Interference in the collective electron momentum in double photoionization of H2

We investigate single-photon double ionization of H(2) by 130 to 240 eV circularly polarized photons. We find a double slitlike interference pattern in the sum momentum of both electrons in the molecular frame which survives integration over all other degrees of freedom. The difference momentum and the individual electron momentum distributions do not show such a robust interference pattern. We show that this interference results from a non-Heitler-London fraction of the H(2) ground state where both electrons are at the same atomic center.     

Laser-induced interference, focusing, and diffraction of rescattering molecular photoelectrons

We solve the time-dependent Schr?dinger equation in three dimensions for H+2 in a one-cycle laser pulse of moderate intensity. We consider fixed nuclear positions and Coulomb electron-nuclear interaction potentials. We analyze the field-induced electron interference and diffraction patterns. To extract the ionization dynamics we subtract the excitations to low-lying bound states explicitly. We follow the time evolution of a well-defined wave packet that is formed near the first peak of the laser field. We observe the fragmentation of the wave packet due to molecular focusing. We show how to retrieve a diffraction molecular image by taking the ratio of the momentum distributions in the two lateral directions. The positions of the diffraction peaks are well described by the classical double slit diffraction rule.     

Two- and three-body kinematical correlation in the dissociative recombination of H(3)(+)

Fragmentation patterns for dissociative recombination of the triatomic hydrogen molecular ion H(3)(+) in the vibrational ground state have been measured using the storage ring technique and molecular fragment imaging. A broad distribution of vibrational states in the H(2) fragment after two-body dissociation and a large predominance of nearly linear momentum geometries after three-body dissociation are found. The fragmentation results are directly contrasted with Coulomb explosion imaging data on the initial H(3)(+) geometry, compared to existing wave-packet calculations, and considered in the light of a simple physical picture.     

强激光场中H2^＋离解行为的研究

在多电子态模型下，利用二阶劈裂传播子算法计算了Ｈ２＾＋在强激光场中的离散行为。结果发现在高频场时分子的多光子离散碎片是近对称的，而对于低频场，在所考虑的激光强度下Ｈ２＾＋是被隧道离散的，因而其离散碎片Ｈ＾＋分布是反对称的，另外，文中还探讨了三脉冲形状对分子离解几率、离解碎片动力学行为的影响。     

Electronic and nuclear responses of fixed-in-space H2S to ultrashort intense laser fields

The Coulomb explosion dynamics of H2S, H2S3+-->H+ +S+ + H+, in ultrashort intense laser fields (12 fs, approximately 2 x 10(14) W/cm2) is studied by the coincidence momentum imaging of the three fragment ions. Different electronic and nuclear responses are identified depending on the direction of laser polarization epsilon in the molecular frame. The dependence can be interpreted in terms of the electronic and bonding characters of charge transfer states of H2S coupled to the electronic ground state.     

低能He2+-He碰撞体系转移电离机理的实验研究

利用冷靶反冲离子动量谱仪,对低能He²⁺-He碰撞反应中产生的反冲靶离子和炮弹离子进行了符合测量,根据反冲靶离子的动量,研究了转移电离过程中的电荷转移机理.实验结果表明：在20—40 keV能量范围内,靶原子上的一个电子俘获到炮弹离子的基态,另一个电子直接发射到靶的连续态的直接电离及另一个电子俘获到炮弹离子的连续态的过程(ECC)是最主要的转移电离机理,且ECC过程主要发生在大碰撞参数条件下;炮弹离子俘获两个电子处在双激发态的自电离过程的贡献很小. 关键词： 冷靶反冲离子动量谱仪 转移电离机理 离子原子碰撞     

氢原子谱线与高质量金刚石薄膜

采用蒙特卡罗方法,对以CH₄/H₂混合气体为源气体的电子助进热丝化学气相沉积(EACVD)中的氢原子发射过程进行了模拟。在模拟中考虑了电子与H₂的弹性碰撞及振动激发、分解、电子激发(包括H_α, H_β, H_γ谱线的激发)、电离及分解电离等非弹性碰撞过程;与CH₄的碰撞考虑了弹性动量传输及振动激发、分解、电子激发、分解激发(包括H_α, H_β, H_γ谱线的激发)、电离及分解电离等非弹性碰撞过程。研究了不同实验条件下产生的H, CH₃的数目与氢原子谱线相对强度的关系,给出了一种利用氢原子谱线来获得最佳成膜实验条件的方法。对于有效控制工艺条件,生长出高质量的金刚石薄膜具有重要意义。     

Deuteron breakup in the 2H(e,e' p) reaction at low momentum transfer and close to threshold

Deuteron breakup has been studied in a 2H(e,e' p) coincidence experiment at low momentum transfer and for energies close to threshold. The longitudinal-plus-transverse ( L+T) and longitudinal-transverse ( LT) interference cross sections are deduced. Nonrelativistic calculations based on the Bonn potential and including leading order relativistic contributions, meson exchange currents, and isobar configurations describe the ( L+T) data well. Surprisingly, large deviations of 30% to 45% are observed for the LT contribution.     

H<Superscript>+</Superscript> and He<Superscript>2+</Superscript> impact charge transfer cross sections of magnesium

H⁺ impact single and He²⁺ impact single and double electron capture cross sections of magnesium atoms have been calculated in the modified binary encounter approximation (BEA). The accurate expressions of ion impact s_DE\sigma _{\Delta {E}} (cross section for energy transfer DE\Delta E) and Hartree-Fock momentum distributions of the target electrons have been used throughout the calculations. On the basis of the present work it is concluded that inner shell captures by H⁺ and He²⁺ ions incident on magnesium atoms contribute partly to single electron capture and partly to transfer ionization cross sections. The calculated He²⁺ impact double electron capture cross sections of magnesium are in reasonably good agreement with the experimental observations. This indicates the success of the present theoretical approach in study of charge transfer cross sections of atoms as indirect mechanisms do not interfere with double electron capture processes in this case.     

Electron beam density imaging system at Beam study using a portable slit the Shanghai Electron Ion Trap

In this work, a portable slit imaging system is developed to study both the electron beam diameter and the profile of the newly developed Shanghai Electron Beam Ion Trap （Shanghai EBIT）. Images are detected by a charge coupled device （CCD） sensitive to both X rays and longer wavelength photons （up to visible）. Large scale ray tracings were conducted for correcting the image broadening effects caused by the finite slit width and the finite width of the CCD pixels. A numerical de-convolution method was developed to analyse and reconstruct the electron beam density distribution in the EBIT. As an example of the measured beam diameter and current density, the FWHM （full width at half maximum） diameter of the electron beam at 81 keV and 120 mA is found to be 76.2 μm and the density 2.00 × 10^3 A.cm-2, under a magnetic field of 3 T, including all corrections.     

Simulating electron momentum spectra of iso-C2H2Cl2： A study of the core electronic structure

Electron momentum spectroscopy(EMS) has been used for the first time to study core electronic structure of isoC2H2Cl2. In the present work, the pronounced difference between ionization energies of two C1 score orbitals(2A1 and 3A1) is seen as a chemical shift of 3 eV, which is due to different chemical environments of the related carbon atoms. Both the calculated spherically averaged core electron momentum distributions(MDs) and three-dimensional electron momentum density maps show that these core molecular orbitals(MOs) 2A1and 3A1 exhibit strong atomic orbital characteristics in real and momentum space. However, the core states 2B2 and 4A1, which are almost degenerate and related to two equivalent atoms, exhibit notable differences between the momentum and position depictions. In contrast to the position space, the momentum density maps of these two core MOs highlight the interference effects which are due to the nuclear positions. The 2B2 orbital of iso-C2H2Cl2 is the antisymmetric counterpart of the 4A1 core orbital in real space. However, it relates to the 4A1 orbital by an exchange of maxima and minima in momentum space. Due to interference effects between electrons scattered from different atomic centers, modulations with a periodicity of 1.12 a.u. can be seen in the computed momentum densities, which tend to decay with increasing electron momenta. Accordingly, the EMS can not only effectively image the electronic structure of compounds by studying valence orbitals, but also provides direct information on the nature of the nuclear geometry by investigating the core states.     

Investigation of ion-atom collision dynamics through imaging techniques

The principle and technique details of recoil ion momentum imaging are discussed and summarized. The recoil ion momentum spectroscopy built at the Institute of Modern Physics (Lanzhou) is presented. The first results obtained at the setup are analyzed. For 30 keV He²⁺ on He collision, it is found that the capture of single electron occurs dominantly into the first excited states, and the related scattering angle results show that the ground state capture occurs at large impact parameters, while the capture into excited states occurs at small impact parameters. The results manifest the collision dynamics for the sub-femto-second process can be studied through the techniques uniquely. Finally, the future possibilities of applications of the recoil ion momentum spectroscopy in other fields are outlined. Supported by the National Natural Science Foundation of China (Grant No. 10434100)     

Fraunhofer diffraction of light with orbital angular momentum by a slit

We study the Fraunhofer diffraction problem while taking into account the orbital angular momentum of light. In this case, the phase singularity of the light beam is incident on the slit in two different cases: in one, it is incident slightly above the slit, and in the other it is centered on the slit. We observed that the symmetry and the fringe formation in the interference pattern strongly depend on the amount of orbital angular momentum and the slit position in relation to the beam.     

Superexcited states of carbon monoxide studied by fast-electron impact

Absolute optical oscillator strength density and double differential cross section spectra of CO below 120 eV are determined by fast electron impact. Some peaks above the first ionization threshold stand out as the momentum transfer square K2 increases. The doubly excited Rydberg states converging to C 2∑^＋, D ^2∏, and F ^2∏ states of CO^＋, respectively, are confirmed in our spectra. Another peak at around 32eV is assigned to the transition of （3σ）^-1（2π）^1^1∏←X^1∑^＋.     

Intense-field double ionization of helium: identifying the mechanism

We present quantum mechanical calculations of the electron and ion momentum distributions following double ionization of a one-dimensional helium atom by ultrashort laser pulses (780 nm) at various intensities. The two-electron momentum distributions exhibit a clear transition from nonsequential to sequential double ionization. We provide strong evidence that rescattering is responsible for nonsequential ionization by calculating the momentum spectrum of the He2+ recoil ions-which we find in excellent agreement with recent experiments-and by analyzing the electronic center-of-mass motion via Wigner transforms.     

Sub-wavelength interference in the field assisted by surface plasmons

We present a theoretical study of the optical transmission from a thin metallic double slit. The second-order correlation function as a function of the displacement of the detectors for different values of slit separation is studied. It is shown that surface plasmons excited at one slit and propagating to the other slit modulate the coincidence counts with the variation of slit separation. Sub-wavelength interference effect has also been observed for the field assisted by surface plasmons. It is also shown that the second order interference-diffraction pattern changes with slit separation and at some particular value of slit separation it changes into the Hanbury Brown and Twiss (HBT) effect.     

相对流量技术及其在电子碰撞微分散射截面测量中的应用

为了提高原子分子动力学参数的测量精度,本文把气体相对流量技术应用到快电子碰撞研究中.并且在此基础上测量了氙原子6s激发的广义振子强度,所得结果在小动量转移区间与前人的结果符合很好,表明气体相对流量技术在电子碰撞方法中的适用性.大动量转移区间6s的表观广义振子强度对电子碰撞能量的依赖行为,说明500 eV的电子碰撞能量还不满足一阶玻恩近似条件.与Hartree-Fock方法相比,考虑电子关联效应更全面的随机相移近似计算结果与本文的实验结果符合更好,说明对于像Xe这样的重原子,电子关联效应十分重要.     

相对流量技术及其在电子碰撞微分散射截面测量中的应用

为了提高原子分子动力学参数的测量精度,本文把气体相对流量技术应用到快电子碰撞研究中. 并且在此基础上测量了氙原子6s激发的广义振子强度,所得结果在小动量转移区间与前人的结果符合很好,表明气体相对流量技术在电子碰撞方法中的适用性. 大动量转移区间6s的表观广义振子强度对电子碰撞能量的依赖行为,说明500eV的电子碰撞能量还不满足一阶玻恩近似条件. 与Hartree-Fock方法相比,考虑电子关联效应更全面的随机相移近似计算结果与本文的实验结果符合更好,说明对于像Xe这样的重原子,电子关联效应十分重要.     

Intense-laser-field ionization of the hydrogen molecular ions H2+ and D2+ at critical internuclear distances

Fragmentation of H2+ and D2+ in ion beams has been studied with short intense laser pulses (100 fs, I=5x10(13)-1x10(15) W/cm2) and by a high-resolution two-dimensional velocity imaging technique. In the Coulomb explosion channel, at intensities just above the threshold for this process, we observe a clear structure in the kinetic energy spectra not previously found or predicted. The peaks can be attributed to single vibrational levels. We interpret this observation as a dissociative allocation of the electron to a proton followed by enhanced ionization at a well-defined "critical" overstretched internuclear distance. When using longer pulses we observe three separate Coulomb explosion velocity groups corresponding to critical distances of about 8, 11, and 15 a.u.     

Coherence characteristics of light-emitting diodes

We report the measurement of coherence characteristics of light-emitting diodes (LEDs). Experiments were performed using red and green color LEDs directly illuminating the Young's double slit kept in the far-zone. Fourier transform fringe analysis technique was used for the measurement of the visibility of interference fringes from which the modulus of degree of spectral coherence was determined. Low degree of spectral coherence, typically 0.4 for red and 0.2 for green LED with double-slit separation of 400 μm was observed. A variable slit was then kept in front of the LEDs and the double slit was illuminated with the light coming out of the slit. Experiments were performed with various slit sizes and the visibility of the interference fringes was observed. It was found that visibility of the interference fringes changes drastically in presence of variable slit kept in front of LEDs and a high degree of spectral coherence, typically 0.85 for red and 0.8 for green LED with double-slit separation of 400 μm and rectangular slit opening of 500 μm was observed. The experimental results are compared with the theoretical counterparts. Coherence lengths of both the LEDs were also determined and it was obtained 5.8±2 and 24±4 μm for green and red LEDs, respectively.