100-femtosecond MeV electron source for ultrafast electron diffraction

A near-relativistic 100-fs MeV electron beam is developed by using a photocathode rf gun for revealing the hidden ultrafast dynamics of intricate molecular and atomic processes in materials through experimentation of ultrafast time-resolved electron diffraction (UED). The transverse and longitudinal dynamics of femtosecond electron beam in the rf gun were studied theoretically by particle simulation. The growths of the emittance, bunch length and energy spread due to the rf and space charge effects were investigated by changing the laser parameters, field gradient and electron charge. The theoretical studies indicate that a 100-fs MeV electron beam with the transverse emittance of 0.1 mm mrad and the relative energy spread of 10⁻³–10⁻⁴ at bunch charge of 0.1–2 pC (10⁶–10⁷ electrons per pulse) is achievable for UED, in which the intensity is three orders of magnitude higher than that produced by the conventional dc or pulsed guns.     

Small-Angle electron scattering and electron density in carbon dioxide

The total (elastic and inelastic) intensity of electrons scattered by CO₂ was measured in the s range of 1 to 12 Å^?1 and compared with the theoretical intensity calculated from the Hartree-Fock molecular wave function and those calculated for the independent-atom-model (IAM ) molecule. In the range of s ? 4 Å^?1 the electron correlation effect on the total scattered intensity was found to be represented by that for the IAM molecule.     

Theory of modulation effects in electron electron double resonance

The nonlinear response of spin systems to intense radiation fields is quantitatively treated by a modification of the stochastic Liouville equation for the spin density matrix. In particular, applied modulation terms are included in this equation. The resulting formalism provides a general method for calculating nonlinear spin response for dilute systems of radicals in a high magnetic field. In this communication, frequency and field swept absorption and dispersion electron-electron double resonance spectra are calculated and compared with experimental spectra recorded under conditions of sinusoidal magnetic field modulation and phase-sensitive detection. Good reproduction of the detailed lineshapes of experimental spectra is observed in all cases. The dependence of ELDOR reduction factors upon modulation frequency is discussed. A theoretical analysis such as employed in the present communication is shown to be essential if ELDOR reduction factors are to be related to relaxation times and hence to molecular dynamics, and if the design of ELDOR experiments is to be optimized.     

Design of electron energy analyzers for electron impact studies

The design and construction of an electron energy analyzer for the study of electron impact processes in atoms, molecules and solids is described. The analyzer incorporates a 180° hemispherical deflector and five-element entrance optics. Focusing characteristics and angular behavior of the analyzer have been investigated by using the electron-ray tracing simulation program, SIMION. The entrance lens system to the hemispherical deflector has been designed to have high collection efficiency for low-energy electrons. The fringing field correction has been done by tilting the input beam angle outward for real aperture configuration.     

Coincidence electron spectrometer for studying electron–atom collisions

The details of our recently developed coincidence electron spectrometer system with the block diagram and characteristics of the coincidence circuit are presented. As a first application we measured the continuous energy spectra of the scattered electron projectiles producing an L_2,3 inner-shell ionization/excitation of argon. The studied energy region was the neighbourhood of the ionization edge, where nearly the total excess energy is taken away by the scattered projectile. Above the edge we identify the peaks of the scattered electrons that produced 2p_3/2→4s or 2p_3/2→3d excitation.     

Charge remote fragmentation in electron capture and electron transfer dissociation

Secondary fragmentations of three synthetic peptides (human αA crystallin peptide 1-11, the deamidated form of human βB2 crystallin peptide 4-14, and amyloid β peptide 25-35) were studied in both electron capture dissociation (ECD) and electron-transfer dissociation (ETD) mode. In ECD, in addition to c and z· ion formations, charge remote fragmentations (CRF) of z· ions were abundant, resulting in internal fragment formation or partial/entire side-chain losses from amino acids, sometimes several residues away from the backbone cleavage site, and to some extent multiple side-chain losses. The internal fragments were observed in peptides with basic residues located in the middle of the sequences, which was different from most tryptic peptides with basic residues located at the C-terminus. These secondary cleavages were initiated by hydrogen abstraction at the α-, β-, or γ-position of the amino acid side chain. In comparison, ETD generates fewer CRF fragments than ECD. This secondary cleavage study will facilitate ECD/ETD spectra interpretation, and help de novo sequencing and database searching.     

Secondary electron microanalysis

Secondary electron microanalysis is described as a nonconventional method to observe microareas at sample surfaces. The method is characterized by a high lateral and depth resolution and additionally by a typical sensitivity to localized electric potentials and electron work functions. The limits of the method for measurement at high vacuum conditions in conventional scanning electron microscopes are described in connection with electron-sample interactions. Examples for investigations of distributions of localized electric potentials, electron work functions at semiconductors, ferroelectrics and electric ceramics are given.     

Highly reproducible secondary electron imaging under electron irradiation using high-pass energy filtering in low-voltage scanning electron microscopy

The reproducibility of contrast in secondary electron (SE) imaging during continuous electron irradiation, which caused surface contamination, was investigated using SE high-pass energy filtering in low-voltage scanning electron microscopy (SEM). According to high-pass energy-filtered imaging, dopant contrast in an indium phosphide remained remarkably stable during continuous electron irradiation although the contrast in unfiltered SE images decreased rapidly as a contamination layer was formed. Charge neutralization and the SE energy distributions indicate that the contamination layer induces a positive charge. This results in a decrease of low-energy SE emissions and reduced dopant contrast in unfiltered SE images. The retention of contrast was also observed in high-pass energy-filtered images of a gold surface. These results suggest that this imaging method can be widely used when SE intensities decrease under continuous electron irradiation in unfiltered SE images. Thus, high-pass energy-filtered SE imaging will be of a great assistance for SEM users in the reproducibility of contrast such as a quantitative dopant mapping in semiconductors.     

Uniform electron gases

We show that the traditional concept of the uniform electron gas (UEG)—a homogeneous system of finite density, consisting of an infinite number of electrons in an infinite volume—is inadequate to model the UEGs that arise in finite systems. We argue that, in general, a UEG is characterized by at least two parameters, viz. the usual one-electron density parameter ρ and a new two-electron parameter η. We outline a systematic strategy to determine a new density functional E (ρ, η) across the spectrum of possible ρ and η values.     

Extremal electron pairs

Extremal pair functions for an n-electron wave function of a closed-shell state are defined as linear combinations of spin-orbital-product pair functions that make some functionals (e.g., r²₁₂ or r⁻¹₁₂) extremal. They are related to the natural spin geminals in the uncorrelated limit and are useful both for an analysis of wave functions in view of an understanding of the chemical bond and for the treatment of electron correlation. Numerical examples are shown and discussed for He₂ as well as the 10-electron systems Ne, HF, H₂O, NH₃, and CH₄. © 1996 John Wiley & Sons, Inc.     

Low-energy electron capture by 6-Aza-2-thiothymine: investigations by electron attachment and electron transmission spectroscopies

The interaction of low-energy (0-10 eV) electrons with 6-aza-2-thiothymine is investigated in the gas phase by studies of sharp structure in the total electron scattering cross section and by mass analysis of the stable or long-lived negative ions produced by electron attachment. The most efficient fragmentation process, occurring at 0.15 eV, involves the ejection of a closed-shell neutral molecule (CH3CN). Ab initio calculations support our proposal that this process leads to ring closure to form a stable four-member heterocyclic anion. A long-lived parent anion with an approximate lifetime of 75 microseconds is observed near zero electron energy, and evidence is also seen for the slow decay of this anion by ejection of CH3CN. Near 3.3 eV, an anion of m/e 41 is produced that is likely to be a metastable valence anion of bent CH3CN, but the dipole-bound anion cannot be ruled out.     

Comparison of rate coefficients for rydberg electron and free electron attachment

New experimental results on attachment reactions involving free electrons at sub-meV resolution allow for the first time a conclusive comparison of measured rate coefficients for Rydberg electron attachment with those calculated from the measured free electron cross sections on the basis of the quasi-free electron model for Rydberg electron collisions. Using classical velocity distributions for the highn Rydberg electrons and our measured free electron attachment cross sections, we calculate Rydberg electron attachment rate coefficientsk _n for the two cases SF₆ and HI for Rydberg binding energies |E _n | of 0.1–40 meV. We find a significant increase ink _n towards lower binding energies, especially for HI, which is due to the deviation of the free electron cross section from the limitings-wave behaviour ₀E ^–1/2. The increase at |E _n |2 meV is in qualitative agreement with our highn Rydberg data (n80) if -mixing due to residual electric fields is taken into account. For low , Rydberg rate coefficientsk _n(|E _n |) are significantly larger than free electron rate coefficientsk _e (E=|E _n |), while for circular orbits (=n–1) they agree. On average, attachment reactions of Rydberg electrons in low orbits proceed with an effective collision energy substantially smaller than the binding energy |E _n |.     

High-resolution electron microscopy and electron energy-loss spectroscopy of giant palladium clusters

Combined structural and chemical characterization of cationic polynuclear palladium coordination compounds Pd₅₆₁L₆₀(OAc)₁₈₀, where L=1,10-phenantroline or 2,2-bipyridine has been carried out by high-resolution electron microscopy (HREM) and analytical electron microscopy methods including electron energy-loss spectroscopy (EELS), zero-loss electron spectroscopic imaging, and energy-dispersive X-ray spectroscopy (EDX). The cell structure of the cluster matter with almost completely uniform metal core size distributions centered around 2.3 ±0.5 nm was observed. Zero-loss energy filtering allowed to improve the image contrast and resolution. HREM images showed that most of the palladium clusters had a cubo-octahedral shape. Some of them had a distorted icosahedron structure exhibiting multiple twinning. The selected-area electron diffraction patterns confirmed the face centered cubic structure with lattice parameter close to that of metallic palladium. The energy-loss spectra of the populations of clusters contained several bands, which could be assigned to the delayed Pd M_{4, 5}-edge at 362 eV, the Pd M₃-edge at 533 eV and the Pd M₂-edge at 561 eV, the NK-edge at about 400 eV, the O K-edge at 532 eV overlapping with the Pd M₃-edge and the carbon C K-edge at 284 eV. Background subtraction was applied to reveal the exact positions and fine structure of low intensity elemental peaks. EELS evaluations have been confirmed by EDX. The recorded series of the Pd M-edges and the N K-edge in the spectra of the giant palladium clusters obviously were related to Pd-Pd- and Pd-ligand bonding.V. Oleshko is on leave from the N.N. Semenov Institute of Chemical Physics of the Russian Academy of Sciences, 117421 Moscow, Russia     

Photoinduced electron transfer reactions of rose bengal and selected electron donors

The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB√−) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10⁸ mol⁻¹ dm³ s⁻¹ and 8.5 × 10⁸ mol⁻¹ dm³ s⁻¹ for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10⁸ mol⁻¹ dm³ s⁻¹ as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions.

It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB√−.     

Average electron tunneling route of the electron transfer in protein media

We present a new theoretical method to determine and visualize the average tunneling route of the electron transfer (ET) in protein media. In this, we properly took into account the fluctuation of the tunneling currents and the quantum-interference effect. The route was correlated with the electronic factor in the case of ET by the elastic tunneling mechanism. We expanded by the interatomic tunneling currents 's. Incorporating the quantum-interference effect into the mean-square interatomic tunneling currents, denoted as , we could express as a sum of variant Planck's over 2pi(2). Drawing the distribution of on the protein structure, we obtain the map which visually represents which parts of bonds and spaces most significantly contribute to . We applied this method to the ET from the bacteriopheophytin anion to the primary quinone in the bacterial photosynthetic reaction center of Rhodobacter sphaeroides. We obtained 's by a combined method of molecular dynamics simulations and quantum chemical calculations. In calculating , we found that much destructive interference works among the interatomic tunneling currents even after taking the average. We drew the map by a pipe model where atoms a and b are connected by a pipe with width proportional to the magnitude of . We found that two groups of 's, which are mutually coupled with high correlation in each group, have broad pipes and form the average tunneling routes, called Trp route and Met route. Each of the two average tunneling routes is composed of a few major pathways in the Pathways model which are fused at considerable part to each other. We also analyzed the average tunneling route for the ET by the inelastic tunneling mechanism.     

Double electron electron resonance as a method for characterization of micelles

Double electron electron resonance (DEER) is an experimental technique used to determine distance between electron spins. In this work, we show that it can be used to study the properties of micelles in solution, specifically their volume and the aggregation number. The feasibility of the method is tested on micelles of Pluronic block copolymers, PEO(x)-PPO(y)-PEO(x), built from chains of poly(ethylene oxide) (PEO), comprising the more hydrophilic corona, and a poly(propylene oxide) (PPO) block constituting the hydrophobic core. In this work, the dimensions of the hydrophobic core of micelles of Pluronic L64 (x = 13, y = 30), P123 (x = 20, y = 70), and F127 (x = 106, y = 70) and their aggregation number were studied. This was done using the spin-probe 4-hydroxy-tempo-benzoate (4HTB), which is hydrophobic and is localized in the hydrophobic core of the micelles and does not dissolve in aqueous solution. The measurements were carried out on frozen solutions, freeze quenched after equilibration at 50 degrees C. It was found that the hydrophobic core radii occupied by 4HTB in 7.5 wt % F127 and 6 wt % L64 are 4.0 +/- 0.05 and 3.8 +/- 0.1 nm, respectively, and the corresponding aggregation numbers are 57 +/- 2 and 206 +/- 14. The micelles of 6 wt % P123 were found to have a rod shape, and the addition of 4HTB at concentrations higher than 0.7 mM resulted in a phase transitioned to spherical micelles. Finally, this study also showed that the micelle structure is preserved upon rapid freezing.     

Study of electron states of solids by techniques of electron spectroscopy

Studies of valence bands and core levels of solids by photoelectron spectroscopy are described at length. Satellite phenomena in the core level spectra have been discussed in some detail and it has been pointed out that the intensity of satellites appearing next to metal and ligand core levels critically depends on the metal-ligand overlap. Use of photoelectron spectroscopy in investigating metal-insulator transitions and spin-state transitions in solids is examined. It is shown that relative intensities of metal Auger lines in transition metal oxides and other systems provide valuable information on the valence bands. Occurrence of interatomic Auger transitions in competition with intraatomic transitions is discussed. Applications of electron energy loss spectroscopy and other techniques of electron spectroscopy in the study of gas-solid interactions are briefly presented.     

Milestones in electron crystallography

Electron crystallography determines the structure of membrane embedded proteins in the two-dimensionally crystallized state by cryo-transmission electron microscopy imaging and computer structure reconstruction. Milestones on the path to the structure are high-level expression, purification of functional protein, reconstitution into two-dimensional lipid membrane crystals, high-resolution imaging, and structure determination by computer image processing. Here we review the current state of these methods. We also created an Internet information exchange platform for electron crystallography, where guidelines for imaging and data processing method are maintained. The server (http://2dx.org) provides the electron crystallography community with a central information exchange platform, which is structured in blog and Wiki form, allowing visitors to add comments or discussions. It currently offers a detailed step-by-step introduction to image processing with the MRC software program. The server is also a repository for the 2dx software package, a user-friendly image processing system for 2D membrane protein crystals.     

Direct electron transfer of hemoglobin founded on electron tunneling of CTAB monolayer

Direct electron transfer and stable adsorption of hemoglobin (Hb) on a carbon paste (CP) electrode were achieved with the aid of a single-chain cationic surfactant, namely, cetyltrimethylammonium bromide (CTAB). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) indicated that CTAB could form a complete monolayer with a high density of positive charges on the surface of the CP electrode, which strongly adsorbed negatively charged protein molecules via electrostatic interactions. The surfactant molecules anchored the protein molecules to align in suitable orientations and acted as electron-tunneling pathways between the protein molecules and the CP electrode. The bioelectrocatalytic activity of the immobilized Hb was confirmed by RAIR and UV-vis spectroscopies, and rapid electrochemical responses to the reduction of oxygen (O2), hydrogen peroxide (H2O2), and nitrite (NO2-) were also obtained.     

Reaction kinetics involving electron tunneling. Dose effect on trapped electron yields

Willard et al. have observed that the concentration of radiation-produced trapped electrons increases to a maximum and then decreases to zero with increasing dose. We interpret this phenomenon by the tunneling of trapped electrons to concurrently produced radicals.