy onto the solute guest molecules whose orientation-dependent magnetic properties can then be explored. The line shape analysis of the porphyrin triplet state EPR spectra – highly sensitive to small conformational changes – confirms the orientation of the zero-field-splitting (ZFS) tensors previously determined for these molecules by magnetophotoselection experiments. A biaxial distribution function is shown to be necessary to simulate the experimental EPR data. The biaxial behaviour, in conjunction with symmetry considerations, allows an unambiguous assignment of the three ZFS tensor axes to the molecular axes. From the determined orientational distributions of the porphyrins in 5CB, the biaxial order parameters for both molecules were calculated.