Coupled-channel optical calculation for positron—lithium scattering

Positron scattering with atomic lithium is investigated by using a coupled-channel optical method.The ionization continuum and positronium formation channels are taken into account via a complex equivalent-local optical potential.The positronium formation cross sections and the ionization cross sections,as well as the total scattering cross sections,are reported at energies above 3 eV and compared with available experimental and theoretical data.     

Elastic and inelastic positron-helium scattering

Elastic and inelastic positron--helium scatterings have been investigated with the coupled-channel optical method (CCO). Ionization continuum and positronium formation channels are included via a complex equivalent-local optical potential. Calculations are reported of cross sections of elastic scattering, total excitation and n=2, 3, and 4 excitations of ground-state helium for incident energies from 30~eV to 400~eV. The present calculation shows that the ionization and Ps-formation channels significantly affect the cross sections of elastic and inelastic positron--helium scatterings.     

Ionization in Ps–H scattering

Coulomb–Born approximation has been used to study three different types of ionization in positronium (Ps) and hydrogen scattering. The present Ps–H system contains four centers; we have considered all the Coulomb interactions and neglected exchange assuming the fact that at relatively higher energies above thresholds, the effect of exchange is not so important. All the important target elastic and inelastic channels are included. The non-unitarity problem in the target elastic channels, discussed by McAlinden et al. in 1996 in their positronium–atom scattering studies is overcomed in the present calculation using the orthonormality properties of target wavefunctions and a simple algebra. Comparative studies are made among ionization cross sections with different target excitations and different Ps-excitations together with summed Ps-ionization, summed H-ionization, both-ionization, summed excitation, total ionization and total cross sections. The present Letter indicates the importance of all the three different types of ionization and the importance of target elastic channels in Ps–H scattering.     

Positron scattering and ionization of neon atoms—theoretical investigations

Although positron scattering with inert gas atoms has been studied in theory as well as in experiment, there are discrepancies. The present work reports all the major total cross sections of e+-neon scattering at incident energies above ionization threshold, originating from a complex potential formalism. Elastic and cumulative inelastic scatterings are treated in the complex spherical e+-atom potential. Our total inelastic cross section includes positronium formation together with ionization and excitation channels in Ne. Because of the Ps formation channel it is difficult to separate out ionization cross sections from the total inelastic cross sections. An approximate method similar to electron-atom scattering has been applied to bifurcate ionization and cumulative excitation cross sections at energies from threshold to 2000 eV. Comparisons of present results with available data are made. An important outcome of this work is the relative contribution of different scattering processes, which we have shown by a bar-chart at the ionization peak.     

Effect of the adiabatic process on the absorption in heavy-ion scattering

We have succeeded in obtaining the optical model potential for ¹²C---¹²C scattering at low incident energies (10 MeV E_cm 20 MeV), by applying the adiabatic approximation to the coupled channels equation. As for this potential inelastic channels of single and mutual excitations of ¹²C^*(2⁺, 4.43 MeV) make a contribution to the real part and not to the imaginary part.     

Investigation of a channeling high-intensity laser beam inunderdense plasmas

The interaction of an intense short pulse laser (>5×10 ¹⁸ Wcm^-2) with underdense plasma was extensively studied. The beam is found to be highly susceptible to the forward Raman scattering instability. At sufficiently high growth rates, this can lead to wavebreaking with the resultant production of a high flux of accelerated electrons (>10¹¹ for E>2 MeV). Some electrons are found to be accelerated well above the dephasing energy, up to 94 MeV. Self-scattered images intimate the presence of high-intensity channels that extend more than 3.5 mm or 12 Rayleigh lengths. These filaments do not follow the axis of laser propagation, but are seen to be emitted within an f4 cone centered around this axis. Spectra of the self-scattered light show that the main contribution of the scattering is not from light captured within these filaments. But there is evidence for self-phase modulation from effects such as ionization and relativistic self-focusing. However, no clear correlation is observed between channel length and the number or energies of accelerated electrons. Evidence for high intensities within the channels is given by small-angle Thomson scattering of the plasma wave generated therein, with this method, the intensity is found to be of the order of 10¹⁸ Wcm^-2 greater than 12 Rayleigh lengths from focus     

Unnatural parity resonance states in positron-excited hydrogen scattering

<正>The coupled-channels optical method for positron scattering has been applied to investigate resonance states with unnatural parities in a positron-excited hydrogen system.The positronium formation channels and continuum channel are included via a complex equivalent local potential.Resonance states with angular momenta L = 1 to L = 2 and parities（—1）^L+1 are calculated.Resonance energies and widths are reported and compared with other theoretical calculations.We found that the opening positronium formation channels play an important role in forming nondipole Feshbach resonances.     

CH_3I~(2+)的二体、三体解离过程的理论研究

采用密度泛函和耦合簇理论从过渡态的观点对CH_3I~(2+)离子的解离过程进行了计算和分析.优化得到了CH_3I,CH_3I+和CH_3I~(2+)的稳定结构及解离过渡态的几何构型并给出了相应能量,计算的第一、二电离能与实验结果符合.基于母体离子、过渡态和解离碎片的几何结构和能量,对CH_3I~(2+)的两体解离过程和三体解离过程进行了详细分析和讨论.给出了二体、三体解离的可能解离通道,并分析了实验上观测到的解离碎片离子的产生机制.计算结果表明,三重和单重绝热势能面上的一些解离过程表现出明显的差异.     

Experimental investigations of positron differential elastic scattering

A review of recent elastic differential cross section (DCS) measurements for positronatom scattering is presented. Most of the earlier measurements are relative DCS determinations which only enable the shapes of DCS curves (versus angle at a given energy) to be compared with theory, while the most recent efforts in Bielefeld and Detroit are directed toward determining absolute DCS values such that magnitudes as well as shapes can be compared with theory. For positron-argon scattering, reasonable agreement has been found between experiment and theory at 300 eV and below 9 eV, while between these energies, significant differences exist, which raises interesting questions concerning possible cross-channel-coupling effects where other scattering channels (e.g. positronium formation and/or ionization) may be having appreciable effects on the elastic scattering channel.     

Coupled channel description of O+Nd scattering around the Coulomb barrier using a complex microscopic potential

Angular distributions of elastic scattering and inelastic scattering from 2⁺₁ state are measured for ¹⁶O+^142,144,146Nd systems at several energies in the vicinity of the Coulomb barrier. The angular distributions are systematically analyzed in coupled channel framework. Renormalized double folded real optical and coupling potentials with DDM3Y interaction have been used in the calculation. Relevant nuclear densities needed to generate the potentials are derived from shell model wavefunctions. A truncated shell model calculation has been performed and the calculated energy levels are compared with the experimental ones. To simulate the absorption, a ‘hybrid’ approach is adopted. The contribution to the imaginary potential of couplings to the inelastic channels, other than the 2⁺₁ target excitation channel, is calculated in the Feshbach formalism. This calculated imaginary potential along with a short ranged volume Woods–Saxon potential to simulate the absorption in fusion channel reproduces the angular distributions for ¹⁶O+¹⁴⁶Nd quite well. But for ¹⁶O+^142,144Nd systems additional surface absorption is found to be necessary to fit the angular distribution data. The variations of this additional absorption term with incident energy and the mass of the target are explored.     

Total cross sections of positronben sodium scattering at low energies

A new calculation for the total cross section of positron-sodium scattering is performed in an energy range down to a few tenths of one electron volt using the coupled-channel optical method. The ionization continuum and the positronium formation channels are included via an equivalent-local complex potential. The role played by the break-up and rearrangement processes in the low energy positron-sodium scattering is also investigated. The total scattering cross section is reported and compared with the available theoretical and experimental data.     

Total and ionization cross-sections of N<Subscript>2</Subscript> and CO by positron impact: Theoretical investigations

In this paper we report our calculations on several important total cross-sections (TCSs) of positron impact on isoelectronic N₂ and CO molecules, treated in the complex spherical potential formalism. Basically the total (complete) cross-section Q _T consists of elastic and inelastic contributions. Our total inelastic cross-section (Q _inel) contains ionization and electronic excitations together with positronium formation. Our goal here is to bifurcate Q _inel further to deduce total ionization cross-section, using the ‘complex scattering potential–ionization contribution’ (CSP-ic) method of electron–atom/molecule scattering. The present range of positron energy is 15–2000 eV. All the resulting cross-sections are in a good general accord with the existing data. This work highlights the importance of various scattering channels in e⁺-N₂ and e⁺-CO interactions at intermediate and high energies.     

Decay Analysis of Ge Isotopes Formed in Reactions Induced by Tightly and Loosely Bound Projectiles

The dynamical cluster-decay model (DCM) is employed to investigate the decay of ^68,70Ge^* compound nuclei formed respectively via tightly (⁴He) and loosely (⁶He) bound projectiles, using ⁶⁴Zn target. The study is carried out over a wide energy range (E_c.m.~5 MeV to 16 MeV) by including the quadrupole deformations (β2i) and optimum orientations (θ_i^opt) of the decaying fragments. The fusion cross-sections, obtained by adding various evaporation channels show nice agreement with the experimental data for ⁴He+⁶⁴Zn reaction. The contribution from competing compound inelastic scattering channel is also analyzed particularly for ⁶⁸Ge^* nucleus at above barrier energies. On the other hand, the decrement in the fusion cross-sections of ⁷⁰Ge^* nuclear system is addressed by presuming that ⁶⁵Zn ER is formed via two different modes:(i) the αn evaporation of ⁷⁰Ge^* nucleus, and (ii) 1n-evaporation of ⁶⁶Zn^* nuclear system, formed via breakup and 2n-transfer channels due to halo structure of the ⁶He projectile. Besides this, the suppression in 2np evaporation cross-sections suggests the contribution of another breakup and transfer process of ⁶He i.e. ⁴He+⁶⁴Zn. The contribution of breakup+transfer channels for ⁶He+⁶⁴Zn reaction is duly addressed by applying relevant energy corrections due to the breakup of " ⁶He" projectile into 2n and ⁴He. In addition to this, the barrier lowering, angular momentum and energy dependence effects are also explored in view of the dynamics of chosen reactions.     

Calculating the Relativistic Wave Functions of 11S0 and 13S1 Positronium

Based on the relativistic Bethe-Salpeter (BS) equation, the positronium wavefunctions in Euclidean momentum space are obtained. Meanwhile the energy levels of positronium ground states 1¹S₀ and 1³S₁ are fitted to be 6.7934 eV and 6.7929 eV respectively, which qualitatively agree with the previous theoretical values. It is shown that the BS theory is valid and reliable to treat positronium.     

Elastic and inelastic scattering of neutrons from Fe and Cu : (II). Scattering and nuclear structure effects from coupled channels calculations

The differential cross sections for neutron scattering from ^{54, 56}Fe and ^{63, 65}Cu at energies between 8 and 14 MeV, which were reported in a companion paper, have been analyzed in the framework of the coupled channels formalism. Neutron potential deformations were extracted and compared with corresponding deformations deduced from the reanalysis of proton scattering measurements and Coulomb excitation results. The size of the Coulomb correction term in the absorptive proton potential is discussed. Matter root-mean-square radii are estimated from our potential and compared with results from -particle folding model calculations.     

Isobar doorway approach to pion scattering on open shell nuclei

Delta-hole approach developed for the pion scattering on closed shell nuclei is extended to be applied to the case of open shell nuclei. The doorway space is spanned by the states of the form |Ψ^(A−1)ψ^(Δ) which correspond to |Δ h states in the usual delta-hole approach for closed shell nuclei. In this approach, the effects of the couplings between elastic and inelastic channels are incorporated in a natural way. As a first application of this approach, some numerical calculations have been done for the elastic and inelastic pion scattering on ¹²C and the elastic scattering on ¹⁴N and ¹⁴C. The effects of the nuclear structure and the coupling between elastic and inelastic channels are discussed.     

NEW AUTOIONIZING STATES NEAR THE FIRST IONIZATION LIMIT OF RARE EARTH ELEMENT Dy

This paper describes the investigation of autoionizing states near the first ionization limit of rare-earth element Dy. The 62, 51, 97 and 25 new autoionizing states resulting from the states of 4f¹⁰(⁵I₈)6s6p(³P⁰₂)³I⁰₇, 4f⁹ (⁶H⁰)5d²(³F)(⁸G⁰)6s⁹G⁰₇, 4f⁹(⁶F⁰)5d6s^{2 7}H⁰₇ and 4f⁹5d6s^{2 7}K⁰₇, respectively, were found by using a laser resonance ionization time-of-flight mass spectrometer (LRI-TOFMS), The photoionization efficiencies for different channels were compared with each other. In addition, the Shore-Fano parameters of autoionizing states were determined by a nonlinear fitting program.     

The role of excitons and substrate temperature in low-energy (5–50 eV) electron-stimulated dissociation of amorphous D2O ice

We have studied the interaction of low-energy (5–50 eV) electrons with nanoscale (10 ML) ice films by probing the yields and quantum-state distributions of the neutral dissociation products using laser resonance-enhanced multiphoton ionization spectroscopy. In particular, we have observed the electron-stimulated desorption (ESD) of D (²S), O (³P₂) and O (¹D₂) from amorphous D₂O films. These products are observed at threshold energies (relative to the vacuum level) between 6.5–7 eV and desorb with low kinetic energies (60–85 meV) which are independent of the incident electron energy. We associate the ESD of atomic fragments from ice with dissociation of Frenkel-type excitons of 4a₁ character which are near the bottom of the ice conduction band. These excitons are created either directly or via electron-ion recombination. Changing the surface temperature from 88 to 145 K results in an increase in the thermal component of the time-of-flight (kinetic energy) distributions and an overall increase in the neutral fragment yield. We suggest that the change in neutral yield with substrate temperature results from a combination of: (1) increased electron-ion recombination; (2) exciton transport to the near-surface region; and (3) dissociation followed by inelastic scattering and desorption.     

icMRCI+Q理论研究CF离子12个-S态和23个?态的光谱性质

采用考虑Davidson修正的内收缩多参考组态相互作用(icMRCI+Q)方法结合相关一致基组aug-ccpV5Z和aug-cc-pV6Z计算了CF~+离子第一离解极限C~+(~2p_u)+F(~2P_u)对应的12个∧-S态(X~l∑~+,a~3∏,1~3∑~+,1~3Δ,1~1∑~-,1~1∑~-,1~3∑~-,2~1∑~+,1~1∏,2~3∏,2~1∏和2~3∑~+)所产生的23个Ω态的势能曲线.计算中考虑了旋轨耦合效应、核价相关和标量相对论修正以及将参考能和相关能分别外推至完全基组极限.基于得到的势能曲线,获得了束缚和准束缚的9个∧-S态和16个Ω态的光谱常数,并且X~1∑~+,a~3∏~(势阱一)∧-S态的光谱常数与已有的实验结果非常符合.此外,计算了CF自由基X~2∏态到CF~+离子束缚和准束缚的9个∧-S态的垂直电离势和绝热电离势,并且CF~+(X~1∑~+)←CF(X~2∏)和CF~+(a~3∏(势阱一))←CF(X~2∏)的垂直电离势和绝热电离势与相应的实验结果也非常符合.由a~3∏,1~1∏态和其他激发∧-S态势能曲线的交叉现象,借助于计算的旋轨耦合矩阵元,分析了a~3∏~(势阱一),1~1∏~(势阱一)和2~1∑~+态的预解离机理.计算的23个Ω态离解极限处的相对能量与实验结果十分吻合.最后计算了(2)0+~(势阱一)(v'=0-5),(1)1~(势阱一)(v'=0-5)和(2)1~(势阱一)(v'=0)到X0~+态跃迁的Franck-Condon因子和辐射寿命.