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Physical Review Letters

Spin Dependence in the Survival Probability of Metastable He ( 2S3 ) Atoms during the Scattering from Ferromagnetic Surfaces

Mitsunori Kurahashi, Taku Suzuki, Xin Ju, Y. Yamauchi

Published Dec 24, 2003• 16 citations
Publisher

Abstract

The use of a spin-polarized metastable helium (${\mathrm{H}\mathrm{e}}^{*}$) atom beam has demonstrated that the survival probability (SP) of ${\mathrm{H}\mathrm{e}}^{*}$ during the scattering from ferromagnetic surfaces depends on the electron spin of ${\mathrm{H}\mathrm{e}}^{*}$. The spin asymmetry in the SP of ${\mathrm{H}\mathrm{e}}^{*}$ scattered from a clean $\mathrm{F}\mathrm{e}/\mathrm{C}\mathrm{u}(100)$ surface changed its polarity with ${\mathrm{H}}_{2}\mathrm{O}$ adsorption. We argue that the reversal is caused by the change in the spin polarization of the surface electronic states at around the Fermi level.

DOI
10.1103/physrevlett.91.267203

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A theoretical study is performed to re-evaluate the scattering length for spin-polarized metastable helium. The Σ5g+ potential is calculated with three huge basis sets, as well as one previously proposed, using two ab initio approaches: a large configuration interaction method and a coupled-cluster method. The influence of a dummy basis set is analyzed and shown to yield spurious behavior in the energy decrease, roughly Gaussian with changing separation of the dummy basis from the molecule. The scattering length is found to be extremely sensitive here even to tiny details of the potential because the potential is very close to supporting a fifteenth rotationless vibrational level. This behavior can be rationalized using semiclassical methods. From this study we conclude that there are certainly 15 vibrational levels and that the dissociation energy is most probably 1045.2±1 cm−1, while our final result for the scattering length, a=291 a0, is best considered as an upper limit.

The Journal of Chemical Physics•Oct 7, 2002•18 citations
The use of a spin-polarized metastable helium (He∗{\mathrm{H}\mathrm{e}}^{*}He∗) atom beam has demonstrated that the survival probability (SP) of He∗{\mathrm{H}\mathrm{e}}^{*}He∗ during the scattering from ferromagnetic surfaces depends on the electron spin of He∗{\mathrm{H}\mathrm{e}}^{*}He∗. The spin asymmetry in the SP of He∗{\mathrm{H}\mathrm{e}}^{*}He∗ scattered from a clean Fe/Cu(100)\mathrm{F}\mathrm{e}/\mathrm{C}\mathrm{u}(100)Fe/Cu(100) surface changed its polarity with H2O{\mathrm{H}}_{2}\mathrm{O}H2​O adsorption. We argue that the reversal is caused by the change in the spin polarization of the surface electronic states at around the Fermi level.
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.
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.
The energy of interacting spin-polarized atomic hydrogen (${\mathrm{H}}_{\mathrm{\ensuremath{\downarrow}}}$) on a helium surface and the density dependence of the adsorption energy have been calculated, permitting a realistic determination of the adsorption isotherms. Surface interactions are shown to allow Bose-Einstein condensation in the gas. At the Bose-Einstein condensation the surface coverage attains a maximum which is essentially independent of the isotherm temperature. Also calculated is the stability condition for surfaces in magnetic fields. Easily met requirements are found.
Low-energy properties of spin-polarized electron-helium (23S)(2\phantom{\rule{0.16em}{0ex}}^{3}S)(23S) and spin-polarized positronium-helium (23S)(2\phantom{\rule{0.16em}{0ex}}^{3}S)(23S) scattering are calculated using the modified confined variational method. To take the van der Waals interaction into consideration, the van der Waals coefficient between positronium and helium (23S)(2\phantom{\rule{0.16em}{0ex}}^{3}S)(23S) is calculated using the explicitly correlated Gaussian basis. Compared with the ground state noble gases, the larger scattering length and zero pickoff annihilation rate indicates that He(23S)\mathrm{He}(2\phantom{\rule{0.16em}{0ex}}^{3}S)He(23S) may be a more suitable cooling gas. In the energy range we considered, no similarity is found for the SSS-wave cross sections between these two scatterings. A Ramsauer-Townsend minimum is observed in spin-polarized electron-helium(23S)(2\phantom{\rule{0.16em}{0ex}}^{3}S)(23S) scattering due to the large polarization of metastable helium. This may open a new way to find and study the Ramsauer-Townsend effect.
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 theoretical study is performed to re-evaluate the scattering length for spin-polarized metastable helium. The Σ5g+ potential is calculated with three huge basis sets, as well as one previously proposed, using two ab initio approaches: a large configuration interaction method and a coupled-cluster method. The influence of a dummy basis set is analyzed and shown to yield spurious behavior in the energy decrease, roughly Gaussian with changing separation of the dummy basis from the molecule. The scattering length is found to be extremely sensitive here even to tiny details of the potential because the potential is very close to supporting a fifteenth rotationless vibrational level. This behavior can be rationalized using semiclassical methods. From this study we conclude that there are certainly 15 vibrational levels and that the dissociation energy is most probably 1045.2±1 cm−1, while our final result for the scattering length, a=291 a0, is best considered as an upper limit.