Kerr effect of molecular oxygen at λ=1064 nm

We report a new measurement of the Kerr effect of molecular oxygen at λ= 1064 nm. The experimental value reported for the anisotropy of the index of refraction Δ n_u^l, (3.15±0.85)×10^-25 m² V^-2 atm^-1, is in good agreement with the value of 3.4×10^-25 m² V^-2 atm^-1 obtained via an ab initio calculation. We show that the dependence of the effect on the pressure is not linear because of the presence of a collision-induced absorption band around 1060 nm due to the transition from the X³ Σ^-_g ground state to the ¹Δ_g state. We also give the value of the quadratic anisotropy Δn_u^q (-1.03±0.68)×10^-25 m² V^-2 atm^-2. We finally compare our ab initio theoretical and experimental results with previous existing data.     

The dynamics of 180° domains in a ferroelectric in the process of polarization reversal and electron emission

A model is proposed to describe the evolution of 180° domains in a ferroelectric. Closed analytic expressions are obtained for the velocity of sideways motion of a 180° domain wall in an electric field and numerical calculations are performed. The connection between polarization reversal and electron emission is discussed.     

A molecular level study of liquid water with a flexible water model

The structural properties of water at different temperatures and pressures have been investigated by using a flexible water model and the inherent structure mechanism. The presence of 60$^\circ$ peak in the O--O--O angle distribution function and the behaviours of the hydrogen bonds in the first shell indicate that some water molecules in the second shell move toward the central molecules through the bending (not breaking) of hydrogen bonds and even become first-shell molecules of the central molecule on the basis of the O--O cutoff distance but not first-shell molecules by means of the hydrogen-bond criterion. The inherent-structure analysis of the O--O radial distribution functions at different pressures shows that the first peak is almost independent of the pressure; the position of second peak moves from 0.45 to 0.32nm as the pressure increases from $1\times 10^5$Pa to $1\times 10^9$Pa. This particularly evident pressure effect, i.e. the constant nearest-neighbours and the transformation of outer-neighbours on the basis of O--O distance, together with the results at different temperatures, gives a positive evidence for the two-state outer-neighbour mixture model: liquid water is a mixture of Ice-Ih-type-bonding and Ice-II-type bonding structures.     

A new unified theory for scattering of a bounded acoustic beam from a liquid-solid interface near the Rayleigh angle

For scattering of a bounded acoustic beam from a liquid-solidinterface,the principal difficulty comes from the treatment of the plane wave reflec-tion coefficient.In the Present paper,following the Singularity Expansion Method,the reflection coefficient is expanded into the sum of an analytic function(constant1)and the principal part of the Laurent expansion in the neighborhood of theRayleigh pole.The scattered field is now evaluated by a double integral in thewavenumber and interface planes.The constant 1 produces a rigid boundary geo-metrical reflected beam,and the expansions at the Rayleigh poles yield the leakyRayleigh waves.It is particularly proved that the residues of the poles are nearlyproportional to the imaginary parts of the poles for most liquid-solid interfaces.Therefore,the complex pole may determine the whole behavior of the leakyRayleigh wave,including the excitation and reradiation efficiencies.For thebackscattered field,since the negative Rayleigh pole plays a major role,a back-ward l     

Combined exciton–electron excitation in quantum wells with a two-dimensional electron gas of low density

A combined exciton–cyclotron resonance is found in the photoluminescence excitation and reflectivity spectra of semiconductor quantum wells with an electron gas of low density. In external magnetic fields an incident photon creates an exciton in the ground state and simultaneously excites an electron between Landau levels. A theoretical model is developed and suggests the dominating contribution of the exchange exciton–electron interaction.     

Elastic differential cross sections of electron scattering by CF_4 at intermediate energies

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Impact of electron–hole pair excitation on the kinetics of exoergic processes at metal surfaces

The effect of energy disposal to metal surfaces, via electron–hole (e–h) pair excitation, on the rate of exoergic processes such as adsorption and adatom recombination, has been investigated. The modeling is based on the mean field rate equations and allows one to couple e–h pair excitation with either recombination or stimulated-desorption processes within a unique kinetic scheme. The energy distribution function of the metal electrons and the number of electrons available for detection as “chemicurrent” have been computed and compared to experimental results on chemicurrent yield for the H–Cu system. The kinetic model can also be applied to interpret experimental data on H(D)-adatom abstraction and recombination and on adsorption stimulated desorption of CO from metal surfaces. Rate coefficients of these desorption processes are proper for non-equilibrium energy distribution functions of the adlayer, which depend on the flux of adsorbing species and correlate to the surface electron density as modified by the adsorbate.     

Synchrotron small angle x-ray scattering study of dye-sensitized／-unsensitized TiO2 nanoparticle colloidal solution

The wide-gap semiconductor TiO_2 nanoparticles with and without dye sensitization have been studied by small angle x-ray scattering using synchrotron radiation. Surface properties of the colloidal TiO_2 nanoparticles have been analysed in terms of the surface fractal dimensions (D_s), showing that D_s changes from 3.25 to 2.34 when TiO_2 nanoparticles are sensitized by ATRA (all-trans-retinoic acid), which reveals that the surface of the particles become relatively smooth after dye sensitization. The size distribution of gyration radius of TiO_2 nanoparticles in the colloids M(R_g) has been successfully determined by the Shull－Roess method. The main peak of M(R_g) for the unsensitized TiO_2 colloid is located at 2.1nm, corresponding to a spherical diameter of 5.4nm, and this value for the ATRA sensitized TiO_2 increases to 2.4nm, indicating a spherical diameter of 6.4nm. Such a size enlargement of TiO_2 nanoparticles suggests that there is a coating of ATRA on the TiO_2 surface, supporting the view that a monolayer of the dye has been attached to the surface of the TiO_2 nanoparticle.     

A new semi-empirical formula for total cross sections of electron scattering from triatomic molecules at 30—5000 eV

Total cross sections (TCSs) for electrons scattering from molecules over a wide energy range are widely used in gaseous lasers, astrophysics, plasma physics, atmospheric physics, and chemical physics, etc.. TCS values repro- duced by analytical or empirical expressions accurately are very useful in computational work related with these applications. Because electron-molecule scattering is a much more complex problem than the corresponding electron-atom scattering, in order to get more accura…     

In-situ transmission electron microscopy study of dislocation processes at precipitate-free zones in a γ ′-strengthened superalloy

In precipitation-strengthened polycrystals, precipitate-free zones (PFZs) often form along grain boundaries. These PFZs lower the yield strength. In this investigation, thin foils of the commercial γ′-strengthened nickel-based superalloy Nimonic PE16 have been strained inside a transmission electron microscope and the relevant dislocation processes in the PFZs and in the γ′-strengthened material next to them have been observed under load. Since the PFZs are only solid solution strengthened, they are softer than the interior of the γ′-strengthened grains. Many different slip systems are activated in the PFZs even at relatively low external stresses. Multiple slip allows for compatible deformation of neighbouring grains. Extensive cross-slip and double cross-slip in the PFZs lead to a high dislocation multiplication rate. Easy creation of dislocations in the PFZs and pile-ups at the border between the PFZs and the γ′-strengthened interior of the grains enhance the propagation of slip across grain boundaries and thus lower the yield strength of the material.     

Numerical simulation of dispersion generated by a 180° turn in a microchannel

A numerical scheme is developed to simulate electro-osmotic flow and mass transport in a microchannel which includes a 180° turn. The model has been used to predict the behaviour of electro-osmotically driven flows. The detailed structure of the flow field in a microchannel in combination with species mass diffusion can explain the concentration dispersion introduced by a 180° turn. The results of our simulations agree both qualitatively and quantitatively with experimental observation. It is demonstrated that an improved electro-osmotic force model could simulate the electrokinetically driven flow well without making detailed calculations of the electric charge density distribution within the electrical double layer. Additionally, because this model applies forces to the liquid as opposed to imposing local velocities, it should also be appropriate for use where pressure gradients exist in the flow field.     

Design of a MeV ultra-fast electron diffraction experiment at Tsinghua university

Time-resolved MeV ultra-fast electron diffraction (UED) is a powerful tool for structure dynamics studies. In this paper, we present a design of a MeV UED facility based on a photocathode RF gun at Tsinghua University. Electron beam qualities are optimized with numerical simulations, indicating that resolutions of 250 fs and 0.01 AA, and bunch charge exceeding 10⁵ electrons are expected with technically achievable machine parameters. Status of experiment preparation is also presented.     

Contribution of electron–magnon scattering to the spin-dependent Seebeck effect in a ferromagnet

We solve the Boltzmann’s transport equations for conduction electrons in a ferromagnet considering electron–magnon scattering. Such scattering gives rise to a spin dependent Seebeck coefficient, which in turn implies that the spin current can be generated by applying a temperature gradient. We estimate the temperature gradient required for switching a nano-magnet by using spin-transfer torque.     

A study on excitation of atmospheric gravity waves from annular solar eclipse

Utilizing the data of microbarometric waves on ground level,radiosonde reports and solar radiations obtained from various observation sta-tions in China during the annular solar eclipse on September 23,1987,themechanism of AGWs excited by the eclipse,the spectral characteristics and thelocal differences have been studied,and the results have been compared withthose from the ionospheric sounding at the same time.It turns out that therewere AGWs excited certainly during this annular solar eclipse,their periods areobviously in accordance with the duration of the fluctuation in the direct solarradiation.     

Pulsed photoconductivity in diamond upon quasi-continuous laser excitation at 222 nm at the formation of an electron–hole liquid

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm² upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm², sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.     

A study of cole diagrams of some alkali halide relaxation maxima appearing at temperatures above 100°C

Relaxation maxima occurring at a frequency of 1 kHz above 100°C in pure LiF, KCl, NaCl and CaCl₂-doped NaCl were investigated and an interesting fact, the dependence of the relaxational loss angle as well as the resp. dielectric constant on the sample thickness, was found and discussed.     

Simulation of multiple scattering in argon ion collisions with an aluminum target at grazing angles of 5°–10°

A model for calculating the atom scattering from a polycrystalline surface consisting of randomly oriented single crystals is proposed. In the calculation it is possible to distinguish the contribution of scattering from any surface layer, and it also takes into account inelastic energy losses in primary ion collisions with target atoms. A comparison with the measurements shows that the proposed model well describes the peak positions in the energy spectra of the scattered atoms. A comparison of the calculated and experimental peak-to-background ratios for scattering from polycrystalline and amorphous targets allows a conclusion to be made about the degree of order of surface layers in the material under investigation.     

Femtosecond laser ablation of metals:a molecular dynamics simulation study

Using molecular dynamics (MD) methods combining with two-step radiation heating model, the mechanisms of ablation and the thermodynamic states at Ni surface under femtosecond laser irradiation are investigated. Simulation results show that the main mechanisms of ablation are evaporation and tensile stresses generated inside the target. The velocity of stress wave is predicted to be nearly equal to sound velocity. The rates of ablation at different fluences obtained from simulations are in good agreement with experimental data.     

A quantum wavepacket study of three-dimensional Ne—H+ 2 scattering

The state-to-state and state-to-all reaction probabilities have been calculated for three-dimensional reactive and inelastic scattering of Ne + H⁺ ₂ (v = 0, j = 0, 1, 2) at zero total angular momentum. The time-dependent Schrödinger equation is solved by means of Fourier grid and discrete variable representation techniques. The inelastic and reactive scattering probabilities for a broad range of energies are calculated. Although the system has an open reactive channel, the results show that a high fraction of the flux is reflected back due to endoergicity and potential barriers leading to inelastic scattering.     

Lattice thermal conductivity of δ-graphyne — A molecular dynamics study

Thermal conductivity of δ-graphyne was investigated using reverse non-equilibrium molecular dynamics simulations. The dependence of the thermal conductivities with the temperature, acetylenic linkages, and external strain were explained by the phonon density of states. Our simulations revealed that as the temperature increased, the thermal conductivity of graphene first increased and then decreased, whereas that of δ-graphyne monotonically decreased. Owing to the presence of the acetylenic linkages, a significant reduction was found in the thermal conductivity of δ-graphyne, which resulted in a phonon vibration mismatch or weakened coupling. Moreover, the temperature profile changed from mono linear to the ladder the number of acetylenic linkages increased. These results play a guidance role in the design and application of thermoelectrics devices using 2D carbon materials.