Electron-impact induced fragmentation of fullerene ions

The C2 fragmentation of fullerene ions C(q+)(60) (q = 1,2,3) induced by electron impact was studied for the first time. The cross sections for the loss of a C2 fragment indicate the presence of two different processes. At low electron energies the projectile electron leads to the direct excitation of the giant plasmon resonance. At electron energies larger than 100 eV the fragmentation of the fullerene ions can be described as an unsuccessful ionization. Only this second part of the cross section shows a dependence on the charge state q of the precursor ion.     

Ionisation and fragmentation of tetraphenyl iron (III) porphyrin chloride induced by slow multiply charged ion impact

Ionisation and ion-induced fragmentation of tetraphenyl iron (III) porphyrin chloride (FeTPPCl) molecules have been studied after slow collisions (v∼ 0.2 a.u.) with multiply charged ions (O³⁺, Ar⁸⁺). Intact molecules and large fragments are observed in charge states up to q=4. For q=1, the intact molecule is the most abundant species, in particular, when projectiles in higher charge states are used. When the internal energy of the singly charged ion is increased by the energy transfer during the collision, the singly charged system de-excites by the emission of a neutral Cl-atom, a free electron or possibly a negative Cl anion. The processes are observed as direct as well as delayed processes on a μs-time scale. For q=2 to 4 the loss of the Cl-atom and some phenyl groups becomes more likely due to the lower stability and the larger energy transfer. The charge state distribution of atomic fragments is found to be very different, in particular, when C^q+ and Cl^q+ ions are compared. In the first case mainly singly charged ions are detected, whereas in the second case ions in charge states up to q=6 are observed with high intensities. These phenomena are discussed in terms of the intramolecular charge mobility.     

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.     

Dissociation of alkane ionized molecules

The subject of investigation is the fragmentation of variously charged molecular ions arising in col-lisions of several kiloelectronvolt H⁺, He²⁺, and Ar⁶⁺ ions with molecules of the simplest alkanes (from methane to butane). Using the method of time-of-flight mass spectrometry, the formation cross sections of dissociation-induced fragment ions are measured. The dissociation takes place when an incident ion captures an electron from a methane, ethane, or propane molecule. The role of additional ionization of the molecule, which accompanies the electron capture by the incident ion, is elucidated. The kinetic energy spectrum for protons resulting from the fragmentation of multiply charged alkane ions is determined. The most plausible kinetic energies of protons depending on the degree of ionization and molecule size fall into the range 1–25 eV. It is shown that, when the molecule loses several electrons, the kinetic energies of protons are governed by Coulomb interaction between all fragment ions and are determined by their flying apart from the relative spatial arrangement of corresponding atoms in a parent molecule.     

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。     

Z oscillations in ion-induced fullerene fragmentation

Multiply charged ion-induced fragmentation and ionization of C60 is governed by electronic and nuclear stopping of the projectile as well as charge exchange. We have studied the collision dynamics as a function of the projectile atomic number Z ( 2相似文献   

Mult ifragment Emission by Nucleation-Evaporation Approach

Nuclear fragmentation events are Monte-Carlo simulated in the nucleation-evaporation picture, with charge and energy conservations. We investigate how the fragment energy spectrum, the charge distribution and the intermediate mass fragment (IMF) multiplicity depend upon the breakup density. The results are compared with some recent 4n experimental data.     

乙胺分子的多光子电离过程质谱研究

报道了乙胺分子在４４０～４７５ｎｍ波长范围内多光子电离（ＭＰＩ）质谱（ＭＳ）研究结果。碎片离子主要由母体离子碎裂模式产生。母体离子ＣＨ３ＣＨ２Ｎ＋·Ｈ２由经３ｓ里德堡态的（２＋２）共振多光子电离产生后，大部分发生β键断裂，形成ＣＨ２＝Ｎ＋Ｈ２离子，还有一部分再吸收一个光子，通过Ｃ－Ｈ（ＣＨ２）键的断裂产生了ＣＨ３ＣＨ＝Ｎ＋Ｈ２离子。ＣＨ３ＣＨ＝Ｎ＋Ｈ２和ＣＨ２＝Ｎ＋Ｈ２离子最容易发生的碎裂过程是脱去氢分子，分别产生Ｃ２Ｈ４Ｎ＋（分子式）离子和ＣＨ≡Ｎ＋Ｈ离子。     

Dissociative ionization of the H2O molecule bombarded by swift singly charged and slow highly charged ions

Fragment ion energy spectra of the water molecule have been measured in conventional crossed-beam experiments by the impact of 46 keV/u energy, singly charged ions (SCIs) and 4.3 keV/u energy, highly charged ions (HCIs). Double differential cross sections have been determined and a comparative analysis has been performed. We found that the fragmentation spectra for SCIs and HCIs are very similar, indicating that both collisions lead to the same fragmentation channels. This suggests that the Coulomb explosion of the water molecule is dominantly determined by the charge state of the transient molecular ions, and it is almost independent from the primary ionization mechanism. Differences were observed not only between the SCI and HCI impact-induced fragmentation cross sections, but between those obtained by the 60 keV N⁶⁺ and 70 keV O⁷⁺ projectiles. The differences were attributed to the selectivity of the electron capture process for HCIs. Multiple target ionization cross sections have been deduced from the fragment ion spectra. We found contributions of up to fivefold ionization for SCIs and up to sixfold ionization for HCIs.     

G-SIMS and SMILES: Simulated fragmentation pathways for identification of complex molecules, amino acids and peptides

G-SIMS is a powerful method for the identification of organics and complex molecules at surfaces. We have previously shown that the molecular structure may be reassembled from fragment ions by studying the evolution of G-SIMS intensities as the surface plasma, with effective temperature T_p, is varied, using a method known as G-SIMS-FPM.Here, we develop a novel approach, based on SMILES (Simplified Molecular Input Line Entry Specification), to assist the reassembly process in an automated way through evaluation of the fragmentation pathways for given molecular structures. A computer program takes a parent structure and goes through every possible fragmentation to provide a tree structure of fragmentation products and simulated fragmentation pathways. For any fragment it is then possible to identify the molecular structure, its mass and a pathway to the parent. We find that there is a good correlation with peak evolution in G-SIMS-FPM data and simulated pathways for two amino acids and a simple peptide. This significantly enhances the application of G-SIMS-FPM to unknown materials.     

Kinetic energy distributions of the fragment ions from multiply ionized C2H6 as functions of the charge state of the intermediate states

Here, we report the kinetic energy distributions (KEDs) of the fragment ions produced from multiply ionized ethane (C₂H₆) molecules in single electron capture collisions with 1.2 MeV C²⁺. To systematically investigate the fragmentation dynamics, the KEDs were obtained as functions of the charge state of the intermediate C₂H₆^r+* ions r transiently generated prior to fragmentation. r was determined from coincidence measurement of the fragment ions and the number of emitted electrons. The KEDs are drastically different depending on the number of broken C–H bonds. The underlying causes are explained by the variation of the relative contributions of the multiply ionized states and preferential fragmentation pathways. For instance, CH_n⁺ fragment ions with smaller n exhibit lower KEs because they are likely to be correlated with H⁺ emission, which carries away a large portion of the KE release. In addition, we report the KEDs of H₃⁺ produced from doubly and triply charged states.     

Nuclear multifragmentation: Basic concepts

We present a brief overview of nuclear multifragmentation reaction. Basic formalism of canonical thermodynamical model based on equilibrium statistical mechanics is described. This model is used to calculate basic observables of nuclear multifragmentation like mass distribution, fragment multiplicity, isotopic distribution and isoscaling. Extension of canonical thermodynamical model to a projectile fragmentation model is outlined. Application of the projectile fragmentation model for calculating average number of intermediate mass fragments and the average size of the largest cluster at different Z _bound, differential charge distribution and cross-section of neutron-rich nuclei of different projectile fragmentation reactions at different energies are described. Application of nuclear multifragmentation reaction in basic research as well as in other domains is outlined.     

Collision-induced fragmentation and neutralization of methanol cluster cations

This contribution addresses the inelastic interaction of positively charged molecular cluster ions with a solid surface at kinetic energies up to 30 eV/molecule. We report experimental results on the scattering of mass-selected, protonated methanol cluster cations (CH₃OH)_nH⁺, n = 4-32, off a diamond-coated silicon surface. In particular we provide fragment size distributions of methanol cluster ions following their impact on the target, as well as surface-induced neutralization probabilities of methanol cluster ions as a function of the size and the kinetic energy of the parent clusters. Received 30 November 2000     

Coulomb explosion of large polyatomic molecules assisted by nonadiabatic charge localization

The electron-nuclear dynamics of the Coulomb explosion of a large polyatomic molecule, anthracene, is probed using kinetic energy distributions of produced H+ ions. The kinetic energy release of ejected protons exceeds 30 eV for anthracene exposed to 10(14) W cm(-2), 800 nm pulses of 60 fs duration. We propose a strong-field charge localization model, based on nonadiabatic dynamics of charge distribution in a (multiply) ionized molecule; the charge localization lasts many laser periods and is sustained through successive ionizations of the molecular ion. The model explains quantitatively the dependence of the H+ kinetic energy on the laser intensity. Dissociative ionization of a polyatomic molecule enabled by long-lived charge localization is a new type of electron-nuclear dynamics and is essential for understanding the pathways of molecular or ionic fragmentation in strong fields.     

丙酮分子的共振增强多光子电离解离过程的实验研究

报道了在280—286.5nm区域内，通过共振增强多光子电离-飞行时间质谱和质量选择光电离激发谱对丙酮分子的光电离和光解离通道进行了研究，并对部分碎片离子的分质量激发谱进行了归属和标识.实验结果表明，在280—286.5nm紫外光波段内，丙酮分子以母体分子的电离通道为主，即首先电离生成母体离子然后母体离子再吸收光子解离生成碎片离子. 关键词： 丙酮 共振增强多光子电离 飞行时间质谱     

Influence of the ion charge sign on the stimulation of plasmochemical etching of silicon

The influence of ions with different charge signs on the stimulation of silicon etching under plasma conditions is studied. Fluorine radicals are produced in a glow discharge with a nonuniform pressure. A beam of positive or negative ions is created using a Penning ion source. The flow of fluorine radicals and the ion beam are superposed on a silicon surface placed in a high vacuum. Positive ions may be converted into fast neutrals via resonance charge exchange in the parent gas. It is shown that fast neutrals have the highest catalytic effect. The catalytic effect of positive ions is about two times less. Negative ions occupy the intermediate position. For the first time, it is found that some kinds of ions (e.g., molecular oxygen) do not accelerate, but rather decelerate the etching process; i.e., they behave as inhibitors.     

Coulomb explosion of CS_2 molecule under an intense femtosecond laser field

We experimentally demonstrate the Coulomb explosion process of CS_2 molecule under a near-infrared(800 nm)intense femtosecond laser field by a DC-sliced ion imaging technique. We obtain the DC-sliced images of these fragment ions S~+, S~(2+), CS~+, and CS~(2+)by breaking one C–S bond, and assign their Coulomb explosion channels by considering their kinetic energy release and angular distribution. We also numerically simulate the dissociation dynamics of parent ions CS_2~(k+)(k = 2–4) by a Coulomb potential approximation, and obtain the time evolution of Coulomb energy and kinetic energy release, which indicates that the dissociation time of parent ions CS_2~(k+) decreases with the increase of the charge number k.These experimental and theoretical results can serve as a useful benchmark for those researchers who work in the related area.     

Capture of an electron by ions in methionine and norleucine molecules

The relative cross sections of processes taking place when H⁺ and He²⁺ ions with an energy of 6z keV (z is the ionic charge) capture an electron from molecules of C₅H₁₁NO₂S methionine (proteogenic amino acid) and C₆H₁₃NO₂ norleucine (nonproteogenic amino acid) are measured by time-of-flight mass spectrometry (a methionine molecule transforms into a norleucine molecule by substituting the CH₂ group for the S heteroatom). The fragmentation pattern of resulting molecular ions is established from correlation analysis of the detection times of all fragment ions. The results are compared with experimental data for fragmentation of the same molecules ionized by electrons and photons. In these amino acids, the pattern of molecular ion fragmentation is found to depend on the type of molecule ionization. However, the detachment cross section of the COOH neutral group or residue (neutral or charged) R of a side chain of the amino acid is invariably among the largest. The relative cross sections of capture with single and double ionization of molecules are measured.