Photon polarization and spin asymmetry in the elementary process of bremsstrahlung

Experimental and theoretical results are reviewed concerning the photon polarization and spin asymmetry in the elementary process of bremsstrahlung. In electron–photon coincidence experiments using an unpolarized primary beam (300 keV) the electron–nucleus bremsstrahlung was found to be almost completely linearly polarized. The same behavior was found in electron–electron bremsstrahlung. By using a transversely polarized electron beam the photon emission asymmetry was measured for fixed direction of the outgoing electrons.     

Measurement of exchange and spin-orbit effects and their interference in elastic e-Cs scattering at 7 eV

Twenty years ago a theoretical analysis showed that electron scattering by high-Z one-electron atoms might lead to interference of exchange and spin-orbit interactions at low electron energies, observable as a cross-section asymmetry if unpolarized electrons are scattered by polarized cesium atoms. By using a highly polarized cesium atomic beam, we studied exchange and spin-orbit effects at different electron energies, starting at 20 eV and going down. We observed the first distinctly non-zero interference asymmetry at 7 eV: Over the angular range of 35 to 145°, it varies between +0.02 and ?0.02 and goes through zero near 110° being negative at larger angles.     

Conductive properties of sintered zinc oxide surface. The influence of electron irradiation and oxygen adsorption

The influence of irradiation with 5 MeV electrons on charge transport phenomena in the surface region of sintered ZnO was studied in the temperature range 200–600 K. Mobility and concentration of electrons were measured in the presence and absence of oxygen as a function of temperature both for irradiated and unirradiated samples. The influence of electron beam processing was found most pronounced for ZnO samples with absorbed oxygen. Radiation induced desorption of O₂ was assumed to be responsible for enhanced effect of irradiation. Increased sensitivity of irradiated ZnO as an oxygen sensor has been observed and attributed to the formation of additional adsorption centres. A correlation between mobility of conduction electrons and their concentration in the surface region of irradiated ZnO with subsequently adsorbed oxygen was observed.     

Evaluation and optimization of electron capture dissociation efficiency in fourier transform ion cyclotron resonance mass spectrometry

Electron capture dissociation (ECD) efficiency has typically been lower than for other dissociation techniques. Here we characterize experimental factors that limit ECD and seek to improve its efficiency. Efficiency of precursor to product ion conversion was measured for a range of peptide (∼15% efficiency) and protein (∼33% efficiency) ions of differing sizes and charge states. Conversion of precursor ions to products depends on electron irradiation period and maximizes at ∼5–30 ms. The optimal irradiation period scales inversely with charge state. We demonstrate that reflection of electrons through the ICR cell is more efficient and robust than a single pass, because electrons can cool to the optimal energy for capture, which allows for a wide range of initial electron energy. Further, efficient ECD with reflected electrons requires only a short (∼500 μs) irradiation period followed by an appropriate delay for cooling and interaction. Reflection of the electron beam results in electrons trapped in or near the ICR cell and thus requires a brief (∼50 μs) purge for successful mass spectral acquisition. Further electron irradiation of refractory precursor ions did not result in further dissociation. Possibly the ion cloud and electron beam are misaligned radially, or the electron beam diameter may be smaller than that of the ion cloud such that remaining precursor ions do not overlap with the electron beam. Several ion manipulation techniques and use of a large, movable dispenser cathode reduce the possibility that misalignment of the ion and electron beams limits ECD efficiency.     

Pulse radiolysis based on a femtosecond electron beam and a femtosecond laser light with double-pulse injection technique

A new pulse radiolysis system based on a femtosecond electron beam and a femtosecond laser light with oblique double-pulse injection was developed for studying ultrafast chemical kinetics and primary processes of radiation chemistry. The time resolution of 5.2 ps was obtained by measuring transient absorption kinetics of hydrated electrons in water. The optical density of hydrated electrons was measured as a function of the electron charge. The data indicate that the double-laser-pulse injection technique was a powerful tool for observing the transient absorptions with a good signal to noise ratio in pulse radiolysis.     

Electron scattering by TIF dimer

The scattering of low energy (0.8 to 15.6 eV) electrons by TIF beans has been measured by beam recoil. Monomer concentrations are calculated and an overestimate of the monomers scattering is subtracted off. Analysis of the dimer component scattering indicates an instantaneous dipole moment of several debye. This result is not easily reconciled with a stiff symmetric linear structure for the dimer.     

Asymmetry of bremsstrahlung from polarized electrons

The triply differential cross section of unpolarized bremsstrahlung produced by polarized electrons in the Coulomb field of a nucleus is calculated with the aid of Sommerfeld-Maue wave functions. The photon asymmetry originating from transversely polarized electrons is calculated for various values of energies and angles, in particular for the parameters of recent experiments. As a function of the atomic number Z of the target nucleus, the asymmetry shows a distinct deviation from a linear dependence, in contrast to a previous calculation exact to lowest order in the Coulomb parameter Z/137. The cross-section formula is integrated numerically over the angles of the outgoing electrons to obtain the photon asymmetry observed in non-coincidence experiments.     

Near threshold absolute TDCS: first results

A new method, and first results for an impact energy 2 eV above the threshold of ionisation of helium, are presented for the measurement of absolute triple differential cross sections (TDCS) in a crossed beam experiment. The method is based upon measurement of beam/target overlap densities using known absolute total ionisation cross sections and of detection efficiencies using known absolute double differential cross sections (DDCS). For the present work the necessary absolute DDCS for 1 eV electrons had also to be measured. Results are presented for several different coplanar kinematics and are compared with recent DWBA calculations.     

Neutron and beta/gamma radiolysis of water up to supercritical conditions. 1. beta/gamma yields for H(2), H(.) atom, and hydrated electron

Yields for H2, H(.) atom, and hydrated electron production in beta/gamma radiolysis of water have been measured from room temperature up to 400 degrees C on a 250 bar isobar, and also as a function of pressure (density) at 380 and 400 degrees C. Radiolysis was carried out using a beam of 2-3 MeV electrons from a van de Graaff accelerator, and detection was by mass spectrometer analysis of gases sparged from the irradiated water. N2O was used as a specific scavenger for hydrated electrons giving N2 as product. Ethanol-d(6) was used to scavenge H(.) atoms, giving HD as a stable product. It is found that the hydrated electron yield decreases and the H(.) atom yield increases dramatically at lower densities in supercritical water, and the overall escape yield increases. The yield of molecular H2 increases with temperature and does not tend toward zero at low density, indicating that it is formed promptly rather than in spur recombination. A minimum in both the radical and H2 yields is observed around 0.4 kg/dm(3) density in supercritical water.     

Measurement of the asymmetry in the emission of bremsstrahlung by transversely polarized electrons

Bremsstrahlung emitted by transversely polarized electrons shows a “right-left” asymmetry in spatial distribution. Measurements and remeasurements of this asymmetry are presented using electrons of 300 and 128 keV, respectively, impinging on a gold target. Within the accuracy of the experiment there are no significant discrepancies to the partial wave calculations of Tseng and Pratt.     

Photoelectron circular dichroism in core level ionization of randomly oriented pure enantiomers of the chiral molecule camphor

The inner-shell photoionization of unoriented camphor molecules by circularly polarized light has been investigated from threshold to a photoelectron kinetic energy of approximately 65 eV. Photoelectron spectra of the carbonyl C 1s orbital, recorded at the magic angle of 54.7 degrees with respect to the light propagation direction, show an asymmetry of up to 6% on change of either the photon helicity or molecular enantiomer. These observations reveal a circular dichroism in the angle resolved emission with an asymmetry between forward and backward scattering (i.e., 0 degrees and 180 degrees to the light beam) which can exceed 12%. Since the initial state is an atomiclike spherically symmetric orbital, this strongly suggests that the asymmetry is caused by final-state effects dependent on the chiral geometry of the molecule. These findings are confirmed by electron multiple scattering calculations of the photoionization dynamics in the electric-dipole approximation.     

Spontaneous and infrared induced electron detachment from negatively charged helium nanodroplets

A beam of helium nanodroplets, with sizes ranging up to N = 10⁷ or more atoms, is produced by fragmentation of a low entropy supersonic expansion. It subsequently is excited by electron impact, producing various charged and metastable droplet states depending on the electron energy. We will describe experiments with negatively charged cluster ions, which are observed for low energy impacts when N >2×10⁵. In these experiments, after a flight time in high vacuum of several milliseconds the droplets pass through a weak transverse field above an electron multiplier. A signal from spontaneously detached electrons is observed, which suggests that the ion, while long lived, is inherently metastable. Furthermore, when the beam is crossed with an infrared light beam above the detector, the detachment rate is significantly increased. The wavelength dependence of this light induced signal has a broad peak near 1.5μm. By deflection measurements it is found that the spontaneous detachment signal comes preferentially from smaller clusters, while the light induced signal comes predominantly from larger ones. By stopping potential measurements one can conclude that both kinds of detached electrons have energies below 1eV, with photo detached electrons the more energetic.     

Electron-donor and electron-acceptor properties of carotenoids: Electrochemical study of carotenes

The methods of polarography, voltammetry at rotating solid electrodes and cyclic voltammetry with triangle and trapezoidal pulses have been employed to study the electrochemical behavior of carotenes in various solvents (in mixtures of dimethyl-formamide and acetonitrile with benzene, in dichloroethane) at different electrodes (Hg, Pt, graphite). The capacity of carotenes to accept or donate electrons reversibly was revealed. Contrary to the general scheme, electrooxidation of carotenes was shown to proceed as a one-stage two-electron reversible process in an available range of time measured (at scan rates up to 1000 V s^?1) Formation of a number of short-living intermediate particles, products of electron transfer (anion-radicals, anions, cations) was recorded. The kinetics of the reactions with the participation of these particles was studied. Normal potentials of reversible redox transitions including those in which unstable intermediates had formed were calculated. Ionization potentials and electron affinity of β-carotene and donor—acceptor activity of carotenes was revealed: carotenes possess stronger electrondonor properties than electron-acceptor. With chlorophylls the similar asymmetry has not been observed.     

Negative-Ion formation in the explosives RDX,PETN, and TNT by using the reversal electron attachment detection technique

First results of a beam-beam, single-collision study of negative-ion mass spectra produced by attachment of zero-energy electrons to the molecules of the explosives RDX, PETN, and TNT are presented. The technique used is reversal electron attachment detection (READ) wherein the zero-energy electrons are produced by focusing an intense electron beam into a shaped electrostatic field which reverses the trajectory of electrons. The target beam is introduced at the reversal point, and attachment occurs because the electrons have essentially zero longitudinal and radial velocity. The READ technique is used to obtain the “signature” of molecular ion formation and/or fragmentation for each explosive. Present data are compared with results from atmospheric-pressure ionization and negative-ion chemical ionization methods.     

Ferromagnetism in one-dimensional vanadium-benzene sandwich clusters

A molecular beam of multilayer vanadium-benzene organometallic complexes Vn(C6H6)m was produced by a laser vaporization synthesis method. The magnetic moments of the complexes were measured by a molecular beam magnetic deflection technique, and were found to increase with the number of vanadium atoms in the cluster, showing that the unpaired electrons, which occupy the nonbonding dsigma orbitals localized on the metal atoms, couple ferromagnetically. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition metal atoms are formally zerovalent.     

Solvation structures of iodide on and below a surface of aqueous solution studied by photodetachment spectroscopy

We investigated solvation structures of I(-) on and below a surface of an aqueous solution by photodetachment spectroscopy. An aqueous solution of an alkali halide was introduced to the vacuum as a continuous liquid flow (liquid beam), and the liquid beam was irradiated with a UV laser pulse. The intensity of electrons emitted from the surface by the laser excitation was measured as a function of wavelength (photodetachment spectroscopy), and we obtained absorption spectrum of I(-) on and below the solution surface. From the absorption spectrum, we found that I(-) starts to appear on the solution surface as the bulk NaI concentration increases. Similar concentration dependence was observed for the KI solution. We also found that I(-) located inside the solution is pushed to the surface, when NaCl is added to the solution. These changes are explained in terms of the difference in the polarizability of halide ions.     

Dose distribution in electron-irradiated PMMA: effect of dose and geometry

Depth-dose distributions for an electron beam are generally determined with either a stack irradiation geometry or a wedge (generally with an angle of about 50°–60°) made from an electrically-conducting material (such as, aluminium) or a non-conducting material (such as, PMMA). If the non-conducting wedge is thicker than the electron range, the stored charge in the material could influence the measured depth-dose distribution. This effect was investigated for 7-MeV electrons for PMMA with the wedge angle varying from 0° to 60°. The maximum-to-surface dose ratio was used as a characteristic parameter of the shape of the distribution. The depth-dose distribution measured by a dosimetry film placed inside the wedge-pair was similar to the standard shape when the wedge angle was larger than about 55° (dose ratio 1.5). However, as the angle was decreased, this ratio sharply increased almost linearly up to about 15°, and then leveled off at about 3. We also studied the effect of the surface dose on this dose ratio for the wedge angle of 0°, where we found that the ratio increased with the dose. Both of these effects can be explained by the presence of the electrostatic field around the dosimetry film due to stored charge in the non-conducting PMMA.     

Rotationally resolved photoelectron angular distributions for H+2(ν′ = 0) at λ = 584 and 736 Å

The photoelectron asymmetry parameter for H⁺₂(ν′ = 0) was measured for the S rotational branch at λ = 584 and 736 Å using supersonic beam of n-H₂. Comparison with the theoretical β values of Itikawa demonstrates the importance of coupling between the p and f photoelectron waves at higher photon energy.     

Monte Carlo simulation of full energy spectrum of electrons emitted from silicon in Auger electron spectroscopy

A Monte Carlo simulation including surface excitation, Auger electron‐ and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true‐secondary‐electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up‐to‐date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam–sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500 eV to 3000 eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantum state-resolved study of the four-atom reaction OH- (X1sigma+) + D2 (X1sigma(g)+, v = 0) --> HOD (X1A', v') + D- (1S)

The D+ transfer reaction between OH- (X1sigma+) and D2 was studied with crossed molecular beam experiments and quantum chemical calculations at collision energies of 89 and 68 kJ/mol. The D- product ions were observed and measured for the first time in the crossed beam experiments. The center-of-mass (c.m.) flux distributions of the D- product ions exhibit significant asymmetry, and their maxima are close to the velocity and direction of the precursor D2 beam. The data are consistent with a direct mechanism that occurs on a time scale significantly less than a rotational period of the transient complex formed by approaching reactants. The D+ transfer results primarily in the excitation of the H-O-D bending vibrational mode of the molecular product. The experimental observation is in agreement with theoretical results showing that, during the D+ transfer, the H-O-D bond angle changes significantly.