Multiple ionization and fragmentation of negatively charged fullerene ions by electron impact

Cross sections for the electron-impact multiple ionization and fragmentation of negatively charged fullerene ions C(-)(n) ( n = 60, 70) to C(q+)(n-m) ( q = 1,2,3 and m = 0,2,4) have been measured for electron energies up to 1 keV. In the case of pure ionization all threshold energies are about 10 eV higher than the values expected. This shift, however, is not observed for the fragment ions. The experimental data indicate that there is no strong electron-electron interaction between the incident electron and the attached electron. A novel ionization mechanism is proposed which can be expected to be valid for all negatively charged molecular or cluster ions which are able to shield the attached electron from the incident electron.     

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.     

高离化态类氖离子的电子碰撞激发特性研究

利用基于多组态Dirac-Fock(MCDF)理论方法的原子结构和性质计算程序GRASP92和全相对论扭曲波电子碰撞激发计算程序REIE06,系统计算了类氖离子(Z=50—57)激发组态2s22p53l和2s2p63l(l=s,p,d)的能级结构和碰撞激发截面,总结了碰撞激发截面随入射电子能量的变化规律,讨论了实验中感兴趣的(2p1/23d3/2)1→2s22p61S0(标记为3C线)与(2p3/23d5/2)1→2s22p61S0(标记为3D线)跃迁线强度比值的沿等电子系列特性和强组态相互作用对高离化态类氖离子截面的影响.     

Synthesis of amorphous boron carbide by single and multiple charged boron ions bombardment of fullerene thin films

In this paper, results of structural modification of fullerene thin films by single and multiple charged boron ions (B⁺, B³⁺) are presented. The applied ion energies were in the range of 15-45 keV. The characterization of as-deposited and irradiated specimens has been performed by atomic force microscopy, Raman and Fourier transform infrared spectroscopy and UV/vis spectrophotometry. The results of Raman analysis have shown the formation of amorphous layer after irradiation of fullerene thin films. Fourier transform infrared spectroscopy has confirmed the formation of new B-C bonds in irradiated films at higher fluences (2 × 10¹⁶ cm⁻²). The morphology of bombarded films has been changed significantly. The optical band gap was found to be reduced from 1.7 to 1.06 eV for irradiated films by B³⁺ ions and 0.7 eV for irradiated films by B⁺ ions.     

A comparison between radiative and non-radiative deexcitation after electron capture by multiple charged ions

For 100 ke V Ne^z+ (z = 1?4) and 200 keV Ar^z+ (z = 1?8) colliding with noble gas atoms the fraction of electron capture processes followed by radiationless deexcitation is shown to increase with rise of z and of target atomic number.     

Structural modification of fullerene thin films by highly charged iron ions

In this paper, the results of structural modification of fullerene thin films bombarded by highly charged iron ions (Fe⁶⁺) are presented. The properties of as-deposited and irradiated fullerene thin films have been investigated by Raman spectroscopy, UV/Vis spectrophotometry and atomic force microscopy (AFM). The results of Raman spectroscopy have indicated structural changes of irradiated thin films depending on fluences. It was found that iron doped fullerene films are dominated by sp³ rather than sp² after bombardment which might be due to formation of nanodiamond structures. AFM analysis showed that the ion beam had destroyed the surface ordering. The optical band gap was found to be in the range of 0.6 to 1.4 eV for irradiated films by Fe⁶⁺ ions at the highest fluences. PACS 61.48.+c; 78.30.-j; 79.20.Rf     

Cross sections for single and double ionization of atomic helium under impact by fast multiply charged ions

Expressions for the cross sections for single and double ionization of atomic helium in collisions with fast multiply charged ions are obtained in the collision parameter range υ ²≫Z≳υ, υ ₀<c, where Z and υ are, respectively, the ion charge and ion velocity, υ ₀ is the characteristic velocity of electrons in the atomic helium ground state, and c is the velocity of light. Zh. Tekh. Fiz. 68, 1–7 (March 1998)     

Inner-shell ionization by positron and electron impact

Measurements of relative cross sections of K- and L-shell ionization of silver and gold targets by positron and electron impact at projectile energies of 30–70 keV are reported. The experiments were performed at the slow positron source TEPOS at the linac of the Strahlenzentrum. This source and the experimental equipment will be described briefly and the experimental results will be presented in detail. In addition, plane wave Born (PWBA) calculations were performed. They include an electron exchange term and the deceleration or acceleration of the incident projectile in the nuclear field of the target atom. The present experimental results agree well with these calculations; for L-shell ionization, they are at variance with a previous experiment where a different energy dependence was observed.     

Inner-shell ionization by relativistic electron impact

K-, L andM-shell ionization cross sections have been measured for 23 elements, 12≦Z≦92, after bombardment with relativistic electrons, 15≦E ₀65MeV, by means of high resolution semiconductor detectors and a recently developed gas-scintillation proportional counter. For constant electron bombarding energyE ₀ the ionization cross sections follow a power law dependence,σ∽Z ^?α, and forE ₀=50MeV we deducedα =2.45±0.02 for theK shell andα=3.00 ±0.09 for theL shell. The observedZ dependence exhibits significant systematic deviations from theoretical predictions which exceed the experimental values up to 15 % at lowZ elements for theK shell and on the average about 11% for theL andM shell. The same behaviour of too low experimental values, i.e. an overestimation by the theory, is observed for the energy dependence of the cross sections for all shells. A scaling behaviour describing theZ andE ₀ dependence for allK-, L andM-shell data points is observed which also predicts the experimental values by other groups at lower and higher energies correctly. The comparsion of the measuredLΒ/Lα, andLγ/Lα intensity ratios for highZ elements with the values obtained by other groups in the energy range 0.3≦E₀≦1,000 MeV exhibits an increase with bombarding energy that cannot merely be explained by the energy dependence of the subshellionization cross sections for theL shell. An attempt to explain this effect with the change of the Coster-Kronig transition probability is described.     

Highly charged ions from laser-cluster interactions: local-field-enhanced impact ionization and frustrated electron-ion recombination

Our molecular dynamics analysis of Xe_{147-5083} clusters identifies two mechanisms that contribute to the yet unexplained observation of extremely highly charged ions in intense laser cluster experiments. First, electron impact ionization is enhanced by the local cluster electric field, increasing the highest charge states by up to 40%; a corresponding theoretical method is developed. Second, electron-ion recombination after the laser pulse is frustrated by acceleration electric fields typically used in ion detectors. This increases the highest charge states by up to 90%, as compared to the usual assumption of total recombination of all cluster-bound electrons. Both effects together augment the highest charge states by up to 120%, in reasonable agreement with experiments.     

Triple ionization of lithium by electron impact

Ejection of the three electrons from lithium in a single electron collision has been observed for the first time. Triply charged lithium was observed in an ion time-of-flight spectrum following electron impact on a sample of ultracold, trapped lithium. The higher signal/background afforded by the trap environment made the observation of Li3+ possible. We measured the ratios of triple-to-double and double-to-single ionization at an impact energy of 1000 eV. The 3+/2+ ratio is approximately 0.001, a value 2 orders of magnitude lower than semiempirical predictions. We present a simple method that uses photoionization data combined with sum-rule analysis to predict the asymptotic charge-state ratios. The sum-rule predictions compare reasonably with experiment and shake calculations, but disagree sharply with the semiempirical estimates.     

Threshold fragmentation of atomic and molecular systems by charged particle impact

The Wannier theory –originally derived for the ionisation of atoms by electrons —is extended to general fragmentation reactions which produce three charged particles of arbitrary masses and charges. The center of mass fragmentation cross section and the angular correlation are shown to be given in terms of two energy-independent, characteristic indices as in Wannier's case. These indices vary drastically as a function of masses and charges of the fragments. It is also shown that these indices may have singularities at critical however physical values of charges and masses. In such cases a generalized Wannier threshold law with a suitable, fractional exponent does not exist. The theory developed in this article applies to energies up to several eV above the fragmentation threshold.     

Triple ionization of lithium-like ions by photon impact

Within the framework of sudden approximation, the non-relativistic cross sections for the triple ionization of Li-like ions caused by absorption and Compton scattering of high-energy photons are calculated. The ratios of the triple-to-single and triple-to-double ionization cross sections are obtained. For neutral lithium atom, our predictions for σ3+/σ⁺ and σ3+/σ2+ are compared both with available experimental and theoretical results.     

Multiple ionization and the charge state evolution of ions exposed to electron impact

Charge state abundances of atoms exposed to an electron flux for a time t are calculated from experimental cross sections by considering either electron impact single ionization only or by including multiple ionization. When multiple ionization is neglected Xe^q+ ion abundances (q = 0,1,…,6) for an electron energy of 700 eV are off by a factor of up to 2 both in peak size and in time necessary to reach the peak value.     

Double ionization of Sc<Superscript>+</Superscript>ions by electron impact

Electron impact double ionization cross-sections of Sc⁺ions have been calculated in the binary encounter approximation (BEA). Accurate expression of σ_ΔE(cross-section for energy transfer ΔE) and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. Direct double ionization from ejection of 3d and 4s electrons has been investigated in the modified double binary encounter model incorporating the focusing action of the target ion on the incident electron. The identification of the 3p shell whose ionization provides a major contribution to double ionization through ionization-autoionization is an interesting aspect of the present investigation. The theoretical results show satisfactory agreement with the experimental observations.