Calculation of Total Cross Sections for Positron Scattering by Lithium at Intermediate Energies

The total cross sections for positron scattering by lithium at intermediate energies (10-200eV) are calculated by using the coupled-channel optical method with a complex equivalent-local polarization potential which incorporates ionization continuum and positronium (Ps) formation channels contributions into the coupled channels framework. The effects of the two-body Ps rearrangement and three-body ionization process on the total crosss ection are found to be significant at lower energies and this effect is not negligible up to 30 eV. Compared to the available theoretical data, the predicted total cross sections agree quite well with the calculations of McAlinden et al. [J. Phys. B: At. Mol. Opt. Phys. 30 (1997)1543] and Campbell et al. [Nucl. Instrum. Methods Phys. B 143 (199s) 41]     

High-Order Above-Threshold Ionization of H2+ in Intense Laser Field

The nonperturbative quantum electrodynamies method proposed by Fu et al. [Phys. Rev. A 75 （2007） 063419] is employed to study the high-order above-threshold ionization （ATI） of a diatomic molecule. Based on this frequency-domain theory, the high-order ATI process can be regarded as ATI followed by laser-assisted collision, where the total transition amplitude is the coherent summation of the contributions from each ATI channel. The angular-resolved ATI spectrum, which agrees with the results by Becket et al. based on the time-domain method, is obtained by this frequency domain theory. Furthermore, it is demonstrated that the interference characteristics representing the molecular structure in the ATI spectrum originates from the recollision of the electron with two-centre ion in each ATI channel.     

S-matrix study on alignment dependence of single ionization of molecular ions with different active orbitals

The orientation-dependent single ionization rate of the diatomic molecular ion H_2^+ with different active orbitals in an intense field is studied by using S-matrix theory. Our results show that the orientation-dependent single ionization probability of H_2^+ is greatly dependent on the symmetry and the electron density distribution of its initial states, and it can be used to identify the excited state of the molecular ion in the dissociation process.     

Influence of laser intensity on the double-resonance multiphoton ionization process of NO molecule

The analytic formula of the ionization efficiency in the process of double resonance enhanced multi-photon ionization （DREMPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the ionization efficiency and the laser power index versus laser intensity in the DREMPI process of NO molecule, via A2E and S2E intermediate resonant states, is numerically simulated. It is shown that the ionization efficiency of NO molecule increases with the laser intensity until getting saturation, while the laser power index decreases with the enhancement of the laser intensity and changes to zero at last. The variation of the laser power index with the laser intensity indicates that the ionization efficiency reaches saturation in the one, two, and three excitation steps respectively. It is also found that the narrower the laser pulse duration is, the higher becomes the laser intensity for saturation.     

Dynamics simulation on interaction of intense laser pulses with atomic clusters

Under classical particle dynamics, the interaction process between intense femtosecond laser pulses and icosahedral noble-gas atomic clusters was studied. Our calculated results show that ionization proceeds mainly through tunnel ionization in the combined field from ions, electrons and laser, rather than the electron-impact ionization. With increasing cluster size, the average and maximum kinetic energy of the product ion increases. According to our calculation, the expansion process of the clusters after laser irradiation is dominated by Coulomb explosion and the expansion scale increases with increasing cluster size. The dependence of average kinetic energy and average charge state of the product ions on laser wavelength is also presented and discussed. The dependence of average kinetic energy on the number of atoms inside the cluster was studied and compared with the experimental data. Our results agree with the experimental results reasonably well.     

Sequential over-barrier ionization of multi-electron atoms in the tens-to-hundreds keV/u energy range

Our previous work on the classical over-barrier ionization model for helium double ionization is extended to the complex multi-electron system of Ne.The total and q-fold ionization cross sections are calculated at energies ranging from a few tens to several hundred keV/u.The calculation results are in good agreement with the experimental data,and the energy dependence of the cross sections suggests that the multi-ionization of a strong perturbated complex atom is probably the sequential over-barrier ionization process.     

Effect of Temperature on the Void Growth in Pure Aluminium at High Strain-Rate Loading

With the environment temperature varying from 273 K to 773 K, the dynamic process of void growth in pure aluminium at high strain-rate loading is calculated based on the dynamic growth equation of a void with internal pressure. The result shows that the effect of temperature on the growth of void should be emphasized. Because the initial pressure of void with gas will increase and the viscosity of materials will decrease with the rising of temperature, the growth of void is accelerated. Furthermore, material inertia restrains the growth of void evidently when the diameter exceeds 10 μm. The effect of surface tension is very weak in the whole process of void growth.     

Ionization and Charge Transfer of Atomic Hydrogen by Highly Charged Ions

Cross sections for charge transfer and ionization of atomic hydrogen by highly charged ions A^q＋ （q =6 9） are evaluated using a simple and classical method based on the previous works by Bohr and Lindhard [K. Dan. Vidensk. Selsk. Mat. Fys. Medd 28 （1954） No 7], Brandt [Nucl. Instrum. Methods Phys. Res. 214 （1983） 93] and Ben-Itzhak et al. [J. Phys. B： At. Mol. Opt. Phys. 26 （1993） 1711]. It is proved that the present calculations are feasible to some extent in comparison with available experimental data and quantum calculations.     

Theoretical analysis on the efficiency of optical-optical double-color double-resonance multiphoton ionization

Analytic formula of the efficiency of optical-optical double-color double-resonance multi-photon ionization （OODR-MPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the influence of characteristic of the pump and probe laser on the ionization efficiency of （1＋2＋1） OODR-MPI process is simulated theoretically. It is shown that the pump laser will affect the ionization efficiency by the number control of the molecules excited to the first resonance state. The ionization efficiency is decided by the probe laser directly. Both of the excited molecules and ionization efficiency increase with the intensity and pulse duration of the laser until saturation. It is also found that the longer the delay time of the probe laser to the pump one is, the lower the ionization efficiency would be. The delay time ought to be smaller than the lifetime of the excited molecule in the practical use of the OODR-MPI technique.     

Jet-Like Long Spike in Nonlinear Evolution of Ablative Rayleigh–Taylor Instability

We report the formation of jet-like long spike in the nonlinear evolution of the ablative Rayleigh-Taylor instability （ARTI） experiments by numerical simulations. A preheating model k（T） = KSH[1＋f（T）], where KSH is the Spitzer Harm （SH） electron conductivity and f（T） interprets the preheating tongue effect in the cold plasma ahead of the ablative front [Phys. Rev. E 65 （2002） 57401], is introduced in simulations. The simulation results of the nonlinear evolution of the ARTI are in general agreement with the experiment results. It is found that two factors, i.e., the suppressing of ablative Kelvin Helmholtz instability （AKHI） and the heat flow cone in the spike tips, contribute to the formation of jet-like long spike in the nonlinear evolution of the ARTI.     

Theory of dust voids in plasmas

Dusty plasmas in a gas discharge often feature a stable void, i.e., a dust-free region inside the dust cloud. This occurs under conditions relevant to both plasma processing discharges and plasma crystal experiments. The void results from a balance of the electrostatic and ion drag forces on a dust particle. The ion drag force is driven by a flow of ions outward from an ionization source and toward the surrounding dust cloud, which has a negative space charge. In equilibrium the force balance for dust particles requires that the boundary with the dust cloud be sharp, provided that the particles are cold and monodispersive. Numerical solutions of the one-dimensional nonlinear fluid equations are carried out including dust charging and dust-neutral collisions, but not ion-neutral collisions. The regions of parameter space that allow stable void equilibria are identified. There is a minimum ionization rate that can sustain a void. Spatial profiles of plasma parameters in the void are reported. In the absence of ion-neutral collisions, the ion flow enters the dust cloud's edge at Mach number M=1. Phase diagrams for expanding or contracting voids reveal a stationary point corresponding to a single stable equilibrium void size, provided the ionization rate is constant. Large voids contract and small voids expand until they attain this stationary void size. On the other hand, if the ionization rate is not constant, the void size can oscillate. Results are compared to recent laboratory and microgravity experiments.     

Modulation of Void Motion Behavior in a Magnetized Dusty Plasma

Based on fluid equations,we show a time-dependent self-consistent nonlinear model for void formation in magnetized dusty plasmas.The cylindrical configuration is applied to better illustrate the effects of the static magnetic field,considering the azimuthal motion of the dusts.The nonlinear evolution of the dust void and the rotation of the dust particles are then investigated numerically.The results show that,similar to the unmagnetized one-dimensional model,the radial ion drag plays a crucial role in the evolution of the void.Moreover,the dust rotation is driven by the azimuthal ion drag force exerting on the dust.As the azimuthal component of ion velocity increases linearly with the strength of the magnetic field,the azimuthal component of dust velocity increases synchronously.Moreover,the angular velocity gradients of the dust rotation show a sheared dust flow around the void.     

Dust Self‐Organized Structures III. Solutions of Master Equations in Presence of Volume Ionization

The shapes of dust self‐organized structures in presence of volume ionization is investigated. Master Equations for small diffusions and presence of an external ionization describing spherical dust structures are solved numerically using asymptotic solutions at the center of the structures. The structures are determined by two parameters, the external plasma flux at the surface of the structures and the power of volume ionizations. It is shown that the range of possible equilibrium states is reduced with an increase of the ionization power. This reduction is due mainly to an increase of the minimum possible value of ion density at the center of the structure for which the equilibrium is possible (an increase of minimum possible external flux). It is found that for certain large ionization power the equilibrium structure without void at the center cannot exist. The critical value of ionization power is found numerically. In the range of ionization power where the equilibria is possible the distribution of the parameters inside the structures are investigated by solving the Master Equations in the limits of small and large ionization power (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dust ion-acoustic shock-wave structures: Theory and laboratory experiments

An evolutionary theoretical model is developed that describes dust ion-acoustic shock waves in dusty plasma consisting of ions (treated in the hydrodynamic approximation), Boltzmann electrons, and variable-charge dust grains. Account is taken not only of ionization, absorption, momentum loss by electrons and ions in collisions with dust grains, and gas-kinetic pressure effects but also of the processes peculiar to laboratory plasmas. It is shown that the model is capable of describing all the main experimental results on dust ion-acoustic shock waves [Q.-Z. Luo et al., Phys. Plasmas 6, 3455 (1999); Y. Nakamura et al., Phys. Rev. Lett., 83, 1602 (1999)].     

Ionization dynamics of helium dimers in fast collisions with He++

By employing the cold target recoil ion momentum spectroscopy technique, we have investigated the (He+, He+) breakup of a helium dimer (He2) caused by transfer ionization and double capture in collisions with alpha particles (E = 150 keV/u). Surprisingly, the results show a two-step process as well as a one-step process followed by electron exchange. In addition, interatomic Coulombic decay [L.?S. Cederbaum, J. Zobeley, and F. Tarantelli, Phys. Rev. Lett. 79, 4778 (1997).] is observed in an ion collision for the first time.     

Single photoeffect on helium-like ions in the non-relativistic region

We present a generalization of the pioneering results obtained for single K-shell photoionization of H-like ions by M. Stobbe [M. Stobbe, Ann. Phys. 7 (1930) 661] to the case of the helium isoelectronic sequence. The total cross section of the process is calculated, taking into account the correlation corrections to first order of the perturbation theory with respect to the electron–electron interaction. Predictions are made for the entire non-relativistic energy domain. The phenomenon of dynamical suppression of correlation effects in the ionization cross section is discussed.     

Size of dust voids as a function of the power input in dusty plasma

A dust void is a dust-free region in dusty plasma. Theory demonstrates that the void results from the balance of the electrostatic and plasma (such as the ion drag) forces acting on a dust particle. In dusty plasma experiments, physical properties of the void show clear dependence on the power input into the plasma (in particular, its size increases with the increase of the applied power). Here, the theory and numerical results are presented for such a dependence. The text was submitted by the authors in English.