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The Journal of Chemical Physics

Focusing and spin polarization of atomic hydrogen beam

Y. Nagaya, H. Nakatsu, Shohei Ogura, K. Shimazaki, Hirokazu Ueta, Kotaro Takeyasu, Katsuyuki Fukutani

Published Nov 2, 2021• 3 citations
Publisher

Abstract

We have developed a spin-polarized-hydrogen beam with a hexapole magnet. By combining the beam chopper and pulsed laser ionization detection, the time-of-flight of the hydrogen beam was measured, and the dependence of the beam profile on the velocity was acquired, which was consistent with the beam trajectory simulations. The spin polarization of the beam was analyzed by using the Stern-Gerlach-type magnet in combination with the spatial scan of the detection laser. The spin polarization was about 95% at a focusing condition due to the hexapole magnet. The polarization was, on the other hand, reduced to about 70% for the beam at higher velocities, which is consistent with simulation results.

DOI
10.1063/5.0068251

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We have developed a spin-polarized-hydrogen beam with a hexapole magnet. By combining the beam chopper and pulsed laser ionization detection, the time-of-flight of the hydrogen beam was measured, and the dependence of the beam profile on the velocity was acquired, which was consistent with the beam trajectory simulations. The spin polarization of the beam was analyzed by using the Stern-Gerlach-type magnet in combination with the spatial scan of the detection laser. The spin polarization was about 95% at a focusing condition due to the hexapole magnet. The polarization was, on the other hand, reduced to about 70% for the beam at higher velocities, which is consistent with simulation results.
In the past, the Stern–Gerlach experiment has been used as a standard method for analyzing the population of magnetic substates contained in spin-polarized and/or state-selected atomic/molecular beams. However, this experiment is quite demanding due to its low signal intensity and difficulty in beam alignment. The present study shows that the use of a hexapole magnet and a spin flipper, together with the time-of-flight analysis, allows us to conduct an almost equivalent analysis while greatly improving the signal intensity. Applications to the analysis of spin-polarized triplet excited helium and state-selected O2( 3 Σg−) beams are presented.
An atomic beam of He in the metastable 2 3 S 1 state can be atomic-spin-polarised by Stern-Gerlach selection using a sextupole magnet. The authors report the first measurements of the polarisation obtained by this method. The authors have determined the metastable beam velocity distribution from a time-of-flight measurement. The distribution was found to be in the range of thermal velocities. The selection efficiency of the sextupole magnet was studied using a Stern-Gerlach deflection with an inhomogeneous dipole field. The experimental results give a vector polarisation close to 95%. Thus, the technique can be useful for many applications.
Results from a novel variation on the Stern–Gerlach method for magnetic separation are presented. A superconducting solenoid lens for spin selection and focusing of a cold atomic hydrogen beam has been built and tested at Brookhaven National Laboratory. The solenoid consists of three coils connected in series, with the current in the outer two coils counter to the current in the middle coil, resulting in a large magnetic field (up to 5.2 T) with strong gradients. Beam density enhancement (focusing) of up to a factor of 10 was achieved. Excellent qualitative agreement exists with ray-tracing calculations. Quantitatively, the experimental results show a lens performance which is a factor of at least 3 better than what these calculations predict.
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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.
An atom interferometer using two Stern-Gerlach magnets as polariser and analyser is described. The interferometer was first operated with a thermal beam of ground state potassium atoms. In that case the beam splitters are two radio-frequency zones within a transverse homogeneous magnetic field building a coherent superposition of Zeeman states. Ramsey fringes are obtained by scanning the RF-frequency through the resonance profile. Pulsing the RF power allows to get the time-of-flight distribution of the beam. Atomic interference fringes (``Stern-Gerlach fringes'') are observed when the magnitude of an inhomogeneous magnetic field located in between the beam splitters is scanned. When the inhomogeneous field is also pulsed, the scalar Bohm-Aharonov effect is observed. The use of metastable helium atoms (He*23S) in place of potassium atoms is proposed. In addition to the usual advantages provided by metastable atoms of high internal energy, this atom offers the specific advantage to behave as an ideal spin-one particle. This allows us to greatly simplify the polarisation scheme of the interferometer. Possible applications involving transversally inhomogeneous magnetic fields are given and limitations due to the finite source size and angular aperture are discussed.