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.     

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.     

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.      

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.     

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     

Recovery of lifetime distributions from frequency-domain fluorometry data by means of the quantified maximum entropy method

The new quantified version of the maximum entropy method allows one to recover lifetime distributions with a precise statement of the accuracy of position, surface, and broadness of peaks in the distribution. Applications to real data (2,6-ANS in aqueous solutions of sodium dodecyl sulfate micelles of -cyclodextrin) are presented.Died on September 1, 1993.     

Pseudorapidity distributions of charged particles in Pb-Pb collisions at super proton synchrotron energies from the NA50 experiment

We present the measurements of charged particle pseudorapidity distributions dN_ch/dη performed by the NA50 experiment in Pb-Pb collisions at the CERN SPS. Measurements were done at incident energies of 40 GeV (√s = 8.77 GeV) and 158 GeV (√s = 17.3 GeV) per nucleon over a broad impact parameter range. The multiplicity distributions are studied as a function of centrality using the number of participating nucleons(N _part ), or the number of binary nucleon-nucleon collisions (N_coll). Their values at midrapidity exhibit a linear scaling withN _part at both energies. Particle yield increases approximately by a factor of 2 betweeny √s = 8.77 GeV and √s = 17.3 GeV.     

Energy and angular momentum transfers from an electromagnetic wave to a copper ring in the UHF band

Electromagnetic waves could carry orbital angular momentum. Such momentum can be transferred to macroscopic objects and can make them rotate under a constant torque. Based on experimental observations, we investigate the origin of orbital angular momentum and energy transfer. Due to angular momentum and energy conservation, we show that angular momentum transfer is due to the change in the sign of angular momentum upon reflection. This leads to a rotational Doppler shift of the electromagnetic wave frequency, ensuring energy conservation.     

如何做好"用油膜法估测分子的大小"实验

介绍了“用油膜法估测分子的大小”实验的准备工作及对学生进行正确实验指导的方法。     

Metrics for probability distributions and the trend to equilibrium for solutions of the Boltzmann equation

This paper deals with the trend to equilibrium of solutions to the spacehomogeneous Boltzmann equation for Maxwellian molecules with angular cutoff as well as with infinite-range forces. The solutions are considered as densities of probability distributions. The Tanaka functional is a metric for the space of probability distributions, which has previously been used in connection with the Boltzmann equation. Our main result is that, if the initial distribution possesses moments of order 2+, then the convergence to equilibrium in his metric is exponential in time. In the proof, we study the relation between several metrics for spaces of probability distributions, and relate this to the Boltzmann equation, by proving that the Fourier-transformed solutions are at least as regular as the Fourier transform of the initial data. This is also used to prove that even if the initial data only possess a second moment, then _v>R f(v, t) v² dv0 asR, and this convergence is uniform in time.     

Measurement of angular distribution of soft X‐ray radiation from thin targets in the tabletop storage ring MIRRORCLE‐20SX

The only available tabletop electron storage rings are the machines from the MIRRORCLE series. The electrons are accelerated in a microtron and injected into the storage ring. During its circulation, each electron passes through a tiny target many times, emitting a photon beam. Both the spectrum and the angular distribution of the radiation depend on the material, the thickness and the shape of the target. In this paper measured angular distributions of the radiation from several different targets in the magnetic field of the 20 MeV storage ring MIRRORCLE‐20SX are presented. The detector comprises a 3 mm × 3 mm × 8.5 µm plastic scintillator (PS) coupled to a photomultiplier by a bundle of optical fibers. The output of the photomultiplier is digitized by an IF converter. This detector is sensitive mostly to soft X‐ray radiation, and its PS is moved by a mechanical system in a plane perpendicular to the radiation axis. The measured angular distributions for Mo and Sn targets contain an annulus which is attributed to transition radiation. The angular distributions for Al, carbon nanotube and diamond‐like carbon (DLC) targets show some suppression of the radiation along the magnetic field. This is the first evidence of observation of the angular distribution of synchrotron Cherenkov radiation, which represents Cherenkov radiation in a magnetic field. The power radiated from the DLC target is estimated.