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AN ELECTRON SPIN POLARIZED HELIUM-EXCITED(TRIPLET-2S) METASTABLE BEAM FOR USE IN SURFACE STUDIES

T. W. Riddle

Published Jan 1, 1982• 0 citations
OpenAlex

Abstract

A source of electron spin polarized He*(2('3)S) metastable atoms is described that provides a thermal energy beam with a flux up to 8 x 10('12) metastables/sec/sterad at a polarization P(,z) (TURN) 50%. The polarization is created by optical pumping and can be simply reversed or modulated without changing the beam trajectory or flux. The source is housed in a system of three differentially pumped vacuum chambers which allows the beam to be used for surface experiments in an ultra high vacuum environment. The polarization is measured with a Stern-Gerlach type analyzer which also serves as a useful diagnostic of beam composition and verifies the purity of the 2('3)S beam. The beam is demonstrated not to contain any significant admixture of 2('1)S metastables, ions, fast neutrals, or photons. Unless care is exercised, however, it is possible to produce a beam of energetic ground state helium which is of much greater intensity than the metastable beam. The conditions under which this occurs are described. A brief overview of metastable deexcitation spectroscopy (MDS) is given and a proposed first experiment involving the investigation of surface magnetism of Ni(110) and the necessary apparatus for this experiment are described.

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A source of electron spin polarized He*(2('3)S) metastable atoms is described that provides a thermal energy beam with a flux up to 8 x 10('12) metastables/sec/sterad at a polarization P(,z) (TURN) 50%. The polarization is created by optical pumping and can be simply reversed or modulated without changing the beam trajectory or flux. The source is housed in a system of three differentially pumped vacuum chambers which allows the beam to be used for surface experiments in an ultra high vacuum environment. The polarization is measured with a Stern-Gerlach type analyzer which also serves as a useful diagnostic of beam composition and verifies the purity of the 2('3)S beam. The beam is demonstrated not to contain any significant admixture of 2('1)S metastables, ions, fast neutrals, or photons. Unless care is exercised, however, it is possible to produce a beam of energetic ground state helium which is of much greater intensity than the metastable beam. The conditions under which this occurs are described. A brief overview of metastable deexcitation spectroscopy (MDS) is given and a proposed first experiment involving the investigation of surface magnetism of Ni(110) and the necessary apparatus for this experiment are described.
Diode laser optical pumping to produce a highly spin-polarized metastable He beam to be used in a spin-polarized metastable atom deexcitation spectroscopy experiment on magnetized surfaces is described. Efficient pumping of the beam is performed by means of an SDL-6702 distributed Bragg reflector diode laser which yields 50 mW of output power in a single longitudinal mode at 1083 nm, the resonance wavelength for the 23 S→23 P0,1,2 (D0, D1, and D2) transitions of He*. The light is circularly polarized by a quarter-wave plate, allowing easy change of the sense of atomic polarization. The laser frequency can be locked to the atomic transition for several hours by phase-sensitive detection of the saturated absorption signal in a He discharge cell. Any of the three transitions of the triplet system can be pumped with the laser but the maximum level of atomic polarization of 98.5% is found pumping the D2 line.
A commercial Spectra Diode Laboratories, 1.083 μ\muμm diode laser has been used to efficiently spin polarize an atomic beam of He(2$\sp3$S) metastable atoms by optical pumping on the $\rm 2\sp3S\sb1\rightarrow2\sp3P\sb1$ (D1) transition. The diode laser is an InGaAs/GaAs strained multiquantum well with a distributed Bragg reflector to insure single longitudinal mode operation. The laser has been frequency stabilized to less than 1 MHz/hr drift. The helium metastable atom beam is produced by electron impact and the polarization is measured using a Stern-Gerlach analyzer. Atomic polarizations exceeding 97% have been obtained with $\sim$25 mW of power from the diode laser. A number of experiments currently under construction may also make use of the diode laser. These include the production of a spin polarized helium ion beam by optically pumping the metastable atoms in an rf discharge, and the trapping of helium metastable atoms in a magneto-optical trap (MOT).
A spin-polarized triplet metastable helium (He*) beam has been used as a probe for surface magnetism, but changes in the spin state during scattering from a surface remain unclear. In the present study, we explored this issue by constructing an apparatus that allows us to direct a spin-polarized He* beam to a surface and measure the spin polarization of He* scattered from the surface. Magnetic hexapoles were used for both the beam polarization and the spin analysis. The results of the spin-dependent He* scattering experiments on clean Fe3O4{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}Fe3​O4​(100), H-terminated Fe3O4{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}Fe3​O4​(100), benzene-adsorbed Fe3O4{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}Fe3​O4​(100), and nonmagnetic Cu(100) surfaces indicated that, although the spin direction of He* was mostly preserved during scattering from these surfaces, spin-flop scattering of surviving He* occurred with a probability up to approximately 0.1. Our results showed that the survival probability was higher when the spins of He* and the Fe3O4{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}Fe3​O4​(100) film were parallel, which can be understood based on the lower He* resonance ionization rate for this spin orientation. Based on our findings, we estimate that a nonpolarized He* gas becomes 10% spin-polarized after a single collision with a clean Fe3O4{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}Fe3​O4​(100) surface.
We developed a radio frequency discharge apparatus for He gas to investigate the spin states of metastable helium (He*) interacting with solid-state surfaces. Our apparatus consisted of a stainless steel vacuum chamber, in which a coil produced He* by discharging introduced He gas. The spin states of the He* were detected using optical pumping and probing techniques. The chamber was designed to accommodate various solid-state samples. We measured the He* polarization produced at a dielectric prism surface by total internal reflection of the circularly polarized pumping light. Our apparatus can be used to investigate possible spin transfer from various solid surfaces to He* atoms.
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.
The excitation of a low-energy ground state helium beam to its metastable states by electron impact is analysed. It is shown that the narrowest velocity distribution for the resulting metastable helium beam is obtained by directing the helium and electron beams parallel or antiparallel. A source of metastable helium atoms was constructed according to this analysis which produced a beam of 3*10 14 metastable atoms/s sr with a velocity resolution of 3% FWHM for a room-temperature beam (E=66 meV). A possible source of errors in time-of-flight experiments with metastable atoms is pointed out.