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Absolute cross-section measurements for resonant double photoexcitation of Li+ions followed by autoionization have been performed at high resolution in the photon energy range from 148 eV, just below the (2s2p, 2(0,1)+2) resonance to 198 eV (the region of the double ionization threshold). The measurements have been made using the photon-ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory, USA. The absolute cross-section measurements show excellent agreement with theoretical results from the R-matrix plus pseudo-state (RMPS) method.

Absolute cross-section measurements for resonant double photoexcitation of Li + ions followed by autoionization have been performed at high resolution in the photon energy range from 148 eV, just below the 2s2p, 2(0, 1) + resonance, to 198 eV (the region of the double ionization threshold). The measurements have been made using the photon–ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory, USA. The absolute cross-section measurements show excellent agreement with theoretical results from the R-matrix plus pseudo-state (RMPS) method. Comparisons between theory and experiment for the Auger resonance energies, autoionization linewidth (� ) and the Fano line profile index q for several members of the principal 2snp, 2(0, 1) + and 3snp, 3(1, 1) + Rydberg series found in the photoionization spectra for the 1 P o symmetry show satisfactory accord. (Some figures in this article are in colour only in the electronic version)

Absolute cross-section measurements for K-shell photoionization of Be-like C2+ ions have been performed in the photon energy range 292 325 eV. These measurements have been made using the photon ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory. Absolute measurements compared with theoretical results from the R-matrix method indicate that the primary C2+ ion beam consisted of 62 percent ground-state (1s22s2 1S) and 38 percent metastable state (1s22s2p 3Po) ions. Reasonable agreement is seen between theory and experiment for absolute photoionization cross sections, resonance energies and autoionization line widths of K-shell-vacancy Auger states.

An adjustable 0.5 to 3.0 MeV monoenergetic positron beam has been developed at Brookhaven National Laboratory. Positrons leaving a 22Na β+ source with a ∼ 6000 A thick W(100) transmission moderator are accelerated to higher energies in a 3.0 MeV Dynamitron. At a target position, located 12 m away from the exit of the accelerator, the beam has a maximum intensity of 5 × 105 e+/s inside a 1.1 mm full width at half maximum (FWHM) spot and is characterized by an energy spread of

The authors extend the formalism introduced by Klar and Schlecht (1976) and more completely developed by Rau and Greene (1982, 1983) to explore the implications of the Wannier threshold theory for the spin dependence of electron impact ionization of atomic hydrogen. By analysing the properties of the contributions to the singlet and triplet cross sections from the multiplicity of (L, S pi ) final states and by combining the results of the investigations of spin-asymmetry measurements of Guo et al. (1990, 1993) and the earlier investigations of spin-averaged cross section measurements of McGowan and Clarke (1968), the authors are able to provide insight into the role played by higher order terms and the possible influence of collisional dynamics. The authors further show that the critical energy above threshold at which the contributions from higher order terms become visible is substantially lower for the singlet and triplet partial cross sections than it is for the spin-averaged cross section.

The authors report on the design of a hybrid transport system for slow positrons based upon the high-intensity magnetically transported positron beam, which existed at the Brookhaven (BNL) high-flux beam reactor (HFBR) in 1986. The resulting modified transport, incorporating an initial electrostatic stage followed by a magnetic stage and completed by brightness enhancement moderation leading to a final electrostatic section, represented the minimum alteration of the existing beam to enable it to be used for crossed-beams differential scattering studies involving a variety of atomic and molecular systems, including atomic hydrogen. A recent proposal for constructing a very high-intensity positron beam at the Paul Scherrer Institute (PSI), based upon magnetic confinement premoderation, has rekindled interest in the conclusions of the study.

We report results of an experiment to search for a resonance enhancement of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ scattering cross section in a metallic Li target utilizing an energy-tunable monoenergetic positron beam. Within statistical uncertainties (0.27%) no evidence has been observed for deviations from Bhabha scattering over the entire invariant-mass region 1560 keV/${\mathit{c}}^{2}$${\mathit{M}}_{\mathit{X}}^{0}$1860 keV/${\mathit{c}}^{2}$ that can be associated with the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ sum-peak energies in GSI heavy-ion experiments. Under the assumptions that the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ channel dominates, we deduce lower limits on lifetimes \ensuremath{\tau}g3.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ sec (J=0) and \ensuremath{\tau}g8.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ sec (J=1), at 90% C.L.

Using beams of polarised electrons and polarised hydrogen atoms the authors measured the spin antiparallel-parallel ionisation asymmetry from threshold to 500 eV. Their measurements are generally more precise than those of earlier studies and, more significantly, are characterised by an energy spread that is at least an order of magnitude smaller than that of the earlier work. They compare their results with several theoretical models as well as with the results from similar experiments using polarised alkali atoms.

Cross-sections for inner-shell photoionization processes in beryllium-like carbon ions are determined both theoretically and experimentally for photon impact energies in the region of the carbon K edge. Measurements of absolute cross-sections, resonance energies, and lifetime/linewidths measured using the merged-beam technique at the Advanced Light Source are in good agreement with R-matrix calculations.

We use beams of polarized electrons and polarized hydrogen atoms to measure the ionization-rate asymmetry, {Delta}{sub {ital I}}=({ital R}({up arrow}{down arrow}){minus}{ital R}({up arrow}{up arrow}))/({ital R}({up arrow}{down arrow})+{ital R}({up arrow}{up arrow})), in the near-threshold region, where {ital R}({up arrow}{down arrow}) and {ital R}({up arrow}{up arrow}) are the ionization rates when the electron spin is antiparallel and parallel, respectively, to the atomic spins. Within 1.7 eV above threshold, our results reveal the presence of structure, which heretofore has not been predicted by any conventional theoretical calculation.

At the Oak Ridge National Laboratory (ORNL) there is a broadly based, interactive fusion program, encompassing theory and atomic physics, computing, the technology of heating and fueling plasmas, superconducting magnets, robotics, materials developments, design studies, and experiments and diagnostics.

Using a Fano-effect polarized electron source and a state-selected thermally dissociated hydrogen beam, we measured the interference between the direct and exchange scattering amplitudes for electron-impact ionization of atomic hydrogen between 13.8 and 30.0 eV. We report the data from these measurements and the results of corrections applied to previously published data.

We report on the development of several operating procedures that promise to make GaAs photoemission electron sources easier to construct, more reliable to operate, and more amenable to use in dynamic vacuum systems. We describe in particular a method for ‘‘ohmically’’ heating a 〈100〉 crystal of GaAs under vacuum to approximately 600 °C. We also discuss our observations of the role of oxygen in the activation of the crystal surface, the use of continuous cesiation, and of the performance of the crystal under varying vacuum conditions.

Spin-dependent asymmetries have been measured in inclusive deep-inelastic scattering of longitudinally polarized electrons by longitudinally polarized protons. Data were obtained at a scattering angle of 10/sup 0/ and for incident energies of 16.2 and 22.7 GeV, which cover the kinematic range 0.18

With use of a Fano-effect polarized electron source and a state selected thermally dissociated hydrogen beam, the interference was measured between the direct and exchange scattering amplitudes for 90/sup 0/ elastic scattering of electrons from atomic hydrogen for energies between 4 and 30 eV.

We report on a new type of high-energy electron-proton scattering experiment in which longitudinally polarized electrons are scattered from longitudinally polarized protons. The asymmetry in elastic scattering at ${Q}^{2}=0.765$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ was measured; our result agrees with the theoretical asymmetry and determines the sign of $\frac{{G}_{E}}{{G}_{M}}$ to be positive.

We have measured the interference between the direct and exchange scattering amplitudes in electron impact ionization of atomic hydrogen over the energy range 15 to 197 eV in a crossed-beams experiment utilizing spin-polarized electrons and spin-polarized hydrogen atoms. The experimental values for the asymmetry $A=\frac{({\ensuremath{\sigma}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}\ensuremath{-}{\ensuremath{\sigma}}^{\ensuremath{\uparrow}\ensuremath{\uparrow}})}{({\ensuremath{\sigma}}^{\ensuremath{\downarrow}\ensuremath{\uparrow}}+{\ensuremath{\sigma}}^{\ensuremath{\uparrow}\ensuremath{\uparrow}})}$ in the total ionization cross section for antiparallel and parallel incident- and atomic-electron spins are inconsistent with all theoretical calculations below 50 eV.

Polarized cesium atoms were ionized by circularly polarized light. The cesium-ion intensities obtained with positive and negative photon helicities were measured at various wavelengths. The results demonstrate that the wave functions for the continuum states of cesium are perturbed by a spin-orbit interaction in accordance with the theoretical interpretations given by Seaton and Fano.

In a photoionization experiment involving polarized alkali atoms and circularly polarized photons we measured the strength of the spin-orbit perturbation in potassium, rubidium, and cesium. Our results are pertinent to the theory of photoionization and to the production of polarized electrons. Extrapolation of the results to photon energies below the ionization threshold provides access to the anomalous doublet line-strength ratio of discrete $P$ states.

Using crossed beams of polarized electrons and polarized hydrogen atoms we have investigated the effect of spin exchange on 90\ifmmode^\circ\else\textdegree\fi{} elastic scattering from 4.4 to 30.3 eV and impact ionization from 14.1 to 197.0 eV. Our results suggest that the range of validity of various theoretical approximation methods is more restricted than had been assumed previously.

A polarized electron source based upon the photoionization of unpolarized Cs atoms by circularly polarized light (Fano effect) has been developed and applied to the study of spin dependence in low-energy electron–atom scattering. Electron intensities of 10 nA with polarizations of 0.63±0.03 have been obtained routinely during continuous runs of up to 75 h. Frequent optical reversal of the direction of the longitudinal electron polarization minimizes systematic effects so that helicity dependent electron-scattering asymmetries smaller than 4×10 to the power of -4 can be measured.

A review is given of our experimental knowledge of the spin dependent structure functions of the proton, which is based on inclusive high energy scattering of longitudinally polarized electrons by longitudinally polarized protons in both the deep inelastic and resonance regions, and includes preliminary results from our most recent SLAC experiment. Implications for scaling, sum rules, models of proton structure, and the hyperfine structure interval in hydrogen are given. Possible future directions of research are indicated.

Depolarization effects in pulsed photoionization of state-selected lithium atoms have been observed. Experimental evidence and theoretical analysis demonstrate that these effects are produced by resonant excitation of the $2^{2}P$ state. Based upon the analysis, it is argued that resonant excitation of the $2^{2}P$ state with circularly polarized cw laser light followed by broad-band pulsed ionizing radiation can be used to produce an intense highly polarized electron beam with longitudinal polarization.

An experiment is being performed at the MIT Bates Linear Accelerator Center to measure the parity‐violating asymmetry in the elastic scattering of longitudinally polarized electrons from Carbon. The asymmetry, which is due to the isoscalar component of the neutral weak current, is predicted to be about 0.6×10−6. We have recently obtained preliminary data with polarized electrons. The status of the experiment is discussed.

We are planning to measure the parity-violating asymmetry A=(σ+−σ#x2212;)/(σ++σ#x2212;) for the differential elastic scattering of positive (+) and negative (#x2212;) helicity electrons from 12C nuclei. This asymmetry determines the axial-electron isoscalar coupling constant (γ in Sakurai’s notation) between electrons and quarks. Since carbon is spinless and isoscalar, it has only one form factor, which cancels in the asymmetry. The interpretation of the result is consequently free of ambiguities arising from the structure of the nucleus. The experiment will take place at the Bates Linear Accelerator Center with a 250 Mev electron beam and at a Q2 of 0.023(GeV/c)2. Polarized electrons will be provided by a photoemission solid state source.

The application of polarized beams to the study of interference phenomena in electron-hydrogen collisions is reviewed. In particular, the results of the first experiment using polarized electrons to study the effects of spin exchange in electron-atom collisions are discussed. Finally, the future of spin exchange studies using polarized electrons is examined.

In order to understand some of the systematic effects associated with conventional Mott-scattering electron polarization measurements, we analyzed asymmetry data obtained with 94-keV polarized electrons scattered at 120\ifmmode^\circ\else\textdegree\fi{} from gold-foil targets ranging in thickness from 27 to 62 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$. Based upon an examination of the influence of multiple and plural scattering, we conclude that the precision of such Mott measurements is fundamentally limited by an experimental uncertainty of greater than \ifmmode\pm\else\textpm\fi{}2.5% and an absolute uncertainty of approximately \ifmmode\pm\else\textpm\fi{}5%, considerably worse than is customarily assumed. We believe that similar conclusions can be drawn for measurements made with ``compact'' cylindrical and spherical Mott polarimeters.

A high‐intensity pulsed source of polarized electrons has been operating at the Stanford Linear Accelerator Center since November 1974. In its normal operating mode, the source, PEGGY, produces a longitudinal polarization of .85 at an intensity of 2.5×109 electrons per 1.6 μs pulse at a repetition rate of 180 pulses/sec. In its alternate mode PEGGY produces up to 9.15×109 electrons/pulse with a polarization of 0.35. Thus far PEGGY has been used in studies of Mo/ller scattering at GeV energies, deep inelastic e‐p scattering with a longitudinally polarized proton target, and parity non‐conservation using deep inelastic e‐p scattering with an unpolarized target.

During the two years of operation of the SLAC Polarized Electron Gun (PEGGY), the electron intensity delivered to the target has increased from 7 x 10/sup 7/ e/sup -//pulse to 1 x 10/sup 9/ e/sup -//pulse. The polarization is 0.85 with no measurable degradation caused by acceleration through the linear accelerator. The predominant cause of downtime is replenishment of lithium, which now averages 43 hours. The lifetime of a lithium load is about 175 hours.

We report on recent asymmetry measurements for inelastic scattering of longitudinally polarized electrons from an unpolarized deuterium target at 19.4 GeV. Using the SLAC 20-GeV/c and 8-GeV/c spectrometers, the helicity-dependent cross-section asymmetries were measured at Q/sup 2/ values of 1.2 and 4.2 GeV/c/sup 2/, and were found to be less than 2 x 10/sup -3/ and 7 x 10/sup -3/, respectively.

The longitudinal polarization of the new Yale University-Stanford Linear Accelerator Center polarized-electron beam has been determined at laboratory energies between 6.47 and 19.40 GeV. Spin-dependent elastic electron-electron scattering (M\o{}ller scattering) has been found to be a practical technique for polarization measurements at high energies. The results are consistent with the energy and angular dependence predicted by quantum electrodynamics and with an energy-independent beam polarization of 0.76\ifmmode\pm\else\textpm\fi{}0.03.

A polarized electron source based upon photoionization of a state-selected 6 Li atomic beam has been developed as an injection gun for the Standford two-mile electron linear accelerator. The source (PEGGY) produces a pulsed beam of electrons with a maximum intensity of 2.6×10 9 electrons per pulse, a polarization of 0.85, a pulse length of 1.6 μs, and a repitition rate of 180 pulses/s. Since its installation at SLAC in July 1974, PEGGY has been used in several energy electron-nucleon scattering experiments.

The spin-orbit interaction for the $P$-state continuum of heavy alkali metals was investigated in a photoionization experiment using spin-polarized alkali atoms and circularly polarized light. From the asymmetry in ion-counting rates corresponding to the two photon helicities, Fano's spin-orbit perturbation parameter $x$ was determined over a range of several hundred angstroms for K, Rb, and Cs. The spin-orbit perturbation was found to increase from K to Rb to Cs as expected, and the nonlinear behavior of $x$ as a function of the photon energy $E$ was demonstrated for K. Knowledge of $x(E)$ was used to establish accurate values for the position of the Cooper minimum and to estimate the magnitude of the cross section at the minimum. In addition, the $x(E)$ data for Cs were used to gain information about the spin polarization of photoelectrons in a Fano-type polarized electron source. Finally, extrapolation of $x(E)$ for cesium into the discrete spectrum indicated the existence of a pole in the function $\ensuremath{\rho}(E)$ which corresponds to the doublet line-strength ratio $\ensuremath{\rho}({E}_{\mathrm{nP}})=\frac{S(n{P}_{\frac{3}{2}})}{S(n{P}_{\frac{1}{2}})}$ at the discrete energies ${E}_{\mathrm{nP}}$. According to our extrapolation, the pole lies in the region of $n=10 \mathrm{to} 15$, in agreement with the early spectroscopic work of Sambursky (1928) and Beutell (1939), whose measurements were discounted by later investigators.