Photoemission and ionization of He+ under simultaneous irradiation of fundamental laser and high-order harmonic pulses

We theoretically study the response of He+ exposed simultaneously to an intense Ti:sapphire laser and its 27th or 13th harmonic pulses. High-order harmonic emission from He+ is enhanced by many orders of magnitude compared with the case of the fundamental pulse alone. Moreover, while an individual 10 fs laser (wavelength lambda(F)=800 nm) or its 27th harmonic pulse with a peak intensity of 3 x 10(14) and 10(13) W/cm(2), respectively, ionizes no more than 5 x 10(-6) of He+, their combined pulse leads to a surprisingly high He2+ yield of 17%.     

On the feasibility of performing non-linear autocorrelation with attosecond pulse trains

We present experimental results in which a second-order effect, namely two-photon ionization of atomic He induced by a superposition of harmonics, is observed. The harmonics are generated in a Xe gas-jet using a 790-nm 10-Hz femtosecond Ti:sapphire laser and are subsequently focused into a He gas-jet with a Kirkpatrick–Baez arrangement. The superposition is formed by using a thin In filter and it comprises the 7th to 13th harmonics. Solving the time-dependent Schrödinger equation for He in a polychromatic laser field, the He ⁺ ion yield is calculated as a function of the total XUV intensity. Using the calculated yield and taking into account the focusing and transmission properties of the arrangement, the number of He ⁺ ions produced per laser pulse is estimated and is found to be in reasonable agreement with its measured value. The total number of ions produced non-resonantly follows a nearly quadratic dependence on the harmonic intensity, thus establishing the feasibility of a second-order auto-correlation measurement of the superposition of harmonics, i.e., of a direct temporal characterization of attosecond pulse trains.     

SIMP钢中H对He热解吸行为的影响研究

为了初步理解核用结构材料中H对He行为的影响,以He离子单独辐照和He和H离子连续辐照作为对比,利用热释放谱(TDS)、透射电子显微镜(TEM)和扫描电子显微镜(SEM)研究了SIMP中H对He的热解吸和滞留行为的影响。TDS结果表明:He释放的主峰主要出现在1 198~1 222 K之间,对应于气泡的迁移释放机制。相对于He单独辐照,H的附加辐照使得He的释放峰向低温移动,且释放量增大。即H促进了He的热解吸。另外,H对He热解吸的促进作用与H的辐照剂量有关。当H注入的峰值浓度(原子分数)从5%增加到50%时,这种促进作用有所减弱。结合TEM和SEM结果发现:H的存在促进了TDS加热过程中材料表面的起泡行为,从而加速了He以气泡迁移机制释放的过程。     

氦原子(e,2e)反应中末通道屏蔽效应的理论研究

在Chen等人工作(2001,Chin. Phys. 10 290)的基础上,进一步对(e,2e)反应的末态He+中核外电子的有效屏蔽给出修正.并用修正后的索末菲参量计算了入射能为40 eV时,共面不对称几何条件下电子离化He原子的三重微分截面.所得结果与其它理论结果及最新绝对测量的实验数据进行比较发现:所得到的理论曲线更接近实验数据.     

Ab Initio NCSM/RGM for Three-Body Cluster Systems and Application to 4He+n+n

We introduce an extension of the ab initio no-core shell model/resonating group method (NCSM/RGM) in order to describe three-body cluster states. We present results for the ⁶He ground state within a ⁴He+n+n cluster basis as well as first results for the phase shifts of different channels of the ⁴He+n+n system which provide information about low-lying resonances of this nucleus.     

Transfer ionization in energetic \alpha-He collisions

Total cross sections σ_if for transfer ionization α + He → He⁺ + α + e are computed at high energies by means of the four-body continuum distorted wave (CDW-4B) approximation. Comparisons between the present results σ_if and the available experimental data at E = 30-600 keV/amu yield satisfactory agreement at impact energies E > 80 keV/amu, in sharp contrast to the previous independent event model. This revised version was published online in August 2006 with corrections to the Cover Date.     

Equation of state of solid 3He

We present results of diffusion Monte Carlo calculations for the bcc and hcp phases of solid 3He, using a recent ab initio interatomic potential, including two- and three-body terms. This potential is found to yield an equation of state for condensed 4He in excellent agreement with experiment, in a wide density range. For 3He, we find a systematic discrepancy, worth 0.7 K, between our computed equation of state and a commonly accepted experimental one. We attribute such a discrepancy to an improper choice of reference energy in the determination of the experimental equation of state.     

电子碰撞激发氢原子和氦离子散射微分截面的计算

本文详细介绍计算电子碰撞激发散射截面的扭曲波玻恩近似（DWBA）理论模型,并对低能DWBA模型进行修正。利用修正的DWBA模型计算了电子碰撞激发氢原子和氦离子1s-2s 和1s-2p的散射微分截面。将关于氢原子由基态到n=2态的电子碰撞激发散射微分截面与绝对实验测量数据比较,发现二者符合得很好,这验证了我们对DWBA修正的正确性。本文工作为拟合强场诱导的氦原子非序列双电离关联电子动量谱提供了有效的理论方法。     

Correlation between the fragmentation modes and light charged particles emission in heavy ion collisions

The correlation between the shape of rapidity distribution of the yield of light charged particles and the fragmentation modes in semi-peripheral collisions for70Zn+70Zn,64Zn+64Zn and64Ni+64Ni at the beam energy of 35 Me V/nucleon is investigated based on Im QMD05 code. Our studies show there is an interplay between the binary, ternary and multi-fragmentation break-up modes.The binary and ternary break-up modes more prefer to emit light charged particles at middle rapidity and give larger values of Rmid yieldcompared with the multi-fragmentation break-up mode does. The reduced rapidity distribution for the normalized yields of p, d, t,3He,4He and6 He and the corresponding values of Rmid yieldcan be used to estimate the probability of multi-fragmentation break-up modes. By comparing to experimental data, our results illustrate that 40% of the collisions events belong to the multifragmentation break-up mode for the reactions we studied.     

利用椭圆流和轻带电粒子产额比探测对称能（英文）

基于最近更新的极端相对论量子分子动力学模型,研究了197Au+197Au产生的中子和氢同位素的横动量依赖的椭圆流和轻带电粒子的产额比。发现中子和氢同位素的椭圆流之比vn H2/v2敏感于对称能的高密行为,而3H和3He的产额比敏感于对称能的低密行为。通过比较FOPI/LAND和FOPI合作组的最新实验数据与模型模拟的结果,发现接近线性对称能的Skyrme参数给出的结果能与这两个实验值都符合。     

35MeV/u~(40)Ar ~(197)Au中的熵产生

根据量子统计模型 (QSM )的计算分析 ,找到了一个提取核反应过程中熵产生的新的可观测量 .核反应过程中约化d的产额d/ (d t 3 He 4 He)和熵有单调的函数关系 ,并且和体系的碎裂密度 (ρ/ ρ0 )及体系的N/Z都无关 ,可以作为提取核反应过程中熵产生的一个观测量 .和目前已经有的其他方法相比 ,约化d产额这一提取熵方法可以用于较低能量的重离子核反应中 ,并且数据处理分析简单 .对于 35MeV/u4 0 Ar 197Au的核反应过程所提取的熵和利用约化带电粒子多重性提取的熵结果一致 .结合后角类靶热核发射体系实验提取的同位素核温度为 4 7±1 2MeV及S/A =2 5± 0 5 ,根据熵和核温度的关联关系 ,可以确定其Breakup密度接近但小于 0 1(ρ/ ρ0 )     

ASYMMETRY PARAMETERS AND DIFFERENTIAL CROSSSECTIONS FOR PHOTOIONIZATION-EXCITATION OF HELIUM

Asymmetry parameterβ2 for photoionization of helium leaving the He+ ion in the n = 2 level and differential cross sections(DCSs) in the n = 2, 3, 4 levels with photon emission angle θ=90°are provided at photon energies 69－76.8 eV employing the R-matrix method with a 20-term target representation, in which five polarization orbitals 6l are included. The asymmetry parameter β2 and the DCS, in the n=2 level in the region of 69－73eV, are in good agreement with available experimental and theoretical results. Above the n = 3 threshold, we present new theoretical results along with measurements. No theoretical and experimental results can be used to compare with the present calculations of the DCSs for photoionization into the He+ ion in the n = 3, 4 levels.     

Crystal effects in the neutralization of He+ ions in the low energy ion scattering regime

Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.     

Mass measurements of few-electron systems in Penning traps

The accuracy of the SMILETRAP mass spectrometer has been verified by a number of mass comparisons involving well-known masses. Our results for H₂ ⁺,Ne⁶⁺,Ne^{9+ ,10+},Si^{12+ ,13+ ,14+},and Ar^{14+ ,16+} all agree within the statistical errors (0.3–1 ppb) with previous determinations. However, all measurements involving He give a deviation. The combined He^1+,2+ data results in a mass deviation of +1.9 ±0.23 ppb. The uncertainty of the accepted He mass is 0.25 ppb, thus this represents a significant deviation. High statistics comparisons (statistical uncertainty <0.5 × 10^-9utilizing different species (excluding He) and charge states agree within ±0.5 ppb. An analysis estimating the contribution from individual systematic error sources and other auxiliary tests does not allow a systematic error larger than ± 0.85 ppb. We conclude that for now we cannot rule out the presence of an unknown systematic error which in the He comparison results in a near 2 ppb deviation. Thus, as a safety measure we should exclude the He data when calculating the proton mass. The He discrepancy also forces us to give a larger limit of the systematic error of the proton mass than motivated by high statistics comparisons. However, due to the consistency of all other measurements and tests, it appears unlikely that this deviation should be present to the same extent in other comparisons. Thus, for now, after a preliminary analysis we report a proton mass = 1.007 276 466 72 ± 16 ± 85 u, where the errors are the weighted statistical errors and the estimated maximal systematical error, respectively. After a complete analysis we expect the systematic error to be reduced below ±0.5 ppb. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crossed beam photodissociation imaging of HeH+ with vacuum ultraviolet free-electron laser pulses

Molecular photofragmentation has been studied by event imaging on HeH+ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into He(1snl,n > or = 2)+H+ was observed to yield significant photodissociation at 32 nm with an absolute cross section of (1.4+/-0.7) x 10(-18) cm2, releasing energies of 10-20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.     

Further property of Lennard-Jones fluid: Thermal conductivity

In this paper, we calculate the thermal conductivity of noble gases, methane, and three noble gas mixtures including He+Kr, He+Xe, and Kr+Xe assuming they obey Lennard-Jones (LJ) (12-6) model potential. One of the required quantities to calculate the thermal conductivity of these systems is the pair correlation function. Therefore, we solve numerically the Ornstein-Zernike (OZ) integral equation using the mean spherical approximation (MSA) to obtain the pair correlation functions. We use these functions to obtain the thermal conductivity, then compare our results with the available data. According to the results obtained from the present work for pure and mixtures of LJ fluids reveals that the integral equations method is suitable for predicting the thermal conductivity of this class of fluid.     

Yield ratios and directed flows of light fragments from reactions induced by neutron-rich nuclei at intermediate energy

The yield ratios of neutron/proton and ~3H/~3He and the directed flow per nucleon for these projectile-like fragments at large impact parameters are studied for ~(50)Ca + ~(40)Ca and ~(50)Cr + ~(40)Ca for comparison at 50 MeV/u using the isospin-dependent quantum molecular dynamics(IQMD) model.It is found that the yield ratios and the directed flows per nucleon are different for reactions induced by the neutron-rich nucleus ~(50)Ca and the stable isobaric nucleus ~(50)Cr,and depend on the hardness of the EOS.The ratios of neutron/proton and ~3H/~3He and the difference of directed flow per nucleon of neutron-proton are suggested to be possible observables to investigate the isospin effects.     

He_2~+团簇结构和解析势能函数的从头计算研究

采用从头计算的耦合簇方法CCSD(T)和He原子Dunning’s相关调和基函数组对He+2 团簇的结构参数、势能曲线进行计算。利用Murrell Sorbie函数和最小二乘法拟合出了解析势能函数 ,并以此为基础计算出光谱常数。通过比较发现 :分子结构和光谱常数计算结果均与实验值符合良好 ,优于文献报道的结果。说明本文所得势能函数解析表达式准确反映了分子中原子间相互作用 ,可用于更广泛的研究     

中等能量入射的质子,α粒子与氦原子碰撞的电荷转移

将最近引入的渐近两态原子轨道展开方法用来研究双电子碰撞体系H⁺+He和He²⁺+He的电荷转移过程．结果表明,由该方法得到的电子俘获截面在中高能区与满足边界条件的一级波恩近似（即B1B近似）和实验结果符合得很好,并且在中低能区,比B1B方法有很大改善,然后指出了由一级微扰理论所得结果在中低能区与实验结果不符的原因． 关键词：     

Evidence for stratification of deuterium-tritium fuel in inertial confinement fusion implosions

Measurements of the D(d,p)T (dd) and T(t,2n)(4)He (tt) reaction yields have been compared with those of the D(t,n)(4)He (dt) reaction yield, using deuterium-tritium gas-filled inertial confinement fusion capsule implosions. In these experiments, carried out on the OMEGA laser, absolute spectral measurements of dd protons and tt neutrons were obtained. From these measurements, it was concluded that the dd yield is anomalously low and the tt yield is anomalously high relative to the dt yield, an observation that we conjecture to be caused by a stratification of the fuel in the implosion core. This effect may be present in ignition experiments planned on the National Ignition Facility.