We have improved a polarized electron source in which unpolarized electrons undergo collisions with a mixture of buffer gas molecules and optically spin-polarized Rb atoms. With a nitrogen buffer gas, the source reliably provides spin polarization between 15% and 25% with beam currents >4 μA. Vacuum pump upgrades mitigate problems caused by denatured diffusion pump oil, leading to longer run times. A new differential pumping scheme allows the use of higher buffer gas pressures up to 800 mTorr. With a new optics layout, the Rb polarization is continuously monitored by a probe laser and improved pump laser power provides more constant high polarization. We have implemented an einzel lens to better control the energy of the electrons delivered to the target chamber and to preferentially select electron populations of higher polarization. The source is designed for studies of biologically relevant chiral molecule samples, which can poison photoemission-based GaAs polarized electron sources at very low partial pressures. It operates adjacent to a target chamber that rises to pressures as high as 10−4 Torr and has been implemented in a first experiment with chiral cysteine targets.

We performed experiments searching for chirality-dependent secondary electron emission for a 141 eV longitudinally spin-polarized electron beam incident on a thick solid cysteine target. We determined the secondary electron yield by measuring the positive current produced when the cysteine target was negatively biased. No spin-dependent effects to a level of 10−3 were found for the secondary electron emission yield.

We outline an experimental technique for measuring the degree of polarization of a positron beam using an optically pumped, spin-polarized Rb target. The technique is based on the production and measurement of the ortho- and para-positronium fractions through positron collisions with the Rb atoms as a function of their polarization. Using realistic estimates for the cross sections and experimental parameters involved, we estimate that a polarization measurement with an uncertainty of 3% of the measured value can be achieved in an hour.

We describe the production of a high-resolution electron beam using a Penning–Malmberg buffer-gas trap, or Surko trap as they have become known. A high-flux beam with an energy width of ~ 30 meV (FWHM) is readily achieved and the efficiency of production is considerably higher than that for positrons in a similar trap configuration. The reasons for this become apparent when one considers the molecular collisions and the respective selection rules involved, for electrons and positrons. We demonstrate the production of the beam and the capacity that it realises for absolute scattering measurements and for high-resolution electron spectroscopy.

High-accuracy optical polarimetry of atomic fluorescence generally requires the use of a collimating collection lens. The orientation of this lens can affect its transmission due to reflective loss, but can also change the polarization state of the light being measured. Current best practices regarding lens orientation are related to minimizing spherical aberration. In this work, we use the ray-tracing software TracePro® to investigate the matter of lens orientation for a plano-convex lens as it relates to light transmission and reflection- and refraction-induced polarization changes. We compare the amount of scattered light for both orientations of the lens with and without anti-reflection coating, and show the effect the lens has on the polarization of the light produced by an unpolarized point-source as well as two point sources simulating highly-polarized atomic fluorescence. We discuss how these effects can be of concern for polarization-sensitive imaging and polarimetry of dim light sources with an accuracy of better than 0.1% of the measured Stokes parameters.

Abstract not available.

The statistical character of electron beams used in current technologies, as described by a stream of particles, is random in nature. Using coincidence measurements of femtosecond pulsed electron pairs, we report the observation of sub-Poissonian electron statistics that are nonrandom due to two-electron Coulomb interactions, and that exhibit an antibunching signal of 1 part in 4. This advancement is a fundamental step toward observing a strongly quantum degenerate electron beam needed for many applications, and in particular electron correlation spectroscopy.

An American football is a rotationally symmetric object, which, when well-thrown, spins rapidly around its symmetry axis. In the absence of aerodynamic effects, the football would be a torque-free gyroscope and the symmetry/spin axis would remain pointing in a fixed direction in space as the football moved on its parabolic path. When a pass is well-thrown through the atmosphere,however, the symmetry axis remains—at least approximately—tangent to the path of motion. The rotation of the symmetry axis must be due to aerodynamic torque; yet, that torque, at first glance,would seem to have precisely the opposite effect. Here, we explain the action of aerodynamics on the ball’s orientation at second glance.

In this note we present the Helmholtz spacing for a pair of thin rectangular coils of arbitrary aspect ratio,and consider how best to use such coils to compensate for Earth’s magnetic field along the coils’ Cartesian symmetry axes. Such coils are frequently used in conjunction with charged-particle beam machines. The Helmholtz spacing varies non-monotonically between that for square coils and that for four optimally-spaced infinite wires. We consider other coil spacings that extend the length over which the field varies by less than some tolerance along the Cartesian symmetry axes. The calculations also provide a convenient means by which to evaluate when the length of the coils is sufficiently long to be considered infinite at the center point within a fixed tolerance.

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It is shown that focusing circularly polarized 800 nm light pulses of duration 100 fs on the tips of p-GaAs crystalline shards having no negative electron affinity (NEA) activation results in electron emission that is both fast and spin-polarized. The 400 fs duration of the emission process was determined by pump/probe measurements. The three samples we investigated produced electron polarizations of 13.1(0.9)\%, 13.3(0.7)\%, and 10.4(0.2)\%. Emission currents ranged between 50 pA and 3 nA with a sample bias of À100 V and an average laser power of 100 mW. The electron emission exhibited linear dichroism and was obtained under moderate vacuum conditions, similar to that of metallic tips. This source of spin-polarized electron pulses is "fast" in the sense that the electron emission process is of comparable duration to the laser pulses that initiate it.

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We have measured electron-circularly-dichroic asymmetries when longitudinally-polarized (chiral) electrons are scattered quasi-elastically by chiral halocamphor molecules: 3-bromocamphor (C10H15BrO), 3-iodocamphor (C10H15IO), and 10-iodocamphor. The proposed dynamic origins of these asymmetries are considered in terms of three classical models related to Mott scattering, target electron helicity density, and spin-other-orbit interactions. The asymmetries observed for 3-bromocamphor and 3-iodocamphor scale roughly as Z2, where Z is the nuclear charge of the heaviest atom in the target molecule, but the scaling is violated by 10iodocamphor, which has a smaller asymmetry than that for 3-iodocamphor. This is in contrast to the asymmetries in the collision channel associated with dissociative electron attachment, in which 10-iodocamphor has a much larger asymmetry. All of the available electron-circularlydichroic data taken to date are considered in an effort to systematically address the dynamical cause of the observed chiral asymmetries.

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The integrated Stokes parameters P1, P2, and P3 of Zn (43P0,1 – 53S1) fluorescence resulting from transversely-spin-polarized electron impact excitation of the Zn (4s5s)53S1 state have been measured. This work was motivated by similar studies reported several years ago, in which non-zero values of the integrated Stokes parameter P2 between the threshold for the (4s5s)53S1 excitation and the first cascading (4s5p)53PJ threshold were measured. We observe optical excitation functions in agreement with previous experimental and theoretical results, but find integrated P2 Stokes parameter values which are consistent with zero and inconsistent with those measured previously.

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All molecular forms of life have chemically-specific handedness. However, the origin of these asymmetries is not understood. A possible explanation was suggested by Vester and Ulbricht immediately following the discovery of parity violation in 1957: chiral beta radiation in cosmic rays may have preferentially destroyed one enantiomeric form of various biological precursors. In the experiments reported here, we observed chiral specificity in two electronmolecule interactions: quasi-elastic scattering and dissociative electron attachment. Using lowenergy longitudinally spin-polarized (chiral) electrons as substitutes for beta rays, we found that chiral bromocamphor molecules exhibited both a transmission and dissociative electron attachment rate that depended on their handedness for a given direction of incident electron spin. Consequently, these results, especially those with dissociative electron attachment, connect the universal chiral asymmetry of the weak force with a molecular breakup process, thereby demonstrating the viability of the Vester-Ulbricht hypothesis.

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Two-photon absorption of 1560 nm light was used to generate polarized electron beams from unstrained GaAs photocathodes of varying thickness: 625 μm, 0.32 μm, and 0.18 μm. For each photocathode, the degree of spin polarization of the photoemitted beam was less than 50\%, contradicting earlier predictions based on simple quantum mechanical selection rules for spherically-symmetric systems but consistent with the more sophisticated model of Bhat et al. (Phys. Rev. B 71 (2005) 035209). Polarization via twophoton absorption was the highest from the thinnest photocathode sample and comparable to that obtained via one-photon absorption (using 778 nm light), with values 40.3 7 1.0\% and 42.6 71.0\%, respectively.

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A compact optical electron polarimeter using a helium target is described. It offers a maximum fluorescence detection efficiency of 20 Hz/nA, which is an order of magnitude higher than that of earlier designs. With an argon target, this device is expected to have a polarimetric figure-of-merit of 270 Hz/nA. By relying on a magnetic field to guide a longitudinally spin-polarized electron beam, the present instrument employs fewer electrodes. It also uses a commercially available integrated photon counting module. These features allow it to occupy a smaller volume and make it easier to operate.

We describe the operation of a prototype polarized-electron source. Rubidium vapor, contained in a cell, is optically pumped in the presence of a buffer gas. Unpolarized electrons from a tungsten filament are injected into the cell and extracted after undergoing spin exchange with the Rb atoms. We compare the performance of the source when different buffer gases are used. We measure a decrease in electron polarization as their injection energy increases, but find an unexpected regime at higher injection energies yielding increased electron polarization accompanied by a 40-fold increase in current, suggesting the production of slow secondary electrons in the target cell. With ethylene, we have measured electron currents of 4 μA simultaneously with electron polarizations of 24%. This work offers the promise of a simple, benchtop, “turnkey” source of polarized electrons.

Laser light with photon energy near the band gap of GaAs and in Laguerre-Gaussian modes with different amounts of orbital angular momentum was used to produce photoemission from unstrained GaAs. The degree of electron spin polarization was measured using a micro-Mott polarimeter and found to be consistent with zero with an upper limit of ∼3% for light with up to ±5¯h of orbital angular momentum. In contrast, the degree of spin polarization of 32.3 ± 1.4% using circularly polarized laser light at the as the same wavelength, which is typical for bulk GaAs photocathodes.

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We have developed a simple heuristic method for determining the polarization of an optically pumped alkalimetal vapor. A linearly polarized probe beam traverses a vapor cell perpendicular to the pump-beam propagation direction, and the transmitted beam intensity is monitored for orthogonal linear polarizations. As the probe beam is scanned in frequency across the D1 transition, its linear-polarization-dependent transmission can be used as a measure of the atomic orientation of the vapor. We analyze these transmission differences and their dependence on the alkali-metal number density in the vapor.

We present a Monte Carlo simulation of a cylindrical luminescent volume and a typical lens–detector system. The results of this simulation yield a graphically simple picture of the regions within the cylindrical volume from which this system detects light. Because the cylindrical volume permits large angles of incidence, we use a modification of the thin-lens approximation for ray tracing. We compare simulation results with concepts from imaging optics, and comment on implications for experimental design.

This paper describes measurements of the beam polarization and quantum efficiency for photoemission using two-photon excitation from unstrained bulk GaAs illuminated with pulsed, high intensity 1560nm laser light. Quantum efficiency is linearly proportional to 1560nm peak laser intensity, which was varied in three independent ways, indicating that the emitted electrons are promoted from the valence to the conduction band via two-photon absorption. Beam polarization was measured using a microMott polarimeter, with a value of 16.8(4)% polarization at 1560nm, which is roughly half the measured value of 33.4(8)% using 778 nm light.

We have measured the production of Lyman α and Balmer α fluorescence from atomic H and D for the photodissociation of H₂, HD and D₂ by linearly-polarized photons with energies between 22 and 64 eV. We discuss systematic uncertainties associated with our data, and compare our results with previous experimental results and ab initio calculations of the dissociation process. We comment on the discrepancies.

We report measurements of linear and circular fluorescence polarizations for molecular transitions in H₂, D₂, and N₂ induced by spin-polarized electron impact. Circular polarizations resulting from some Fulcher-α transitions in H₂ and D₂ are found to be significant, while the null results from nitrogen’s second positive system are consistent with an earlier measurement by the M¨unster group. We compare this nitrogen data to that from our previous study of nitrogen’s first negative system. Emphasis is placed on understanding the mechanisms that cause the values of circular polarization from N2 to be relatively small compared to those observed from H₂ and D₂ molecules.

Nuclear physics experiments at Thomas Jefferson National Accelerator Facility’s CEBAF rely on high polarization electron beams. We describe a recently commissioned system for prequalifying and studying photocathodes for CEBAF with a load-locked, low-voltage polarized electron source coupled to a compact retarding-field Mott polarimeter. The polarimeter uses simplified electrode structures and operates from 5 to 30 kV. The effective Sherman function for this device has been calibrated by comparison with the CEBAF 5 MeV Mott polarimeter. For elastic scattering from a thick gold target at 20 keV, the effective Sherman function is 0.201(5). Its maximum efficiency at 20 keV, defined as the detected count rate divided by the incident particle current, is 5.4(2) x 10⁴, yielding a figure-of-merit, or analyzing power squared times efficiency, of 1.0(1) x 10⁵. The operating parameters of this new polarimeter design are compared to previously published data for other compact Mott polarimeters of the retarding-field type.

We have studied the optical pumping of mixtures of Rb vapor and N₂ buffer gas by laser light tuned to the D₁ transition having a spectral width of ∼500 MHz. The Rb densities are of the order of 1013 cm^-3, while the buffer-gas pressures range from 0.1 to 10 torr. As the frequency of the right-hand circularly polarized laser is varied across the D₁ absorption profile, the electron spin polarization of the Rb is found to take on negative values for small negative values of pump detuning from the absorption profile center. This occurs for N₂ pressures below ∼1 torr; at 10 torr the electron spins consistently point in the same direction as the angular momentum of the pump light. The spin-reversal effect can be understood in terms of populations of the F = 2 (85Rb) and F = 1 (87Rb) states caused by small unpolarized fractions in the pump beam and its elimination in terms of pressure broadening caused by the N₂ buffer gas. We speculate that this effect could be used for fast Rb spin modulation.

We have studied simultaneous photoionization and excitation of Ar in the range of incident photon energies between 36.00 and 36.36 eV

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This monograph reviews developments in the general area of polarized electron scattering from atoms and molecules since 1991, the date of the last such review in this series of monographs [Kessler, J. (1991). Advances in Atomic, Molecular and Optical Physics, 27, 81]. The physics of spin dependence in electronic collisions with atomic and molecular targets is outlined, with emphasis on the qualitative effects that can be probed using polarized beams and/or targets as well as analysis of the residual target and the scattered electron polarizations. Using the categories of exchange scattering, spin–orbit coupling, and interference between the two, experiments which elucidate these interactions are discussed for atomic and molecular targets, respectively. Developments in polarized electron sources and electron polarimeters since 1991 are also reviewed, and promising new technologies discussed.

We have measured the linear and circular polarization of 391.4 nm N₂⁺ B ²∑⁺_u (v'=0)→X ²∑⁺_g (v''=0) P-branch fluorescence produced by spin-polarized electron impact excitation of N₂ X² ∑_g⁺(v''=0) ground states. The resulting nonzero values of the circular polarization indicate that spin-rotation coupling, in conjunction with electron exchange excitation, acts to produce oriented rotational angular momenta in the excited molecular state prior to decay.

We have studied simultaneous photoionization and excitation of Ar in the range of incident photon energies between 36.00 and 36.36 eV, where the resonant production of doubly excited neutral Ar states imbedded in the ionization continuum is dominant. By measuring the relative Stokes parameters of the fluorescence from residual Ar^+∗ (3p⁴ [³P] 4p) ions (²P_1/2, 465.8 nm transition; ²P_3/2, 476.5 nm; ²D_3/2, 472.7 nm; ²D_5/2, 488.0 nm; ⁴P_5/2, 480.6 nm; ⁴D_5/2, 514.5 nm) we demonstrate a technique for determining individual partial-wave cross sections in photoionizing collisions. This procedure is shown to be important in sorting out competing dynamical ionization mechanisms, particularly with regard to resonant production of intermediate doubly excited autoionizing states. Comparison with theoretical photoionization cross sections demonstrates that spin–orbit coupling between different states of Ar II needs to be accounted for in the calculations.

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A method for the active feedback reduction of optical instrumental intensity asymmetries is presented. It is based on the fast chopping of two spatially separated beams of light with orthogonal linear polarizations that are recombined and passed through a quarter-wave plate to yield a single beam with rapidly flipping helicity. Active electro-optic feedback has been successfully employed to maintain this asymmetry below 10⁻⁵.

The measurement of optical excitation functions excited by electron impact is typically accomplished by recording atomic fluorescence emitted into a small solid angle perpendicular to the incident electron beam. This measured intensity is not proportional to the emission cross section because the fluorescence exhibits an angular distribution and polarization that varies with the energy of the exciting electrons. Typically, a polarizer is set at the “magic angle” (54.7°) with respect to the electron beam axis to remove this polarization dependence. The literature for the derivation of the magic angle value assumes the polarizing element is perfect. An expression for the angle that accounts for the use of a partial polarizer is presented.

We have made a systematic study of rubidium optical pumping in a simple cylindrical cell geometry with a high-power 10 W diode laser array, low magnetic fields, and buffer-gas pressures of less than 50 torr. We have determined rubidium polarizations experimentally for H₂, N₂, He, and Ar buffer gases, with Rb number densities from 10¹² to 10¹³ cm⁻³. Comparison to a relatively simple optical pumping model allows us to extract useful information about radiation trapping and quenching effects.

We discuss recent experiments that study the transfer of angular momentum from a projectile to the residual target in collisions between the simple diatomic molecules H2 and N2 and spin-polarized electrons or circularly-polarized photons. We observe the fluorescence of both the atomic fragments and excited molecular states, and measure the circular polarization fraction of this light, P3. The incident electron energies range from 10 to 100 eV; the incident photon energies from 33 to 38 eV.

We investigate the causes of electron-circular dichroism (ECD) in bromocamphor and dibromocamphor, focusing specifically on the electron helicity density of the target. Using electron transmission spectroscopy (ETS) and quantum chemical calculations, we have observed and assigned temporary negative ion states of bromocamphor and dibromocamphor. Further calculations were conducted to determine the helicity densities of these compounds. Large helicity densities are found in the regions of large wavefunction amplitude of the normally unoccupied molecular orbitals responsible for resonances in the scattering cross sections. We relate our ETS assignments and helicity density results to the chiral asymmetry data observed in electron-circular dichroism experiments by the Munster group (Nolting et al 1997 J. Phys. B: At. Mol. Opt. Phys. 30 5491). Our results support helicity density as a possible source of chiral asymmetry at certain resonancepositions in bromocamphor and dibromocamphor.

We have investigated helium (1s3d) ³D → (1s2p) ³P (588 nm) fluorescence produced by electron impact excitation in the vicinity of the (2s²2p) ²P and (2s2p2) ²D negative-ion resonances at 57.2 and 58.3 eV, respectively. In contrast to previous work, we use spin-polarized incident electrons and report the relative Stokes parameters P₁, P₂ and P₃ in the 55–60 eV region. Our failure to see discernable resonance effects in P₂ indicates that even though the lifetime of these resonances is significant (∼10 fs), magnetic forces acting on the temporarily captured electron are small. Resonant structures in the values of P₁ and P₃ are observed because the polarization contributions of resonant states are generally different than those from direct excitation of the 3 ³D state.

We have measured the production of both Lyα and Hα fluorescence from atomic H and D for the photodissociation of H₂ and D₂ by linearly polarized photons with energies between 24 and 60 eV. In this energy range, excited photofragments result primarily from the production of doubly excited molecular species which promptly autoionize or dissociate into two neutrals. Our data are compared with ab initio calculations of the dissociation process, in which both doubly excited state production and prompt ionization (nonresonant) channels are considered. Agreement between our experimental data and that of earlier work, and with our theoretical calculations, is qualitative at best.

We have investigated collisions between transversely polarized electrons and Ar, in which the Ar is simultaneously ionized and excited to the Ar^+*[3p⁴(¹D)4p] states. The Stokes parameters of the fluorescence emitted in the following transitions was measured: (¹D)4s ²D_5/2−(¹D)4p ²F_7/2 (461.0 nm), (¹D)4s ²D_5/2−(¹D)4p ²F_5/2 (463.7 nm), (¹P)3d ²D_5/2−(¹D)4p ²F_5/2 (448.2 nm), and (¹D)4s ²D_3/2−(¹D)4p ²P_3/2 (423.7 nm). We develop the angular momentum algebra necessary to extract from these data, starting from the overall atomic J multipoles, the partitioning of orbital angular momentum into the ¹D core electric quadrupole and hexadecapole moments, and the outer 4p electric quadrupole moment. The magnetic dipole of the outer electron is also determined. This procedure requires the assumption of good LS coupling for these states, which is justified. We recouple these individual core- and outer-electron moments to calculate the initial electric quadrupoles, hexadecapoles, and hexacontatetrapoles of the initial excited-state manifold. The detailed time structure of the electron-atom collision is considered, as well as the time evolution of the excited ionic state. The Rubin-Bederson hypothesis is thus shown to hold for the initial ionic L and S terms. The consequences of the breakdown of LS coupling are considered. From the circular polarization data, estimates of the relative importance of direct and exchange excitation cross section are made. We discuss experimental issues related to background contributions, Hanle depolarization of the fluorescence signal, and cascade contributions. Nonlinearity of the equations relating the Stokes parameters to the subshell multipole moments complicates the data analysis. Details of the Monte Carlo terrain-search algorithm used to extract multipole data is discussed, and the implications of correlation between the various subshell multipole moments is analyzed. The physical significance of the higher-order multipole moments is discussed, and graphical representations of the effects of these multipoles on the excited ionic charge clouds is presented.

We have measured the circular polarization of light emitted from both atomic H and molecular H₂ after bombarding H₂ with longitudinally polarized electrons. For both atomic and molecular fluorescence near threshold we observe a circular polarization as great as 10% of the electron polarization. This represents the first direct observation of spin transfer in electron-molecule collisions.

We have used a new configuration for a noble gas optical electron polarimeter. This polarimeter is part of an experiment involving dichroic scattering of longitudinally polarized electrons from chiral molecules. Our polarimeter sits along the electron beam axis at the end of the apparatus and measures the polarization of noble gas fluorescence emitted at 0°. The polarimeter is maximally sensitive to longitudinal electron polarization, and it maintains the axial symmetry of our experiment.

We obtain relative cross sections for the production of photoelectrons with specific angular momentum quantum numbers. These cross sections are obtained from the polarization analysis of the visible fluorescence of ions produced when circularly polarized vacuum ultraviolet radiation photoionizes ground state Ar. The ratio of cross sections for the production of photoelectrons with the same orbital angular momentum but different total angular momenta shows strong deviations from the statistical ratio, demonstrating the importance of relativistic interactions in many-electron photoionization dynamics.

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Using the computer program SIMION 6, we have studied the effects of spurious insulator charging on the trajectories of electrons through electrostatic tube lenses. We considered lens elements with flat ends, spaced a distance g apart, whose inner diameter D=10g. For the standard cases of drift tubes, two-element lenses, and einzel lenses, we found that charging effects are eliminated if t/g>3.5, where t is the distance between the lens inner diameter and the charged spacing insulator.

We present the results of a series of model Monte Carlo calculations of the scattering of spin-polarized electrons from gold foil targets. Our calculations examine the behavior of the left-right scattering asymmetry A as a function of various parameters conventionally used in extrapolation of the left-right asymmetry to singleatom and/or elastic scattering conditions. These parameters include target thickness, scattered count rate, and the maximum energy that an electron can have lost in the target and still be detected. Data are obtained at incident electron energies of 10–120 keV, with detector-subtended half-cone angles of 5°, 10°, and 20°, and gold foils of average thickness varying from 3 to 1000Å. Both elastic and inelastic electron scattering effects have been considered. Comparisons of our results are made with existing measurements and theoretical models. We make recommendations concerning extrapolation algorithms and for future experiments to test the present Mott scattering Monte Carlo model.

We report our work in developing new turn-key sources of polarized electrons. These sources operate by extracting the electrons from a discharge and polarizing them through optical pumping. Preliminary work demonstrates that beams of 4 µA with greater than 20% polarization are possible. Such devices could enormously simplify the running of many experiments in the fields of atomic structure, magnetic materials and biophysics.

We present the results of a rigorous quantum-mechanical calculation of the propagation of electrons through an inhomogeneous magnetic field with axial symmetry. A complete spin polarization of the beam is demonstrated assuming that a Landau eigenstate can be inserted into the field. This is in contrast with the semiclassical situation, where the spin splitting is blurred.

We have studied polarized electron collisions with Ar in which the target is simultaneously ionized and excited to form Ar⁺[3p⁴(¹D)4p] states. We measured the integrated Stokes parameters of the subsequent fluorescence emitted by the ²F_7/2, ²F_5/2, ²D_5/2, and ²P_3/2 states along the direction of electron polarization. The Rubin-Bederson hypothesis is shown to hold for the L and S multipoles of these states. The electric quadrupole and hexadecapole of the ¹D core are derived. By recoupling these moments with the electric quadrupole moment of the 4p electron, we calculate higher moments of the total ionic orbital angular momentum, including its hexacontatetrapole (64-pole) moment.

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We report an attempt to make an optical observation of Matt scattering, involving the first studies of inelastic collisions between polarized electron\textasciicircum and a heavy noble gas. Polarization fractions of light emitted by the 5ps6p[\textasciitilde],('D,) state of Xe following impact excitation in an axial collision geometry were measured as a funclion of the incident electron energy, and compared with distorted-wave Bom calculations. The theoretical and experimental results agree qualitatively in the energy range where cascade contributions to the photon signal are small. We failed to measure non-zero values of the linear polarization fraction 7 , . which would have constituted unambiguous evidence for Matt scattering and/or the importance of higher-order excitation processes in this system.

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The spectra of electrons emitted in collisions between H+ and He2+projectiles and He, Ne and Ar targets at energies of 50 and 100 keV amu-’ have been studied. The data are in qualitative agreement with results of Irby et ai, but are in disagreement with recent measurements of Bernardi et al. It is shown that the observed electron spectra have a dependence on both target-ion and projectile effective charge that can be understood qualitatively in terms of ‘saddle-point’ ionization. Several issues relevant to saddle-point ionization are discussed.

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We have made absolute and relative measurements of differential cross sections for single-electron transfer in collisions between Mg+ (30-150 keV) and Be+ (56.25 keV) ions and He atoms. The behaviour of transfer probability as a function of impact parameter can be understood qualitatively from recent molecular orbital calculations of quasi-oneelectron systems.

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We present measurements of the intensity and polarisation of light emitted from two excited states of neutral helium, after break-up of the HeH+ molecule in thin foils. We vary the foil thickness to study their dependence on the internuclear separation of the He and proton at the final foil surface and postulate that the variations observed are sensitive to the molecular orbitals of the dissociating HeH' molecule. These vicinage effects provide the distance scale of the final state interaction at the foil surface.

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Mat erial-dependent variations of alignment in beam-foil excitation. H . G. Berry, G. Gabrielse, T. Gay and A. E. Livingston (Department of Physics, Uni versity of Chicago, Chicago, USA and Argonne National Laboratory, Argonne, USA).

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