Oscillations in the double-photoionization cross section of Li near threshold

The threshold region of the double-photoionization cross section of lithium was investigated using monochromatized synchrotron radiation and ion time-of-flight spectrometry. While the overall energy dependence can be described by the Wannier power law, we found oscillations in the cross section which are in good agreement with a modulated threshold law as proposed by Temkin [Phys. Rev. Lett. 49, 365 (1982)]]. This behavior may be due to the unequal binding energies of the electrons involved in the double-photoionization process.     

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.     

Experimental evidence for modulations in the relative double-photoionization cross section of C60 from threshold up to 280 eV

The relative double-photoionization cross section of neutral C60 clusters was investigated using monochromatized synchrotron radiation between 18 and 283 eV. Our measurement of the double-to-single photoionization ratio reveals two modulating components that are superimposed on a smooth ratio curve from threshold (19.0 eV) up to 280 eV, when inner-shell excitations become possible. The maxima in the modulation can be related to geometrical dimensions of the C60 cluster.     

一维库仑三体系统中电子碰撞导致的(e,2e)电离截面

用经典力学的方法,研究了三种不同类型的一维库仑三体模型在总能量大于零时(e,2e)电离截面与总能量的关系.发现了不同的模型的(e,2e)电离截面有不同的阈值行为;即s波模型,共线e^-Ze^-和Ze^-e^-模型的(e,2e)电离截面在阈值附近分别遵守线性律,Wannier定律和平方根律.还对所得数值结果作了理论分析. 关键词： 库仑三体系统 经典动力学 (e;2e) 电离截面     

Raman scattering of light by 2LO-phonons

A theory of resonance Raman scattering of light by 2LO-phonons in semiconductor crystals is presented. Wannier excitons are considered as intermediate states. Analytic expressions are obtained that take into account contributions from different chains of intermediate states. The scattering cross section is shown to be weakly dependent on the wave vector imparted to the phonon system. The theoretical model permits the calculation of the scattering cross section for the energy of the exciting radiation photon below the level of exciton resonances.     

Threshold Laws for Multiparticle Fragmentation

 The threshold behaviour of the cross section for break-up of an atomic particle into N charged fragments is described by a power law, , where E is the energy excess above the threshold. The threshold index μ reflects the dynamics of long-range Coulomb correlation. The general features of the theory are discussed. The charge and mass dependence of the threshold index is analyzed for some particular systems. Received October 29, 2001; accepted for publication November 9, 2001     

Measurements of the electron concentration and conductivity of a partially ionized inert gas plasma

A theoretical model for the resonant hyper-Raman scattering in semiconductor crystals is presented. The Wannier excitons are considered as intermediate states. The expressions for the resonant hyper-Raman scattering cross section are obtained. The theoretical model developed allows taking into account different mechanisms of the exciton-photon and exciton-phonon interactions.     

Near-threshold positron-hydrogen ionization

Ionization of hydrogen by positron impact near the breakup threshold is considered. The two-centre convergent close-coupling method is used. We find that convergence in the cross section is only practically obtained if two near-complete expansions are used, one centred around H and the other around Ps. The contribution to the breakup cross section becomes the same for both centres as the threshold is approached. The calculations are found to be in good agreement with the threshold law predicted by Ihra et al. (1997) Phys Rev Lett 78:4027.     

Modern methods for calculating photoionization and electron-impact ionization of two-electron atoms and molecules

A number of recently developed methods for calculating multifold differential cross sections for photoionization and electron impact ionization of atoms and molecules with two active electrons are reviewed. The methods are based on unique approaches to the calculation of three-body Coulomb wave functions. The exterior scaling method and the driven Schrödinger equation formalism are considered. The effectiveness of the time-dependent approaches to the scattering problem, such as the paraxial approximation and the time-dependent scaling, is demonstrated. A novel numerical method is formulated, which has been developed by the authors to solve the six-dimensional Schrödinger equation for an atom with two active electrons on the basis of the Chang-Fano transformation and the discrete variable representation. The threshold behavior manifested by the angular distributions of the two-electron photoionization of a negative hydrogen ion and a helium atom and the multifold differential cross sections for the electronimpact ionization of hydrogen and nitrogen molecules are analyzed on the basis of numerical simulations. The Wannier law for the angular distribution of double ionization is demonstrated to be incorrect even at very low energies.     

Exponential decay properties of Wannier functions and related quantities

The spatial decay properties of Wannier functions and related quantities have been investigated using analytical and numerical methods. We find that the form of the decay is a power law times an exponential, with a particular power-law exponent that is universal for each kind of quantity. In one dimension we find an exponent of -3/4 for Wannier functions, -1/2 for the density matrix and for energy matrix elements, and -1/2 or -3/2 for different constructions of nonorthonormal Wannier-like functions.     

A comprehensive description of the photon structure function and photon-photon total cross section data

We fit the photon structure function and photon-photon total cross section obtained from two photone ⁺ e ^? experimental data. We show that a combination of the QPM point-like cross section and an hadronic cross section, given by a simple energy power expansion modified by a threshold factor, is in excellent agreement with the data.     

Cw oscillation at 1.05 and 1.32 μm of LiNd(PO3)4 lasers in external resonator and in resonator with directly applied mirrors

The room temperature cw oscillation at 1.05 and 1.32 μm of lithium neodymium tetraphosphate [LiNd(PO₃)₄, LNP] lasers in an external resonator and, for the first time, also in a Fabry-Perot resonator with directly applied mirrors is reported. The growth and fluorescence properties of the LNP crystals are briefly described. Laser parameters such as threshold pump power, output power, differential efficiency, optical gain, emission cross section and laser modes are both measured and calculated. Laser experiments show the threshold pump power to be lower for 1.05 μm than for 1.32 μm emission because of the smaller emission cross section; the pump power density is lower for the external resonator than for the Fabry-Perot resonator because of its higher mode losses.     

Threshold behavior of e-H ionizing collisions

We present accurate ab initio numerical solutions of the full Schr?dinger equation for the electron-impact ionization of hydrogen near threshold using the propagating exterior complex scaling method. They provide strong support for the Wannier threshold law [Phys. Rev. 90, 817 (1953)], giving sigma proportional to E(1.122+/-0.015), and also give the energy dependence of the electrons' angular distribution as (pi-theta12) FWHM approximately 3.0E(1/4), in general agreement with classical and semiclassical predictions.     

Near-threshold positron-impact ionization of atomic hydrogen

We consider the positron-impact ionization (breakup) of atomic hydrogen utilizing the full and S-wave model calculations, concentrating on the near-threshold energy region. Unlike the corresponding electron-impact case, the S-wave model does support the Wannier-like threshold law predicted by Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)10.1103/PhysRevLett.78.4027]. It is found that convergent S-wave model cross sections are obtained only if complete expansions are utilized on both the atomic and the positronium centers. Furthermore, we suggest that, in the model and full calculations, the separate contributions to the breakup cross section from both centers become equal at threshold.     

Ab initio calculation of double ionization of atoms

The Solov’ev-Vinitsky method was used to perform an ab initio calculation of the triple-differential cross section for the double single-photon photoionization of helium for the case of equal emitted-electron energies. A Gaussian width γ describing angular electron-electron correlations at the total electron energy E taking values in range between 0.1 and 100 eV was obtained for this cross section. The results agree with available experimental data, but they raise a doubt as to whether the well-known Wannier law γ ∝ E ^1/4 is applicable at experimentally accessible energies. The Gaussian width γ was investigated as a function of the total emitted-electron energy for targets that have a strongly asymmetric configuration of the initial state—specifically, a negative atomic-hydrogen ion H^? and heliumin the 1s2s ¹ S and 1s3s ¹ S excited states. It was found that this function, γ(E), had a maximum at low energies. It was also shown that, at low energies, the dependence of the double-differential cross section on the angle between the emitted-electron momenta for the targets indicated above differed substantially from the Gaussian dependence, featuring maxima whose number was equal to the number of radial nodes in the initial state. This opens new possibilities for a qualitative analysis of the electron structure of targets.     

Scaling law in the electrodisintegration of deuterons around 1 (GeV/c)2

Starting from ?t around 1 (GeV/c)², the electrodisintegration cross section of the deuteron near the threshold is shown to follow a scaling law when the contributions from the nucleon form factors are properly treated.     

The nonlinear Wannier function in square Kronig-Penney model

The nonlinear Wannier functions in square Krnig-Penney potential are investigated with the help of a set of exact nonlinear Bloch solutions. The nonlinear interaction makes the Wannier functions fall off as non-exponential law with distance and enhances the tunneling coupling between the neighbor sites.     

Photodetachment of H- near an elastic surface in a magnetic field

This paper investigates the photodetachment of the negative hydrogen ion H near an elastic wall in a magnetic field.The magnetic field confines the perpendicular motion of the electron,which results in a real three-dimensional well for the detached electron.The analytical formulas for the cross section of the photodetachment in the threedimensional quantum well are derived based on both the quantum approach and closed-orbit theory.The magnetic field and the elastic surface lead to two completely different modulations to the cross section of the photodetachment.The oscillation amplitude depends on the strength of the magnetic field,the ion-wall distance and the photon polarization as well.Specially,for the circularly polarized photon-induced photodetachment,the cross sections display a suppressed(E E th) 1/2 threshold law with energy E in the vicinity above Landau energy E th,contrasting with the(E E th) 1/2 threshold law in the presence of only the magnetic field.The semiclassical calculation fits the quantum result quite well,although there are still small deviations.The difference is attributed to the failure of semiclassical mechanics.     

内F-P腔条波导光双稳器件

首次在CdSλSe1-x玻璃上成功地研制出内F—P腔结构条型波导光双稳器件.实现了低功耗和超快速本征光双稳运转.