Jet-like structures in photoelectron angular distributions

The photoelectron angular distributions (PADs) of hydrogen atoms in an intense laser field of linear polarization are studied using the S-matrix theory in the length gauge. The PADs show main lobes along the laser polarization and jet-like structures sticking from the waist of main lobes. Our previous prediction, based on a nonperturbative scattering theory of photoionization developed by Guo et al, showing that the number of jets on one side of PADs may increase by one, three, or other odd numbers and may decrease by one when one more photon is absorbed, is confirmed by this treatment. Within the strong-field approximation, good agreement is obtained between these two quite different treatments. We further study the influence of the Coulomb attraction to PADs, by taking a Coulomb--Volkov state as the continuum state of photoelectrons. We find that under the influence of the Coulomb attraction, the PADs change greatly but the predicted phenomena still appear. This study verifies that the jet-like structures have no relation with the angular momentum of photoelectrons.     

Imprints of molecular orbitals using photoelectron angular distribution by strong laser pulses of circular polarization

We theoretically investigate the strong-field ionization of H+2 molecules in four different electronic states by calculating photoelectron angular distributions in circularly polarized fields. We find that the structure of photoelectron angular distribution depends on the molecular orbital as well as the energy of the photoelectron. The location of main lobes changes with the symmetric property of the molecular orbital. Generally, for molecules with bonding electronic states, the photoelectron's angular distribution shows a rotation of π/2 with respect to the molecular axis, while for molecules with antibonding electronic states, no rotation occurs. We use an interference scenario to interpret these phenomena. We also find that, due to the interference effect, a new pair of jets appears in the waist of the main lobes, and the main lobes or jets of the photoelectron's angular distribution are split into two parts if the photoelectron energy is sufficiently high.     

氖原子光电子角分布的理论计算

本文利用密度矩阵理论和Racah代数推导出了光电子角分布的一般计算公式, 并在多组态Dirac-Fock方法基础上发展了计算原子光电离过程中产生的光电子角分布的相对论程序, 利用该程序对氖原子2s和2p光电子角分布的偶极和非偶极参数进行了具体计算, 所得结果与已有文献具有很好的一致性. 在此基础上, 本文讨论了光子与电子相互作用多级展开中的非偶极项以及入射光的极化性质对光电子角分布的影响.     

Monte Carlo simulation of photoelectron angular distribution

A practical simulation method has been performed for studies of the influence of surface excitations on the angular distributions of photoelectron peak intensities. The surface effects have been incorporated into simulations by using the surface excitation parameters (SEPs) which have been calculated with the extended Drude dielectric function. Also, elastic scattering cross sections are calculated using the finite difference method for a Hartree-Fock-Wigner-Seitz potential in the Dirac equation to take into account the solid-state effect. Results of Monte Carlo simulations reveal that surface effects lead to a reduction of the intensities at small detection angles and a sharp decrease at large angles since the surface excitation is most probable for glancing electrons. The calculated results taking into account surface effects are in better agreement with the experimental data.     

Multiple-scattering calculations for 1s photoelectron angular distributions from single oriented molecules in the energy region above 50 eV

1s photoelectron angular distributions from fixed-in-space CO₂, NO₂, BF₃ and CH₃F molecules have been calculated by X-ray photoelectron diffraction (XPD) theory with muffin-tin-type molecular potential. For all the molecules, the calculated results show good agreements with those by density functional theory in the energy region ?100 eV. Furthermore, for all the molecules experimental data on the angular distributions in such energy region are well reproduced by the XPD theory. These intensive studies lead to a rather general rule that the XPD theory is an adequate tool to describe high-energy photoelectron angular distributions for any single oriented molecules.     

Photoelectron angular distributions: developments in applications to isolated molecular systems

The last decade has seen photoelectron angular distributions from isolated molecules used for an increasing variety of purposes, including examining details of electron correlation, demonstrating electron diffraction as a structural probe of single molecules, and probing photochemical processes. In this article these developments are reviewed and it is shown that the stage is set for another decade of innovation in which we can expect to see exciting results from pump–probe experiments using the emerging XUV and X-ray free-electron laser sources.     

Ar原子序列双光双电离产生光电子角分布的理论计算

基于多组态Dirc-Fock方法和密度矩阵理论,给出了原子序列双光双电离光电子角分布的计算表达式,发展了相应的计算程序.利用该程序对Ar原子3p壳层序列双光双电离过程进行了理论研究,给出了光电离的总截面、磁截面、剩余离子取向以及光电子角分布的各向异性参数与入射光子能量的函数关系.结果显示在光电离截面的Cooper极小位置附近取向参数出现极大值,而光电子角分布的各向异性参数在该位置附近出现极小值.进一步给出了33.94和55.34 eV光子能量下序列双光双电离过程中第一步的Ar原子和第二步的Ar^+离子3p壳层光电子角分布,分析了序列双光双电离光电子角分布与单光电离光电子角分布的差异.将计算结果与文献已有的数据进行了比较,具有很好的一致性.本文的研究结果对揭示光与物质相互作用的非线性动力学机制具有重要的参考价值.     

Polarization measurements of laboratory VUV light: a first comparison between multilayer polarizers and photoelectron angular distributions

The use of photoelectron angular distributions to determine the linear polarization of VUV light over a wide range of photon energies is demonstrated. Light at wavelengths from 256 to 736 Å, partially polarized by large angle reflections in a toroidal grating monochromator and at a refocus mirror, has been analyzed. The results are validated by comparison measurements at spot wavelengths using multilayer polarizers.     

Energy and angular distributions of backscattered electrons from collisions of 5 keV electrons with thick Al,Ti, Ag,W and Pt targets

The energy and angular distributions of backscattered electrons produced under the impact of 5 keV electrons with thick Al, Ti, Ag, W and Pt targets are measured. The energy range of backscattered electrons is considered between E _B = 50 eV and 5000 eV. The angle of incidence α and take-off angle θ are chosen to have values α = 0 and 10 and θ = 100, 110 and 120 respectively. The measured energy spectra are compared with the available theoretical models for α = 0 and 10. The elastic peak intensity of backscattered electrons is found to be a function of angle of incidence, take-off angle and atomic number of the target material. The considered theories are reasonably in good agreement with experiment for the energy spectra of the backscattered electrons having their reduced energies ∈ (= E _B/E ₀) in the range of 0.20 to 1.00.      

Photoelectron momentum distributions of Ne and Xe dimers in counter-rotating circularly polarized laser fields

The strong-field ionization of dimers is investigated theoretically in counter-rotating circularly polarized laser fields. By numerically solving the two-dimensional (2D) time-dependent Schrödinger equation (TDSE) with the single-electron approximation (SEA) frame, we present the photoelectron momentum distributions (PMDs) and photoelectron angular distribution (PADs) of aligned Ne and Xe dimers. It is found that the PMDs and PADs strongly depend on the time delays by counter-rotating circularly polarized laser pulses. The results can be explained by the ultrafast photoionization model and the evolution of electron wave packets for Ne and Xe dimers. Besides, We make a comparison of PMDs between Ne atom and Ne dimer.     

Fragment angular distributions in fission and fission-like reactions

Fragment angular distributions in fission is one of the oldest and well understood aspects of fission theory. However, recent heavy ion-induced fission and fission-like reactions have added a new dimension to this problem. We review here our present understanding of the fragment angular distribution theory in fission and fission-like reactions.     

Effects of the internuclear vector on the photoelectron angular distributions of H<Subscript>2</Subscript><Superscript>+</Superscript>

We study the photoelectron angular distributions (PADs) of diatomic molecule H₂ ⁺ irradiated by intense laser fields using a nonperturbative scattering theory. We find that the internuclear vector may change the PADs. The PADs have qualitative changes with the increasing of the internuclear distance. The molecular orientation affect the symmetry of the PADs. When the internuclear vector is vertical or parallel to the laser polarization vector, the PADs are four-fold symmetric; for other case the PADs are two-fold symmetric. Due to the modulation effect resulting from the molecular multi-core nature, the size of the jet and the main lobe can be enlarged or reduced. The molecular modulation effect become obvious for large internuclear distance.     

Calculated photoelectron angular distributions of ω-(n-pyrrolyl)alkanethiol self-assembled monolayers for distinguishing between different arrangements of the pyrrole groups

Photoelectron angular distributions from ω-(n-pyrrolyl)alkanethiol self-assembled monolayers (SAMs) were calculated within a single-scattering approximation of the photoemission process. The calculations were carried out on two different surface structures with face-stacked and herringbone arrangements of the pyrrole groups, which were deduced from molecular dynamics calculations. The characteristic angular patterns calculated for the molecular orbital originating from the pyrrole group involve information on the orientations of the pyrrole groups, which allows distinction between these arrangements. The photoelectron angular distributions from substituted SAMs can be used as a clue for studying the surface structures of the substituent groups.     

Distance distributions and dynamics of a zinc finger peptide from fluorescence resonance energy transfer measurements

Time-resolved fluorescence resonance energy transfer (FRET) measurements were used to measure distance distributions and intramolecular dynamics (site-to-site diffusion) of a 28-residue single-domain zinc finger peptide in the absence and presence of zinc ion. Energy transfer was measured between TRP₁₄ and a N-terminal DNS group. As expected, the TRP-to-DNS distance distribution for zinc-bound peptide is shorter and narrower (R _av=11.2 Å,hw=2.8 Å) than the metal-free peptide (R _av=20.1 Å,hw=14.5 Å). The degree of mutual donor-to-acceptor diffusion (D) was also determined for these distributions. For zinc-bound peptide there is no detectible diffusion (D0.2 Ų/ns), whereas for metal-free peptide a considerable amount of motion is occurring between the donor and the acceptor (D=12 Ų/ns). These results indicate that the zinc-bound peptide folds into a unique, well-defined conformation, whereas the metal-free conformation is flexible and rapidly changing. The absence of detectible mutual site-to-site diffusion between the donor and the acceptor in the metal-bound zinc finger peptide indicates that intramolecular motion is essentially frozen out, on the FRET time scale, as a consequence of zinc coordination.Dedicated to the memory of Barbara D. Wells.     

End-to-end diffusion coefficients and distance distributions from fluorescence energy transfer measurements: Enhanced resolution by using multiple donors with different lifetimes

We describe a method to improve the resolution of donor-to-acceptor distance distributions in molecules which are flexing on the timescale of the fluorescence lifetime. We measured the timedependent donor decays of two donor (D)-acceptor (A) pairs, where the donor lifetimes were substantially different. The donors were an indole residue (5.7 ns) and a naphthalene residue (24.4 ns). The same dansyl acceptor was used for both D-A pairs. The donor decays are complex due to both a distribution of D-A distances and D-A diffusion. Using the donor decay data for each D-A pair alone, it is difficult to resolve both the distance distribution and the D-to-A diffusion coefficient. However, these values are unambiguously recovered from global analysis of the data from both D-A pairs. The increased resolution from the global analysis is apparently the result of the complementary information content of the data for each D-A pair. The shorter-lived indole donor provides more information on the time-zero distance distribution because there is less time for D-A diffusion, and the longer-lived naphthyl donor is quenched to a greater extent than indole due to the longer time for diffusion-enhanced energy transfer. Simulations were also used to demonstrate the increased resolution of global analysis with different lifetime donors to obtain distance distribution parameters in the presence of D-A diffusion.     

Exact solutions to the angular Teukolsky equation with s ≠ 0

We first convert the angular Teukolsky equation under the special condition of τ ≠ 0, s ≠ 0, m=0 into a confluent Heun differential equation (CHDE) by taking different function transformation and variable substitution. And then according to the characteristics of both CHDE and its analytical solution expressed by a confluent Heun function (CHF), we find two linearly dependent solutions corresponding to the same eigenstate, from which we obtain a precise energy spectrum equation by constructing a Wronskian determinant. After that, we are able to localize the positions of the eigenvalues on the real axis or on the complex plane when τ is a real number, a pure imaginary number, and a complex number, respectively and we notice that the relation between the quantum number l and the spin weight quantum number s satisfies the relation l=∣s∣+ n, n=0, 1, 2···. The exact eigenvalues and the corresponding normalized eigenfunctions given by the CHF are obtained with the aid of Maple. The features of the angular probability distribution (APD) and the linearly dependent characteristics of two eigenfunctions corresponding to the same eigenstate are discussed. We find that for a real number τ, the eigenvalue is a real number and the eigenfunction is a real function, and the eigenfunction system is an orthogonal complete system, and the APD is asymmetric in the northern and southern hemispheres. For a pure imaginary number τ, the eigenvalue is still a real number and the eigenfunction is a complex function, but the APD is symmetric in the northern and southern hemispheres. When τ is a complex number, the eigenvalue is a complex number, the eigenfunction is still a complex function, and the APD in the northern and southern hemispheres is also asymmetric. Finally, an approximate expression of complex eigenvalues is obtained when n is greater than ∣s∣.     

洛伦兹力公式是怎样给出的

在电子尚未发现,并无任何实验证据的1892年,洛伦兹力公式是怎样给出的?回顾了电磁学历史上超距作用和近距作用观点的长期论争以及"什么是电"的长期论争后指出,洛伦兹力公式是场观点和"电就是带电粒子"相结合的产物.一切磁作用力都应归结为磁场对运动带电粒子的作用力--洛伦兹力.     

Retardation corrections to the angular distributions for the free-bound transitions in H-like atoms

The non-relativistic matrix elements and cross-sections of the radiative recombination for H-like atoms are calculated by considering the retardation corrections up to the first order in . These corrections are evaluated for the transitions to the spherical states using recursion relations which lead to fast and accurate calculations of the cross-sections. Received 4 June 1999