Dynamics of molecular superrotors in an external magnetic field

Abstract

We excite diatomic oxygen and nitrogen to high rotational states with an optical centrifuge and study their dynamics in external magnetic field. Ion imaging is employed to directly visualize, and follow in time, the rotation plane of molecular superrotors. The two different mechanisms of interaction between the magnetic field and the molecular angular momentum in paramagnetic oxygen and non-magnetic nitrogen lead to the qualitatively different behaviour. In nitrogen, we observe the precession of the molecular angular momentum around the field vector. In oxygen, strong spin-rotation coupling results in faster and richer dynamics, encompassing the splitting of the rotation plane in three separate components. As the centrifuged molecules evolve with no significant dispersion of the molecular wave function, the observed magnetic interaction presents an efficient mechanism for controlling the plane of molecular rotation. PACS numbers:Magnetic field is one of the most powerful tools for controlling atomic and molecular dynamics. Both translational and rotational degrees of freedom of molecules have been successfully manipulated via various types of magnetic interaction. Alignment of molecular axes has been accomplished by creating pendular states in paramagnetic molecules at low temperature [1,2]. Gyroscopic precession along the direction of the applied magnetic field has been suggested for non-magnetic molecules in dispersionless "cogwheel" states [3,4]. Spin-rotational coupling has been recently utilized for converting unidirectional molecular rotation into axial alignment, manifested through magneto-rotational optical birefringence under ambient conditions [5,6].Recent progress in creating and detecting unidirectional rotation of molecules [7][8][9][10][11]18] revived the interest in controlling molecular rotation with external magnetic fields. Magnetic effects could further broaden the controllability provided by tunable rotational excitation, including control of molecular collisions [12,13] and scattering of molecules at gas-solid interfaces [14], molecular trajectories [15] and formation of gas vortices [16], optical [5, 17] and acoustic [19,20] properties of a gas of rotating molecules.With the invention of the optical centrifuge technique [21,22], a very flexible control over the degree of rotational excitation became available. The technique proved capable of producing synchronously rotating molecules with narrow rotational state distribution in a wide variety of species up to extremely high angular frequencies [5,23]. However, the spatial orientation of the induced angular momentum is often defined by the propagation direction of the excitation laser beam, and is therefore not easy to manipulate.Here we demonstrate how an applied magnetic field can be used to rotate the plane of molecular rotation. The described method is applicable to a wide variety of molecules and relies on the coupling between the rotation of a molecule and its magnetic moment. An applied magnetic field causes this moment to precess, "...

A theoretical study of the dynamics of paramagnetic superrotors in external magnetic fields

“…For strong fields, it was shown that the angular momentum becomes evenly distributed in the plane perpendicular to the magnetic field, and the precession is independent of the field strength. A comparison of the analytical derivations on one hand, and numerical simulations (reported here) as well as experimental observations (reported in [35,36]) on the other hand, showed a quantitative agreement for the weak field case and a qualitative agreement for the strong field case.…”

“…The respective g-factor is of the order of 10 −4 [42]; for the considered field strengths of a few Tesla, this would correspond to precession periods of 100 ns and longer, clearly too slow to explain the observed effect (such a slow precession has been observed experimentally for nitrogen superrotors [36]). Instead, the interaction between N and B is mediated by the electronic spin S. The spin interacts with the magnetic field via the Zeeman interaction H ms , and with N via the spinrotation interactions H ss and H so .…”

“…In the timescale of hundreds of picoseconds, this disc splits into three discs, corresponding to the three spin-states: One remaining in the yz-plane, and two rotating in opposite directions around z. In a very recent experiment, Milner et al [36] succeeded to observe this behaviour.…”

“…In this article, a detailed theoretical analysis of the magneto-rotational effect reported in [35] is provided. Further insights using an experimental approach are provided in a companion article [36].…”

Adiabatic Field-Free Alignment of Asymmetric Top Molecules with an Optical Centrifuge

“…Optical centrifuge has recently been used for reaching extreme rotational states, known as molecular "superrotors" [23,24]. High degree of planar alignment has been demonstrated in the ensembles of centrifuged linear molecules [25]. Here, we extend this technique to the adiabatic alignment of asymmetric rotors.…”