王友年，学者凯发k8国际首页主页-凯发网

a self-consistent model is proposed to study nonlinear phenomena, such as secondary resonance and hysteresis in the vertical oscillations of a charged microparticle in a radio-frequency sheath. the motion of a single microparticle in the sheath is simulated by solving newton's equation in which various forces acting on the particle are taken into account. the particle charging and the sheath electric field are described by a self-consistent model of the collisional radio-frequency sheath dynamics. it is found that the nonlinearity is related to the particle's charge, the sheath electric field, and the external excitation force, as well as the ion drag force and neutral-gas friction on the particle.

the molecular dynamics method is used to simulate coulomb explosion of fast c60 ion clusters in an al target, taking into account dynamical screening of interionic interactions by the electron gas of the target. it is found that the wake forces in the medium are strong enough, depending on cluster speed, to stabilize the whole cluster against coulomb explosion, or to compress the trailing part of the cluster, for prolonged times of penetration through the target. this is encouraging news for such cluster-ion beams applications where massive energy depositions in small volumes of targets are desired.

the interactions of charged particles with carbon nanotubes are studied by means of the linearized hydrodynamic theory for electronic excitations on the nanotube surface. general expressions are derived for the induced potential, the self-energy, and the stopping power for a charged particle moving paraxially in a nanotube. numerical results are obtained showing the influence of the damping factor, the nanotube radius, and the particle position on its self-energy and the stopping power. results for stopping power in the linearized hydrodynamic model are compared with those obtained by means of the dielectric formalism in random-phase approximation, showing a close agreement between the two approaches for high speeds of charged particles.

the interactions of charged particles with cylindrical tubules are studied within the framework of the dielectric theory. elementary excitations on a tubule are modeled by an infinitesimally thin layer of freeelectron gas, uniformly distributed over the surface of the tubule. the dielectric function of such a system, obtained from the random-phase approximation, exhibits a dimensional crossover from two-dimensional to one-dimensional electron gas, when the radius of the tubule decreases. energy loss of a charged particle, moving paraxially in a tubule, can be divided into a single-particle excitation part and a resonant excitation part. it is shown that the resonant excitation modes on a tubule, which dominate the energy loss in the high-velocity regime, are quite different from those in a cylindrical cavity in a solid, described by a bulk dielectric function of the surrounding three-dimensional electron gas.

the angle-resolved energy spectra of the electrons emitted from a metal surface during specular reflection of heavy ions have been studied by means of the first-order, time-dependent perturbation theory, with the electron states obtained from a finite step-barrier potential model. the surface response has been described by the dielectric formalism within the specular reflection model, where the local-field correction and the plasmon-pole approximation have been used for the dielectric function in the cases of low and high ion velocities, respectively. the ion trajectory has been evaluated based on the surface continuum potential and the ion image potential, whereas the screening of the ion by the bound electrons has been taken into account by means of the brandt-kitagawa model. it has been found that the electron spectra are dominated by the decay of surface plasmons for fast incident ions, whereas the electron emission is dominated by the single-electron excitation mechanism for slow incident ions.

a theoretical model is developed to study the energy loss of heavy ions specularly reflected on a solid surface at a glancing angle using the dielectric response theory and the specular reflection model. a local-field correction and a plasmon-pole approximation for dielectric function are employed in the low- and highvelocity regimes, respectively. also, the brandt-kitagawa model is used to express the distribution of the electrons bound to the projectiles. we obtain analytical expressions for the position-dependent stopping power and the surface image potential. the energy losses, dependent on the charge state of the ions and the incident angles, are calculated and compared with the corresponding experimental results.

the dielectric response formalism is used to evaluate the dynamically screened interaction potential among the ions in a fast c20 cluster passing through a solid target. this potential is further used to calculate the individual ion charge states in the cluster by means of a statistical–variational theorywhich takes into account the effects of the vicinity of the neighbouring ions. coulomb explosion and the energy losses of the cluster are simulated by solving equations ofmotion for individual ions while taking into account, in a self-consistent manner, the variation of ion charges in the course of explosion. moreover, a monte carlo method is used to simulate the effects ofmultiple scattering of cluster constituent ions on target atoms. it is found that, owing to the wake-like asymmetry of the inter-ionic potential, both the distribution of ion charges in the cluster and the coulomb explosion patterns exhibit strong spatial asymmetries in the direction of motion. it is also shown that the cluster energy losses exhibit characteristic interferences due to the vicinity effects, which diminish after long dwell times. (some figures in this article are in colour only in the electronic version)

a hydrodynamic model is established to study the interactions of a dust particle with a radio-frequency (rf) sheath, taking into account the influence of the spatial inhomogeneity of the ~rf! sheath, as well as the influence of the ion-neutral collisions. numerical results are obtained for the charge on the dust particle and for the spatial distribution of the induced potential around this particle, based on a self-consistent modeling of the sheath parameters such as the sheath electric field, the ion velocity, and the ion and electron densities. the induced potential exhibits the familiar oscillatory structure of a wake potential, which is, however, strongly damped due to the collisional effects. the spatial inhomogeneity of the rf sheath gives rise to a further damping of the induced potential, as well as to a reduction of the oscillations at large distances from the dust particle.

a hybrid theoretical model, capable of describing the characteristics of a collisional sheath driven by a sinusoidal current source and determining the energy and angular distributions of ions incident onto the substrate, is proposed. the model consists of one-dimensional time-dependent fluid equations coupled with the poisson equation determining spatiotemporal evolution of the sheath and the monte carlo simulation predicting the energy and angular distributions of ions striking the electrode, in which charge-exchange collisions between ions and neutrals are included. additionally, an equivalent circuit model in conjunction with the fluid equations is adopted to self-consistently determine the relationship between the instantaneous potential at a rf-biased electrode and the sheath thickness. it is found that the collisional effects would influence the height of the energy peaks in the ion energy distributions and the ion angular distributions.

a self-consistent fluid model is proposed for describing collisionless radio-frequency ~rf! sheaths driven by a sinusoidal current source. this model includes all time-dependent terms in ion fluid equations, which are commonly ignored in some analytical models and numerical simulations. moreover, an equivalent circuit model is introduced to determine self-consistently the relationship between the instantaneous potential at a rf-biased electrode and the sheath thickness. the time-dependent voltage wave form, the sheath thickness and the ion flux at the electrode as well as the spatiotemporal variations of the potential, the electric field and the ion density inside the sheath are calculated for various rf powers and ratios of the rf frequency to the ion plasma frequency. the ion energy distributions ~ ieds! impinging on the rf-biased electrode are also calculated with the ion flux at the electrode. the numerical results show that the frequency ratio is a crucial parameter determining the spatiotemporal variations of the rf sheath and the shape of the ieds.