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Review of Scientific Instruments

Use of hexapole magnet and spin flipper combined with time-of-flight analysis to characterize state-selected paramagnetic atomic/molecular beams

Mitsunori Kurahashi

Published Jan 1, 2021• 7 citations
Publisher

Abstract

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.

DOI
10.1063/5.0031903

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AIP conference proceedings•Jan 1, 1982•0 citations
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.
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.
O2 is one of the most important chemical species in every field of natural science. In the present study, a state-selected triplet molecular oxygen beam, in which nearly 100% of the molecules are in the spin-rotational state of (J, M)=(2, 2), has been produced by combining a supersonic seeded O2 beam with a hexapole magnet. The (2, −2) beam with >90% purity has also been obtained by a controlled non-adiabatic transition. The (2, ±2) beam, for which we can determine the spin and rotational angular momenta of O2 almost independently, is the most promising probe for studying the spin effects as well as the steric effects in O2 molecular scattering.
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.
A state-selected O(2)((3)Sigma(g)(-)) molecular beam, in which nearly 100% of the molecules are in the spin-rotational state of (J,M) = (2,2), has been produced by combining a supersonic seeded O(2) beam with a hexapole magnet. The (2,-2) beam has also been obtained by the state inversion of the (2,2) beam through the nonadiabatic passage in a reversing longitudinal magnetic field along the beam axis. The intensities of the other (2,M) states, which appear when applying additional transverse magnetic fields to the reversing field region, were well reproduced by the Majorana's formula for J = 2. The (2,+/-2) beam, for which we can determine the spin and rotational angular momenta of O(2) almost independently, is the most promising probe for studying the spin effects as well as the steric effects in O(2) molecular scattering.
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.
The spin flipper presented here is based on the method of diabatic passage. It allows for a rapid reversal of electronic as well as nuclear polarization of a thermal atomic beam without affecting beam intensity, beam profile, or magnetic fields in the interaction region. The flipper will be useful in spin dependent measurements where instrumental asymmetries and systematic errors have to be kept small. The spin flipper was tested with a polarized lithium beam in an atomic physics experiment and the reversal efficiency was found to by 0.98. The flipper can easily be adapted for other spin‐polarizable atoms, e.g. for a polarized atomic hydrogen beam target.