G. K. Walters

Simple, compact, medium‐energy Mott polarization analyzer

A simple, compact, UHV compatible Mott polarization analyzer is described that employs electron accelerating voltages in the range 20–40 keV.The analyzer provides excellent discrimination against inelastically scattered electrons and has the advantage that the major portion of the apparatus, and the scattered electron detectors, are operated near ground potential. The efficiency of the analyzer, ∼2×10−5, is competitive with those provided by other polarimeters. The analyzer can measure the polarization of electron beams having currents as low as ∼10−13 A and is suitable for use in a wide variety of experiments involving polarized electron beams.

Simple and compact low‐energy Mott polarization analyzer

A UHV compatible Mott polarization analyzer is described that employs electron accelerating voltages of ∼20 kV. The efficiency of the analyzer, ∼3×10−5, is competitive with those provided by other polarimeters. The present analyzer is considerably simpler and more compact than earlier designs and can be used to undertake energy‐ and angle‐resolved polarization measurements with input beam currents below ∼10−14 A.

Polarized He (23S) thermal metastable atom source

A source of polarized He (23S) metastable atoms is described that provides a thermal energy beam with a flux of the order of ∼1014 metastables s−1 steradian−1 at a polarization Pz∼50%. The polarization of the beam can be simply reversed, or modulated, without changing the beam trajectory or flux. The beam contains no significant admixtures of He(21S) atoms, photons, or fast neutrals and is suitable for use in a wide variety of collision and surface physics experiments.

Compact low‐energy Mott polarimeter for use in energy‐ and angle‐resolved polarization studies

A simple low‐voltage (20‐kV) UHV‐compatible Mott polarimeter is described that incorporates an in‐line retarding potential energy analyzer that, at incident electron energies ≤50 eV, provides an energy resolution of ≤0.5 eV. The polarimeter is very compact and is suitable for use in a wide variety of applications requiring energy‐ and angle‐resolved polarization measurements.

Intense source of spin‐polarized electrons using laser‐induced optical pumping

A source of spin‐polarized electrons based on a laser‐pumped flowing helium afterglow is described. He(23S) atoms contained in the afterglow are optically pumped using circularly polarized 1.08‐μm (23S→23P) radiation provided by a NaF (F2+)* color‐center laser. Spin angular momentum conservation in subsequent chemi‐ionization reactions with CO2 produces polarized electrons that are extracted from the afterglow. At low currents, ≲1 μA, polarizations of ∼70%–80% are achieved. At higher currents the polarization decreases, falling to ∼40% at 50 μA. The spin polarization can be simply reversed (P→−P) and the source is suitable for use in the majority of low‐energy spin‐dependent scattering experiments proposed to date.

Improved source of polarized electrons based on a flowing helium afterglow

The performance of the Rice source of spin polarized electrons, which is based on an optically pumped flowing helium afterglow, has been substantially improved. He(23S) metastable atoms contained in the afterglow are optically pumped using 1.08 μm 23S1↔23P1 radiation from an LNA laser. Spin conservation in subsequent chemi‐ionization reactions with CO2 results in the production of free polarized electrons that are extracted from the afterglow. At low currents, ≲1 μA, polarizations of 80%–90% are achieved. This decreases to ∼75% at 10 μA and to ∼50% near 100 μA. The polarization can be simply reversed (P→−P). The energy spread in the extracted beam is <0.4 eV, and the beam emittance is <4 mrad cm−1 at 270 eV. This source is suitable for use in a wide variety of applications, and is particularly attractive for use with the new generation of high‐duty factor electron accelerators that are currently being developed.

Optical pumping of He(23S) atoms by a color‐center laser

A color‐center laser operating at 1.08 μm has been used to optically pump a thermal He(23S) atom beam via 23S→23P transitions. Even modest laser powers (∼10 mW) result in more efficient optical pumping than is obtained using a 400‐W rf‐excited helium resonance lamp. The laser also permits optical pumping via transitions to selected 23P fine structure levels.

Use of thick gold foils in retarding-potential Mott polarimeters

Data are presented showing that thick scattering foils (or solid targets) can be used in retarding‐potential Mott polarimeters without degrading their performance. Use of thick foils can simplify the design and operation of such polarimeters. A straightforward calibration procedure is described.

Measurement of the spin polarization of an optically pumped ensemble of He (23S) atoms

A technique is described that enables simple, direct determination of the spin polarization of an optically pumped ensemble of He (23S) atoms. The polarization is obtained from measurements of the attenuation of the circularly polarized output of a color‐center laser tuned to the 1.08 μm He (23S1→23P0) transition.

Temperature dependence of electron spin polarization in low‐energy electron diffraction from W(001)

The temperature dependence of electron spin polarization and intensity in low‐energy electron diffraction (LEED) from a clean W(001) surface has been measured over the range 1000°–300°C. The effect of temperature on polarization is found to be greatest at energies for which the intensity is low and the polarization, and rate of change of polarization with energy, is large. The present results may be interpreted in terms of a shift of polarization features toward lower energies as a result of thermal expansion when the crystal is heated.