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Spin-Polarized Triplet Helium.

Jonas Stasys Zmuidzinas

Published Apr 1, 1984• 3 citations
OpenAlex

Abstract

Abstract : It is pointed out that bulk quantities of spin-polarized Triplet He* (2 Superscript 3 subscript 1) and Triplet He2* He2(a Superscript 3 sigma subscript u (+) should form insulating classical solids, respectively denoted as He IV-A and He IV-M. Estimates of some basic physical properties of He IV-A are given. A metallic phase of He IV-A is considered and modeled by a two-component Fermi fluid of spin-polarized He(+), ions and electrons. This metallic phase is predicted to occur at r subscript s approx. 2.3 and to lie considerably lower in energy than insulating He IV-A(M), making it the preferred phase to be looked for experimentally. A number of deexcitation mechanisms of atomic, dimerized, and bulk He* are identified, and the open problem of stability is discussed. A method is proposed to produce He IV-M, and ultimately metallic He IV-A, by optically pumping the He2* species produced by electron bombardment of superfluid helium. (Author)

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Review of Scientific Instruments•Aug 1, 1995•14 citations
Abstract : It is pointed out that bulk quantities of spin-polarized Triplet He* (2 Superscript 3 subscript 1) and Triplet He2* He2(a Superscript 3 sigma subscript u (+) should form insulating classical solids, respectively denoted as He IV-A and He IV-M. Estimates of some basic physical properties of He IV-A are given. A metallic phase of He IV-A is considered and modeled by a two-component Fermi fluid of spin-polarized He(+), ions and electrons. This metallic phase is predicted to occur at r subscript s approx. 2.3 and to lie considerably lower in energy than insulating He IV-A(M), making it the preferred phase to be looked for experimentally. A number of deexcitation mechanisms of atomic, dimerized, and bulk He* are identified, and the open problem of stability is discussed. A method is proposed to produce He IV-M, and ultimately metallic He IV-A, by optically pumping the He2* species produced by electron bombardment of superfluid helium. (Author)
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.
Abstract A theory of the helium atom was developed by Heisenberg, following his famous principle of “resonance.” His work is based upon Schrodinger’s equation, and in order to allow for the spin of the electrons he introduces extra energy terms, expressing the classical energy of two appropriate magnets. It is the perturbation by this spin energy which produces the triplet separations. Heisenberg’s calculation of these separations is open to three criticisms: (1) He assumes that certain simple forms are correct first approximations to the wave-functions; whereas, owing to degeneracy, it is necessary to take linear combinations of these forms. (2) He estimates the mean values of the spin energy by means of a model built up of precessing vectors. (3) He neglects the radius of the inner orbit in comparison with that of the outer orbit, even when the principal quantum number of the latter is only 2; this is the least satisfactory of several necessary simplifications. Schrödinger’s equation is likely to be supplanted as the foundation of wave-mechanics by the equation recently put forward by Dirae. This fits the spin of the electron neatly into the theory of relativity, and produces the doublets of the hydrogen-like atom in a beautiful manner. Dirac’s q-number theory has been translated into wave-mechanics by Darwin. Apart altogether from the above criticisms of Heisenberg, it seemed expedient to proceed to the theory of an atom with two electrons on the basis of the new equation. Calculations which are independent of the spin, such as the approximate energylevels, and the separation between ortho- and para-terms, are the same on either theory, and are not the concern of this paper. We deal here with spin effects, such as the fine structure of the triplets and intercombinations between ortho- and para-states.
We briefly review the field of spin polarized fluids. The study of spin polarized hydrogen was initially motivated by the hope to observed a Bose–Einstein condensation in a dilute gas. This goal has not yet been reached but, on the way, a lot of beautiful physics has been done, including atom–wall interactions studies and the observation of nuclear spin waves. Spin polarized helium can be studied in gas, liquid and solid phases. The two major polarization methods are fast melting of polarized solid or fast condensation of optically pumped helium three gas. We briefly discuss heat conductivity changes under the effect of nuclear spin polarization, sound velocity, and finally liquid–gas phase diagrams.
By D1{\mathit{D}}_{1}D1​-line optical pumping of alkali-metal atoms (Cs and Rb) in superfluid helium (He II), we could achieve large spin polarization of the atoms in the ground states, more than 50% for both electron and nuclear spins, especially for the Cs atoms. The alkali-metal atoms were implanted into He II by laser sputtering. We detected optically the magnetic resonances between the ground-state Zeeman sublevels, and found that the g values and hyperfine constants in He II were the same as in vacuum within the experimental error. We discuss possible applications of these spin-polarized atoms in He II.
Some years ago Kastler pointed out that the equilibrium constant for alkali metal atom dimer formation could be suppressed by polarizing the electrons so that the atoms interacted via the weak triplet potential. That basic idea has now been realized with atomic hydrogen, but using a polarization technique involving high magnetic fields and low temperature rather than optical pumping. By achieving high polarization, the gas can be stabilized at densities a few seconds over 10^18 atoms cm-3, and for minutes or hours at lower densities.
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.