MULTIPLE IONIZATION PROCESS STUDIED WITH COINCIDENCE TECHNIQUE BETWEEN SLOW RECOIL IOH AND PROJECTILE ION IN 42 MeV Ar~(q )-Ar COLLISIONS

Slow Ar recoil ion production cross sections by 42MeV Ar~(q )(q=4-14)projectiles were measured using a projectile ion-recoilion coincidence technique in order to provide information onmechanisms of multiple ionization of target atoms through pureionization as well as of that accompanied simultaneously with mul-tiple electron loss or capture of projectiles. The present resultssuggest that inner-shell electron processes caused through electrontransfer into projectiles and also electron ionization by projectilesplay a key role in the production of multiply charged recoil ions.     

Multiple ionization of atoms and molecules impacted by very high-q fast projectiles in the strong coupling regime （q/v 〉 1）

In this paper, we present a simple theoretical approach to calculate the multiple ionization of big atoms and molecules induced by very high-q fast projectiles in a strong coupling regime （q/v 〉 1）. The results obtained from this approach are in excellent agreement with the available experimental data. A probable scenario of molecular multiple ionization by fast and very high-q projectiles is discussed. The very small computational time required here and the good agreement with the existing experimental data make it a good candidate for studying the multiple ionization of complex molecules under high linear energy transfers.     

Multiple ionization of argon accompanied by electron loss and capture of 0.22-6.35 MeV C^q＋ ions

In this paper a projectile ions recoil ions coincidence technique is employed to investigate the target ionization and projectile charge state changing processes in the collision of 0.22-6.35 MeV Cq^＋ （q = 1 - 4） ions with argon atoms. The partial cross section ratios of the double, triple, quadruplicate ionization to the single ionization （or the single capture） of argon associated with single electron loss （or single electron capture） by the projectile are measured and compared with the previous experimental results. In the present experiment, it is observed that the ratios of ionization cross sections R associated with single loss and single capture depend strongly on the projectile charge state and vary significantly with different reaction channels as impact energy increases. In addition, this paper gets empirical scaling laws for the ionization cross section ratios R corresponding to the projectile single loss and finds that the ratios of the double ionization to the single ionization associated with single electron capture remain constant in the present energy range.     

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.     

Ellipticity-dependent ionization/dissociation of carbon dioxide in strong laser fields

Ionization and dissociation of linear triatomic molecules, carbon dioxide, are studied in 50-fs 800-nm strong laser fields using time-of-flight mass spectrometer. The yields of double charged ions CO2+2 and various fragment ions(CO+,O n+, and C n+(n = 1, 2)) are measured as a function of ellipticity of laser polarization in the intensity range from 5.0 ×1013W/cm2 to 6.0 × 1014W/cm2. The results demonstrate that non-sequential double ionization, which is induced by laser-driven electron recollision, dominates double ionization of CO2 in the strong IR laser field with intensity lower than2.0 × 1014W/cm2. The electron recollision could also have contribution in strong-field multiple ionization and formation of fragments of CO2 molecules. The present study indicates that the intensity and ellipticity dependence of ions yields can be used to probe the complex dynamics of strong-field ionization/dissociation of polyatomic molecules.     

Reduce of Threshold of Laser Inducing Breakdown in Atmosphere by Introducing an Electric Spark

We report laser-generated plasmas in atmosphere with electrical spark generated by a synchronization circuit. The breakdown thresholds under the conditions that the electrical spark is used and not used are compared. The breakdown threshold has a distinct decrease after the electrical spark is used. Breakdown thresholds as a function of atmosphere pressure have also been measured at laser wavelengths 532nm and 1064 nm for the laser pulse width of 15ns. We also discuss the principle and performances of the ionized atmosphere by Nd：YAG laser under the condition of electrical spark introduction. Multiphoton ionization and cascade ionization play important roles in the whole process of atmosphere ionization. The free electron induced by electrical spark can supply the initialization free electron number for multiphoton ionization and cascade ionization. A model for breakdown in atmosphere, which is in good agreement with the experimental results, is described.     

Laser-Induced Particle Jet and Its Ignition Application in Premixed Combustible Gases

A hot particle jet is induced as a laser pulse from a free oscillated Nd：YAG laser focused on a coal target. The particle jet successfully initiates combustion in a premixed combustible gas consisting of hydrogen, oxygen, and air. The experiment reveals that the ionization of the particle jet is enhanced during the laser pulse. This characteristic is attributed to the electron cascade process and the ionization of the particles or molecules of the target. The initial free electrons, which are ablated from the coal target, are accelerated by the laser pulse through the inverse Bremsstrahfung process and then collide with the neutrals in the jet, causing the latter to be ionized.     

Effect of electron correlation on high-order harmonic generation in helium model atom

High-order harmonic generation (HHG) of a helium model atom in an intense laser field has been numerically investigated. The influence of electron correlation on HHG is analysed by changing the strength between the electrons. The numerical results show that as the electron interaction strength becomes small, the first ionization energy increases rapidly, which results in the decrease in ionization. So the conversion efficiency of the high harmonic lying in the plateau decreases greatly, while the cutoff harmonic order in the harmonic spectrum increases.     

Generation of Fabry–Pérot oscillations and Dirac state in two-dimensional topological insulators by gate voltage

We investigate electron transport through Hg Te ribbons embedded by strip-shape gate voltage through using a nonequilibrium Green function technique. The numerical calculations show that as the gate voltage is increased, an edgerelated state in the valence band structure of the system shifts upwards, then hangs inside the band gap and merges into the conduction band finally. It is interesting that as the gate voltage is increased continuously, another edge-related state in the valence band also shifts upwards in the small-k region and contacts the previous one to form a Dirac cone in the band structure. Meanwhile in this process, the conductance spectrum displays as multiple resonance peaks characterized by some strong antiresonance valleys in the band gap, then behaves as Fabry–P′erot oscillations and finally develops into a nearly perfect quantum plateau with a value of 2e~2/h. These results give a physical picture to understand the formation process of the Dirac state driven by the gate voltage and provide a route to achieving particular quantum oscillations of the electronic transport in nanodevices.     

非全裸氟离子与氖原子碰撞中的转移电离研究

采用位置灵敏探测和散射离子 反冲离子飞行时间技术测量了强相互作用区F2 + 和F3 + 离子与Ne原子碰撞中的转移电离截面与单电子俘获截面之比 .与Fq+ He实验结果进行了比较 ,并进行了定性讨论. The ratios of the cross-section of the transfer ionization to the single electron capture of Neon induced by F 2+ and F 3+ ions are measured by means of the time-of-flight technologie. In the present energy range, the transfer ionization C1I1 of neon can be considered as a two-step process, in which one electron is captured by projectile and another one is directly ionized, and the transfer ionization C1I2 process should be considered as a subsequent rearrangement of neon following...     

非全裸氟离子与氦原子碰撞中的转移电离过程

采用位置灵敏探测和散射离子 反冲离子飞行时间测量技术 ,测量了强相互作用区氟离子与氦原子碰撞中的转移电离截面与单电子俘获截面之比 .实验发现 ,该比值随作用强度的增加而减小 .在本工作能区 ,转移电离现象可视作一个电子被俘获、另一个电子被电离的两步过程 ;He原子二重电离的主要机制为两步机制. The cross-section ratios of the transfer ionization to the single electron capture of helium induced by fluorine ions in the strong-interaction region were measured by means of the position-sensitive-detecting and the time-of -flight (TOF) technologies. Is is found that the cross section ratios decrease with the increasing of the interaction-strength κ in this work. In the energy range of this work, the transfer ionization can be considered as a two-step process that one electron is captured by projectile...     

Electron Acceleration and Bunch Generation by Intense Femtosecond Laser Pulse in Preplasma of a Target

We present analytical studies of electron acceleration in the low-density preplasma of a thin solid target by an intense femtosecond laser pulse. Electrons in the preplasma are trapped and accelerated by the ponderomotive force as well as the wake field. Two-dimensional particle-in-cell simulations show that when the laser pulse is stopped by the target, electrons trapped in the laser pules can be extracted and move forward inertially. The energetic electron bunch in the bubble is unaffected by the reflected pulse and passes through the target with small energy spread and emittance. There is an optimal preplasma density for the generation of the monoenergetic electron bunch if a laser pulse is given. The maximum electron energy is inverse proportion to the preplasma density.     

Interpulse interference of electron emission from an atom irradiated by sinusoidally phase-modulated pulse

We theoretically investigate the photoelectron emission from an atom irradiated by an amplitude modulated sinusoidally phase-modulated pulse through solving the time-dependent Schr¨odinger equation in the momentum space. By controlling the phase amplitude of the pulse in the frequency domain, it can be found that the photoelectron spectra appear as explicit interference phenomena, which originated from the interference between the directly ionized electron and the ionization of the pre-excited atom from different subpulses.     

Transfer Ionization of Isocharge Sequence Ion and Ar Collisions

The study of multi-electron process has been a very active area of atomic physics research. The simultaneous electron emission and electron transfer is one of the most interesting two-electron processes in the study Of ionatom collisions, In last decade much experimental and theoretical attention has been devoted to the process, named transfer ionization (TI). This process is normally considered as the successive process of the single electron capture and the ionization of the second or more target electrons. The studies of transfer ionization are motivated by the understanding of the role of the electron correlation in ion-atom collisions which is of great interest in basic atomic physics and may provide important information for the applications both in astrophysics and fusion plasmas researches. Different authors have investigated the transfer ionization process by measuring the angular distribution of the emitted electrons. Up to now, the systematic study of the transfer ionization and the dependences of these processes on the collision parameters for the low energy ion-atom collisions are still rarely reported in the literatures.     

Synthesis of high-TC ferromagnetic Mn-doped ZnO nanorods by thermal evaporation

This paper reports that a large amount of Mn-doped ZnO nanorods have been synthesized through thermal evaporation. The morphologies and properties are studied with x-ray diffraction, a scanning electron microscope, transmission electron microscope and Raman spectroscope. The results indicate that the manganese atoms occupy the zinc vacancies in the wurtzite lattice of ZnO without forming secondary phases. The exact manganese content has been studied by the x-ray fluorescence spectrum. Meanwhile, the magnetic moment versus temperature result proves that the as-prepared Mn-doped ZnO nanorods show ferromagnetic properties at temperatures as high as 400 K. These studies provide a good understanding of the origin of magnetic properties in diluted magnetic semiconductors.     

Monte Carlo simulation of prompt γ-ray spectra from depleted uranium under D-T neutron irradiation and electron recoil spectra in a liquid scintillator detector

To overcome the problem of inefficient computing time and unreliable results in MCNP5 calculation, a two-step method is adopted to calculate the energy deposition of prompt γ-rays in detectors for depleted uranium spherical shells under D-T neutron irradiation. In the first step, the γ-ray spectrum for energy below 7 MeV is calculated by MCNP5 code; secondly, the electron recoil spectrum in a BC501 A liquid scintillator detector is simulated based on EGSnrc Monte Carlo Code with the γ-ray spectrum from the first step as input. The comparison of calculated results with experimental ones shows that the simulations agree well with experiment in the energy region 0.4–3 MeV for the prompt γ-ray spectrum and below 4 MeVee for the electron recoil spectrum. The reliability of the two-step method in this work is validated.     

CuO Nanosheets Synthesized by Hydrothermal Process

CuO nanosheets are synthesized by oxidation of commercial Cu substrates through the hydrothermal process at 150℃. The as-synthesized nanosheets are characterized by powder x-ray diffraction, transmission electron microscopy, selected area electron diffraction and x-ray photoelectron spectroscopy. For comparison, Cu substrates are also oxidized without NaOH added in precursor solution. The results show that the morphology of CuO could be controlled by NaOH, which demonstrates that NaOH can serve as a cosolvent and modifier in the reaction system. The possible mechanism of the growth of CuO nanosheets is also discussed.     

Local electron mean energy profile of positive primary streamer discharge with pin-plate electrodes in oxygen nitrogen mixtures

Local electron mean energy (LEME) has a direct effect on the rates of collisional ionization of molecules and atoms by electrons. Electron-impact ionization plays an important role and is the main process for the production of charged particles in a primary streamer discharge. Detailed research on the LEME profile in a primary streamer discharge is extremely important for a comprehensive understanding of the local physical mechanism of a streamer. In this study, the LEME profile of the primary streamer discharge in oxygen-nitrogen mixtures with a pin-plate gap of 0.5 cm under an impulse voltage is investigated using a fluid model. The fluid model includes the electron mean energy density equation, as well as continuity equations for electrons and ions and Poisson’s electric field equation. The study finds that, except in the initial stage of the primary streamer, the LEME in the primary streamer tip tends to increase as the oxygen-nitrogen mole ratio increases and the pressure decreases. When the primary streamer bridges the gap, the LEME in the primary streamer channel is smaller than the first ionization energies of oxygen and nitrogen. The LEME in the primary streamer channel then decreases as the oxygen-nitrogen mole ratio increases and the pressure increases. The LEME in the primary streamer tip is primarily dependent on the reduced electric field with mole ratios of oxygen-nitrogen given in the oxygen-nitrogen mixtures.     

Controlling electron collision by counterrotating circular two-color laser fields

Electron collision as well as its controlling lies in the core of study on nonsequential double ionization(NSDI).A single collision occurred in a convergent time is important to disclose the essential features of the electron correlation.However,it is difficult to form such a collision.By using counterrotating circular two-color(CRTC)laser fields,we show that a single electron collision can be achieved in a convergent time and a net electron correlation is set up within the sub-femtosecond time scale in the NSDI process of Ar atoms.The proposed method is also valid for other atoms,provided that one chooses the frequency and intensity of the CRTC field according to a scaling law.     

Terahertz emission in tenuous gases irradiated by ultrashort laser pulses

Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scaling with the laser amplitude within a large range is also discussed.