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Review of Scientific Instruments

Production of a single spin-rotational state [(J,M)=(2,2)] selected molecular oxygen (Σ3g−) beam by a hexapole magnet

Mitsunori Kurahashi, Yasushi Yamauchi

Published Aug 1, 2009• 26 citations
Publisher

Abstract

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.

DOI
10.1063/1.3206299

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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.
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.
Cold samples of oxygen molecules in supersonic beams have been decelerated from initial velocities of 390 and 450 m s−1 to final velocities in the range between 150 and 280 m s−1 using a 90-stage Zeeman decelerator. (2 + 1) resonance-enhanced-multiphoton-ionization (REMPI) spectra of the 3sσ g 3Π g (C) two-photon transition of O2 have been recorded to characterize the state selectivity of the deceleration process. The decelerated molecular sample was found to consist exclusively of molecules in the J ′′ = 2 spin–rotational component of the X ground state of O2. Measurements of the REMPI spectra using linearly polarized laser radiation with polarization vector parallel to the decelerator axis, and thus to the magnetic-field vector of the deceleration solenoids, further showed that only the magnetic sublevel of the N′′ = 1, J ′′ = 2 spin–rotational level is populated in the decelerated sample, which therefore is characterized by a fully oriented total-angular-momentum vector. By maintaining a weak quantization magnetic field beyond the decelerator, the polarization of the sample could be maintained over the 5 cm distance separating the last deceleration solenoid and the detection region.
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.
Multistage Zeeman deceleration is a technique used to reduce the velocity of neutral molecules with a magnetic-dipole moment. Here we present a Zeeman decelerator that consists of 100 solenoids and 100 magnetic hexapoles that is based on a short prototype design presented recently [Phys. Rev. A 95, 043415 (2017)]. The decelerator features a modular design with excellent thermal and vacuum properties and is robustly operated at a 10 Hz repetition rate. We use this decelerator to demonstrate the state-selective deceleration of atomic oxygen to final mean velocities in the range 500--125 m/s. We further characterize our decelerator with molecular oxygen, which, despite its heavier mass, is velocity tuned in the range 350--150 m/s. This corresponds to a maximum kinetic-energy reduction of 95% and 80% for atomic and molecular oxygen, respectively. The long multistage Zeeman decelerator presented here demonstrates that the concept of using alternating hexapoles and solenoids is truly phase stable. This Zeeman decelerator is ideally suited for applications in crossed-beam scattering experiments; the state-selected and velocity-controlled samples of O atoms and O2{\mathrm{O}}_{2}O2​ molecules are particularly relevant for studies of inelastic and reactive processes.
We theoretically investigate three-dimensional (3D) focusing of pulsed molecular beam using a series of hexapoles with different orientations. Transversely oriented hexapoles provide both the transverse and longitudinal focusing force and a longitudinally oriented one provides only the transverse force. The hexapole focusing position are designed to realize the simultaneous focusing in three directions. The additional longitudinal focusing compared with the conventional hexapole can suppress the effect of chromatic aberration induced by the molecular longitudinal velocity spread, thus improving the state-selection purity as well as the beam density. Performance comparison of state selection between this 3D focusing hexapole and a conventional one is made using numerical trajectory simulations, choosing CHF 3 molecules as a tester. It is confirmed that our proposal can improve the state-selection purity from 68.2% to 96.1% and the beam density by a factor of 2.3.
We study the dynamics of paramagnetic molecular superrotors in an external magnetic field. An optical centrifuge is used to create dense ensembles of oxygen molecules in ultrahigh rotational states. In is shown, for the first time, that the gas of rotating molecules becomes optically birefringent in the presence of a magnetic field. The discovered effect of "magneto-rotational birefringence" indicates the preferential alignment of molecular axes along the field direction. We provide an intuitive qualitative model, in which the influence of the applied magnetic field on the molecular orientation is mediated by the spin-rotation coupling. This model is supported by the direct imaging of the distribution of molecular axes, the demonstration of the magnetic reversal of the rotational Raman signal, and by numerical calculations.