Quantum momentum distribution and kinetic energy in solid 4He

We present measurements of neutron scattering from solid 4He at high momentum transfer. The solid is held close to the melting line at molar volume 20.87 cm3/mol and temperature T=1.6 K. From the data, we determine the shape of the momentum distribution, n(k), of atoms in the solid and the leading final state contribution to the scattering. We show that n(k) in this highly anharmonic, quantum solid differs significantly from a Gaussian. The n(k) is more sharply peaked with larger occupation of low momentum states than in a Maxwell-Boltzmann distribution, as found in liquid 4He and predicted qualitatively by path integral Monte Carlo calculations. The atomic kinetic energy is =(24.25+/-0.30) K. If n(k) is assumed to be Gaussian, as is usually the practice, a 10% smaller is obtained.     

截面依赖动量转移条件下屏蔽效应的理论研究

利用BBK模型及修正方案, 对电子入射电离氦原子(e,2e)反应过程中截面依赖动量转移条件下的三重微分散射截面(TDCS)进行了理论计算, 把计算结果与实验测量结果进行比较, 分析了截面结构, 并研究了末通道屏蔽效应对三重微分截面所产生的影响.     

Interpretation of momentum distribution of recoil ions from laser induced nonsequential double ionization

We show that the recently reported momentum distribution of recoil ions from laser induced nonsequential double ionization can be interpreted as a combined effect of interelectron correlation and final state field interaction ("Volkov dressing") of the two outgoing electrons. We also find a cutoff formula for the recoil momentum that gives the maximum momenta observed in the experiments.     

Schwinger variational calculation of ionization of hydrogen atoms for large momentum transfers

Schwinger variational principle is used here to study large momentum transfer cases of electron and positron impact ionization of atomic hydrogen from the ground state at intermediate and moderately high energies. The results appear somewhat better compared to other theories.     

Photoproduction of the omega meson on the proton at large momentum transfer

The differential cross section, dsigma/dt, for omega meson exclusive photoproduction on the proton above the resonance region (2.6相似文献   

Angular momentum distribution in strong-field frustrated tunneling ionization

Using the classical-trajectory Monte Carlo model, we have theoretically studied the angular momentum distribution of frustrated tunneling ionization(FTI) of atoms in strong laser fields. Our results show that the angular momentum distribution of the FTI events exhibits a double-hump structure. With this classical model, we back traced the tunneling coordinates, i.e., the tunneling time and initial transverse momentum at tunneling ionization. It is shown that for the events tunneling ionized at the rising edge of the electric field,the final angular momentum exhibits a strong dependence on the initial transverse momentum at tunneling.While for the events ionized at the falling edge, there is a relatively harder recollision between the returning electron and the parent ion, leading to the angular momentum losing the correlation with the initial transverse momentum. Our study suggests that the angular momentum of the FTI events could be manipulated by controlling the initial coordinates of the tunneling ionization.     

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.     

ELECTRON MOMENTUM SPECTRA OF EXCITED He(2~1S) AND He(2~3S)

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Constraints on the high momentum behavior of inelastic scattering amplitudes

The ground state contributions to the leading high momentum behavior of the breakup amplitude of a composite system by a weak probe are shown to be exactly cancelled by final state interactions. The cancellation is a manifestation of the orthogonality of the target and continuum final states. The remaining large momentum behavior falls more rapidly and is more difficult to interpret directly in terms of properties of the target.     

The role of Volkov waves in laser-assisted electron momentum spectroscopy

We analyze theoretically laser-assisted electron-impact ionization of atomic hydrogen at high impact energy and large momentum transfer. Considering linearly polarized laser radiation with frequency ω=10.4 fs−1 and intensity W=4×¹⁰¹² W/cm², we describe the field-dressed hydrogen state within time-dependent perturbation theory. We find that even at such moderate field parameters the laser effects on the incident, scattered, and ejected electrons with energies of 1-2 keV can in a dramatic way influence the dependence of the field-assisted differential cross sections on the recoil proton momentum.     

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     

Electron-induced neutron knockout from 4He

The differential cross section for electron-induced neutron knockout in the reaction 4He(e,e(')n)(3)He has been measured for the first time with a statistical accuracy of 11%. The experiment was performed in quasielastic kinematics at a momentum transfer of 300 MeV/c and in the missing-momentum range of 25-70 MeV/c. The comparison of the data with theoretical calculations shows an impressive increase of the cross section resulting from final state interaction effects. Specifically, the p-n charge-exchange process dominates the cross section in this kinematical regime.     

Correlated two-electron momentum spectra for strong-field nonsequential double ionization of He at 800 nm

We report on a kinematically complete experiment on nonsequential double ionization of He by 25 fs 800 nm laser pulses at 1.5 PW/cm;{2}. The suppression of the recollision-induced excitation at this high intensity allows us to address in a clean way direct (e,2e) ionization by the recolliding electron. In contrast with earlier experimental results, but in agreement with various theoretical predictions, the two-electron momentum distributions along the laser polarization axis exhibit a pronounced V-shaped structure, which can be explained by the role of Coulomb repulsion and typical (e,2e) kinematics.     

Electron scattering off simple atoms for large momentum transfer collisions

In a previous paper this author examined the Born expansion and isolated those parts of the expansion that contribute most significantly to the scattering amplitude for large momentum transfer collisions in inelastic collisions from the ground state of both hydrogen and helium. It turned out that certain terms where the scattering electron interacts once with the nucleus and once with the other electron dominate. The physical reason is that large momentum transfer collisions require the nucleus to take the bulk of the incident momentum but require an interaction with the one of the bound electrons to change the state of the atom. The arguments are quite general and this paper will extend this analysis by comparing the inelastic results obtained by this method for hydrogen and helium to a close coupling calculation with many intermediate states. Further, we will extend this analysis to the correction to the 1st Born result for elastic electron-hydrogen and electron-helium collisions and provide some results for scattering from the initial metastable states of hydrogen for large momentum transfer collisions. A comparison of the results of this analytic approach will be made to the numerical close coupling approach and experiments where available. The agreement is remarkable.     

Partitioning of the linear photon momentum in multiphoton ionization

The balance of the linear photon momentum in multiphoton ionization is studied experimentally. In the experiment argon and neon atoms are singly ionized by circularly polarized laser pulses with a wavelength of 800 and 1400 nm in the intensity range of 10(14)-10(15) W/cm2. The photoelectrons are measured using velocity map imaging. We find that the photoelectrons carry linear momentum corresponding to the photons absorbed above the field free ionization threshold. Our finding has implications for concurrent models of the generation of terahertz radiation in filaments.     

Heat and momentum transport to particulates injected intolow-pressure (~80 mbar) nonequilibrium plasmas

An analytical model for plasma flow around a spherical particle has been developed using a two-temperature chemical nonequilibrium approach that includes rarefaction and particle-charging effects. The contribution of each component of the plasma to the heat and momentum transfer to injected particulates is studied by simulating the plasma flow around a sphere for conditions typical of thermal spray processes at pressures from 80 mbar to 1 bar. A parametric study of the 80-mbar case is also presented for free-stream velocities in the 0-3000 m/s range and free-stream temperatures in the 8000-12000 K range for both ionization equilibrium and nonequilibrium. The results demonstrate that there are departures from kinetic and ionization equilibrium in the plasma around a particle, and that heat transfer to a particle can be enhanced by the increased ion flux to the particle surface when the surrounding plasma is in ionization nonequilibrium     

Neutron scattering from superfluid 4He at very large momentum transfer

Measurements of inelastic neutron scattering from liquid ⁴He were performed at very large momentum transfer (about 150 Å⁻¹) using an eV spectrometer with a pulsed spallation neutron source. A difference in the scattered neutron spectrum measured at 1.2 and 2.5 K was observed for the first time within this momentum-transfer range.     

Bifurcation of ion momentum distributions in sequential double ionization by elliptically polarized laser pulses

Using a fully classical model, we have studied sequential double ionization (SDI) of argon driven by elliptically polarized laser pulses at intensities well in the over-barrier ionization region. The results show that the width of the ion momentum along the major elliptical axis increases with the ellipticity while that in the laser propagation direction decreases with the ellipticity. The ion momentum in the minor elliptical axis bifurcates from one peak structure to three-peak, then to four-peak, finally to six-peak structure. Analysis shows that this ellipticity dependence of the ion momentum distribution is a result of the subcycle nature of the ionization dynamics in SDI. By changing the ellipticity and wavelength of the driving pulses, this subcycle ionization dynamics is more observable. This provides a simple and efficient way to study the subcycle ionization dynamics in strong field processes.