高电荷态离子引起的富勒烯离子的碎裂

离子在与富勒烯的相互作用过程中会导致C60分子的激发。处于低激发态的C60r+离子通过发射中性C2分子或带电的轻团簇碎片Cn+等非对称碎裂方式来耗散激发能,但如果激发能很高,笼形的C60r+离子可能会彻底崩溃,而发生多重碎裂。C60r+离子的碎裂过程与其电荷态r及分裂势垒密切相关。低电荷态的C60r+(r≤3)离子蒸发一个C2分子需要克服10.3 eV左右的势垒。随着电荷态的升高,发射带电的Cn+会变得越来越容易,并逐渐过渡到多重碎裂过程。另一方面,C60r+离子的碎裂机制还与激发方式有关,在直接正碰过程中,将C60分子当作固体薄靶来处理,通过分析不同价态的C60r+离子的碎片谱,发现母核的初始电荷态决定碎裂方式,由此获得一个可以表征激发能大小的可观测量——发射电子个数。     

高电荷态离子引起的富勒烯禹子的碎裂

离子在与富勒烯的相互作用过程中会导致C60分子的激发。处于低激发态的C60^r＋离子通过发射中性C2分子或带电的轻团簇碎片Cn^＋等非对称碎裂方式来耗散激发能，但如果激发能很高，笼形的C60^r＋离子可能会彻底崩溃，而发生多重碎裂。C60^r＋离子的碎裂过程与其电荷态r及分裂势垒密切相关。低电荷态的C60^r＋（r≤3）离子蒸发一个C2分子需要克服10．3eV左右的势垒。随着电荷态的升高，发射带电的Cn^＋会变得越来越容易，并逐渐过渡到多重碎裂过程。另一方面，C60^r＋离子的碎裂机制还与激发方式有关，在直接正碰过程中，将C60分子当作固体薄靶来处理，通过分析不同价态的C60^r＋离子的碎片谱，发现母核的初始电荷态决定碎裂方式，由此获得一个可以表征激发能大小的可观测量——发射电子个数。     

纳秒激光诱导C60分子碎裂中轻碎片离子C+n(n《30)的产生机理研究

采用飞行时间质谱计测量了纳秒激光诱导C60分子碎裂中轻碎片离子C n(n≤11)的初始平均动能,结果显示轻碎片离子具有相同的初始平均动能(约为0.34 eV),并且该动能在一定范围内不随激光通量的变化而明显改变.结合前人的实验结果,对纳秒激光诱导C60分子碎裂中轻碎片离子C n(n<30)的主要产生模式作了新的阐述,即C60分子级联发射15个C2分子和一个电子形成自身不稳定的C 30离子,在皮秒时间尺度内C 30离子的笼形结构塌陷,进而轻碎片离子产生.     

激光辐照C60分子中光束发散度对产物分布的影响

利用焦距为300 mm和1500 mm的透镜对纳秒激光束进行聚焦,产生了在空间发散度上具有较大差别的两激光束,采用飞行时间质谱计分别测量了两光束诱导C60分子电离和碎裂时的离子产物分布随激光通量的变化特征.实验结果表明,激光束的空间发散度越大,轻碎片离子Cn,+(n<25)的总产额、C60分子的碎裂程度以及延迟电离产额随激光通量的变化趋势就越快,延迟电离成分占总电离成分的百分比就越高.该实验结果可用多光子吸收的内能分布理论给予合理解释.     

Na离子和Cl离子引起He原子直接多重电离过程的研究

利用2～8 MeV的Naq+、Clq+(q=2,3,4,5)轰击氦原子,对碰撞的直接多重电离过程进行研究.实验采用反冲离子-散射离子飞行时间符合技术,通过反冲离子飞行时间谱区分不同价态反冲离子;利用静电偏转和位置灵敏探测技术区分不同电荷态散射离子;结合CAMAC-PC多参数获取系统得到一定价态散射离子所对应的反冲离子电荷态分布谱;经分析该谱得到直接多重电离截面与直接单电离截面之比R21.讨论了R21随入射离子速度和电荷态的变化关系.     

近Bohr速度129Xeq+离子入射Ta靶发射近红外光谱线

用动能一定的高电荷态¹²⁹Xe^q+(17≤q≤27)离子，分别入射洁净的Ta靶表面，测量中性化的激发态Xe原子从组态5p⁵(²P°_3/2) nl退激到组态5p⁵(²P°_3/2) ml’过程中辐射的近红外光谱线.实验结果表明:多激发态的空心原子退激发射其特征光谱线，部分典型的跃迁按照阶梯方式退激.Xe原子发射的谱线的单粒子荧光产额和激发的Ta原子发射谱线的单粒子荧光产额随入射离子电荷态的增加而增加，其增加的趋势与入射离子携带的势能随电荷态增加的趋势一致.证明在近Bohr速度的能区，经典过垒模型是成立的.     

Ar+和He+离子与C60碰撞后富勒希离子的激发机制

C60在与重离子作用下的激发机制与入射离子能量、质量及电荷态有关.核阻止主要出现在低能重离子与C60的碰撞中;而高能轻离子作用下,电子阻止迅速增强,成为主要的激发方式.本文中直接观察到由弹性碰撞引起的C+峰,及其丰度依赖于入射离子的质量.同时我们还发现电子阻止随入射离子能量(7～20?keV)增大相应增加,这与绝热量子分子动力学计算的结果一致.     

中能Ne~(4+)离子诱导的羰基硫分子三体碎裂动力学分析

利用反应显微成像谱仪开展了56 keV/u的Ne~(4+)离子与羰基硫(OCS)气体的交叉碰撞实验,研究了Ne~(4+)离子诱导的OCS~(3+)的碎裂动力学.通过符合探测三个末态离子,重构了OCS~(3+)离子三体碎裂对应的牛顿图和Dalitz图,并明确区分了直接解离和次序解离两种碎裂过程.重构了OCS~(3+)离子解离过程的动能释放(KER)分布,发现其峰值在25 eV处,同时在18 eV处有肩膀结构的存在,其中25 eV左右的峰来源于直接解离过程,18 eV处的肩膀结构来源于次序解离和非次序解离两种过程.通过分析不同能量和不同电荷态下重离子碰撞实验所得到的KER谱,发现微扰强度不是影响态布居的主要因素.OCS~(3+)次序解离中的第二步KER的峰值在6.2 eV处.结合以往的实验结果,我们得出结论:多电离态的分子发生次序碎裂的根源在于二价离子碎片存在亚稳态,而重构得到的第二步KER可以反映亚稳态离子的电子态信息.     

高电荷态离子Arq+与不同金属靶作用产生的X射线

研究了高电荷态离子Ar^q+(q=16，17，18)入射金属Be，Al，Ni，Mo，Au靶表面产生的X射线谱.实验结果表明，Ar的Kα-X射线是离子在与固体表面相互作用过程中固体表面之下形成空心原子发射的.电子组态1s²的高电荷态Ar¹⁶⁺离子在金属表面中性化过程中，存在的多电子激发过程使Ar¹⁶⁺的K壳层电子激发产生空穴，级联退激发射Ar的Kα 特征X射线.Ar¹⁷⁺离子在金属表面作用过程中产生的X射线谱形与靶材料没有明显的关联,入射离子的Kα-X射线产额与其最初的电子组态有关，靶原子的X射线产额与入射离子的动能有关. 关键词： 高电荷态离子 空心原子 多电子激发 X射线     

高电荷态离子Arq+与不同金属靶作用产生的X射线

研究了高电荷态离子Arq+(q=16,17,18)入射金属Be,Al,Ni,Mo,Au靶表面产生的X射线谱.实验结果表明,Ar的Kα-X射线是离子在与固体表面相互作用过程中固体表面之下形成空心原子发射的.电子组态1s2的高电荷态Ar16+离子在金属表面中性化过程中,存在的多电子激发过程使Ar16+的K壳层电子激发产生空穴,级联退激发射Ar的Kα特征X射线.Ar17+离子在金属表面作用过程中产生的X射线谱形与靶材料没有明显的关联,入射离子的Kα-X射线产额与其最初的电子组态有关,靶原子的X射线产额与入射离子的动能有关.     

Charge localization in collision-induced multiple ionization of van der Waals clusters with highly charged ions

Charge localization in multiple ionization and fragmentation of small argon clusters is reported. The processes are initiated by interaction of the neutral cluster with highly charged Xe(q+) (5< or =q< or =25). Products are detected by means of multicoincidence time-of-flight methods. A strong dependence of the fragmentation pattern on the Xe charge state q is observed. In particular, we find evidence for formation of multiply charged atomic Ar(r+) fragment ions up to r = 7. Such high charge states have neither been observed in fission of multiply charged van der Waals clusters nor in ion-induced fragmentation of fullerenes or metal clusters. This hints at fundamentally different excitation and fragmentation dynamics.     

Effect of collective response on electron capture and excitation in collisions of highly charged ions with fullerenes

Projectile deexcitation Lyman x-ray emission following electron capture and K excitation has been studied in collisions of bare and Li-like sulphur ions (of energy 110 MeV) with fullerenes (C(60)/C(70)) and different gaseous targets. The intensity ratios of different Lyman x-ray lines in collisions with fullerenes are found to be substantially lower than those for the gas targets, both for capture and excitation. This has been explained in terms of a model based on "solidlike" effect, namely, wakefield induced stark mixing of the excited states populated via electron capture or K excitation: a collective phenomenon of plasmon excitation in the fullerenes under the influence of heavy, highly charged ions.     

Atomic collisions involving C60 and collective excitation

Here we review and discuss some of our recent investigations on collective excitation in a free C₆₀ molecule and its influence on the atomic collisions. In particular, emphasis has been given for collisions with fast highly charged ions. It is demonstrated, from the charge-state-dependence studies of recoil-ion spectra, that the plasmon excitation plays a dominant role in the single and double ionization process. The observed linear charge-state-dependence is in contrast to the expected behavior predicted by ion-atom collisions models. This behavior was observed for different projectiles and at different energies. The time-of-flight recoil-ion mass spectroscopy experiments involve 1–5 MeV/u C, O, F and Si ion beams with different charge states, ranging between 4⁺ and 14⁺. In addition, the influence of the collective excitation on the electron capture process was also investigated. The wake-field induced Stark-mixing and splitting of sub-levels of projectile-ions following electron capture from C60 carries signature of the collective plasmon excitation. For the electron capture studies X-ray spectroscopic technique was used for collisions with bare and dressed S and Cl ion beams. The results on the TOF data on fullerene target obtained in last few years will be summarized.     

On the excitation mechanism of the He-Ar hollow cathode laser

Investigations were carried out on the Ar-II 476.5 nm laser to obtain information on possible excitation mechanisms. The He-Ar hollow cathode discharge was excited by square wave current pulses. From the results of measurements it could be concluded that collisions of the second kind between He 2³ S metastable atoms and ground state Ar ions are dominant in the excitation of the upper laser level. This is also supported by results obtained in the case of the He-Kr hollow cathode laser, where the upper level of the Kr-II 469.4 nm laser is excited by a similar collision process.     

单、双电荷离子与原子碰撞中的激发态和发射截面比较

本文给出了单、双电荷离子和He,Ne,Ar,碰撞过程中产生的激发态的实验结果。He^q+,Ar^q+(q=1,2)离子束实验室能量为(70—170)×q keV。光学测量由光学多道分析系统(OMA)完成,波长范围为200—800nm。观察到单、双电荷离子和原子碰撞中各种不同的激发过程,讨论了发射截面和入射离子电荷数、势能亏损的依赖关系。 关键词：     

Decay time characteristics of the U-like excited nuclei

The unified energy dependence of the induced fission times obtained by the crystal blocking technique for heavy nuclei with Z=91–94 in the range of initial excitation energy from 5 to 250 MeV was analyzed. It was demonstrated that, for excitation energies of the investigated heavy fissionable nuclei up to 60–70 MeV, the fission times can be described in the framework of the statistical theory of nuclear reactions taking into account the double-humped structure of the fission barrier and the lifetimes of both classes of excited nuclear states realized in the first and second potential wells. However, for excitation energies above 70 MeV, there is a need to consider the dynamical effects in the fission channel.     

Correlation between the fragmentation modes and light charged particles emission in heavy ion collisions

The correlation between the shape of rapidity distribution of the yield of light charged particles and the fragmentation modes in semi-peripheral collisions for70Zn+70Zn,64Zn+64Zn and64Ni+64Ni at the beam energy of 35 Me V/nucleon is investigated based on Im QMD05 code. Our studies show there is an interplay between the binary, ternary and multi-fragmentation break-up modes.The binary and ternary break-up modes more prefer to emit light charged particles at middle rapidity and give larger values of Rmid yieldcompared with the multi-fragmentation break-up mode does. The reduced rapidity distribution for the normalized yields of p, d, t,3He,4He and6 He and the corresponding values of Rmid yieldcan be used to estimate the probability of multi-fragmentation break-up modes. By comparing to experimental data, our results illustrate that 40% of the collisions events belong to the multifragmentation break-up mode for the reactions we studied.     

Phenomenological analysis of fission induced by high-energy protons

High-energy proton induced fission is studied in the framework of a two-step model. In the first step, the projectile penetrates the target nucleus, knocks out few nucleons and leaves the residual nucleus with a spectrum of excitation energies depending upon the number of projectile-nucleon collisions. This stage is described in terms of a simplified version of Glauber's multiple-scattering theory. The second stage in which the residual nucleus fissions, is treated by assuming phenomenological expressions for the dependence of the fission probability on excitation energy which take into account the onset of fragmentation at a certain “crack” energy. Comparison with experimental data suggests that high energy fission of heavy nuclei proceeds in a way similar to low-energy fission. Light nuclei, however, require a more violent fission mechanism.