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Radio frequency discharge apparatus for studying spin transfer from solid surfaces to metastable helium gas

Haruka MARUYAMA, Keisuke Shaku, Eiji Saitoh, A. Hatakeyama

Published Mar 1, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.1063/5.0189499

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•Jan 1, 1995•0 citations
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.
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The alignment of He4{\mathrm{He}}^{4}He4 atoms in the (n=2n=2n=2, metastable) 3S1^{3}S_{1}3S1​ state is described. Metastable atoms are produced by an rf discharge in a glass tube containing a few mm of pure helium, and the one micron pumping light ($2^{3}P\ensuremath{-}2^{3}S$) is provided by a helium lamp. A resonance signal is obtained from radio frequency disorientation by monitoring the transmitted pumping light. The double maximum line shape of this signal for strong rf magnetic fields is discussed. Included also is a discussion of the angular dependence of the signal when unpolarized light is used and an explanation of the inversion of the resonance signal for certain densities of the metastable helium atoms.The measured relaxation time of the oriented metastable atoms in the discharge is about 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ second and the pumping time is about a millisecond. A method is proposed and initial measurements are given for the cross section for destruction of metastable helium atoms by collision with foreign gas atoms. The application of optical pumping in helium to the measurement of weak magnetic fields is also discussed.
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.
A polarized He{sup +} ion source based on spin conservation in Penning ionization reactions involving polarized He(2{sup 3}S) atoms contained in an optically-pumped, rf-excited helium discharge is being developed for surface studies. To optimize source performance, the He(2{sup 3}S) polarization must be determined. The authors have devised a technique for doing this that involves measuring the attenuation of a circularly-polarized probe beam provided by a 1.083 {mu}m DBR diode laser whose output frequency is scanned by varying the drive current to allow separate measurements on the 2{sup 3}S{sub 1}-2{sup 3}P{sub 0}, 2{sup 3}P{sub 1} and 2{sup 3}P{sub 2} transitions. As will be discussed, several independent expressions for the polarization (and alignment) can be derived from the experimental data thus providing valuable internal consistency checks. The technique is simple to implement and reliable in operation.
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