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arXiv:physics/0606027 · Physical Review Letters

Controlling Electronic Spin Relaxation of Cold Molecules with Electric Fields

Timur V. Tscherbul, Roman V. Krems

Published Aug 22, 2006• 119 citations
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Abstract

We present a theoretical study of atom-molecule collisions in superimposed electric and magnetic fields and show that dynamics of electronic spin relaxation in molecules at temperatures below 0.5 K can be manipulated by varying the strength and the relative orientation of the applied fields. The mechanism of electric field control of Zeeman transitions is based on an intricate interplay between intramolecular spin-rotation couplings and molecule-field interactions. We suggest that electric fields may affect chemical reactions through inducing nonadiabatic spin transitions and facilitate evaporative cooling of molecules in a magnetic trap.

DOI
10.1103/physrevlett.97.083201

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We present a theoretical study of atom-molecule collisions in superimposed electric and magnetic fields and show that dynamics of electronic spin relaxation in molecules at temperatures below 0.5 K can be manipulated by varying the strength and the relative orientation of the applied fields. The mechanism of electric field control of Zeeman transitions is based on an intricate interplay between intramolecular spin-rotation couplings and molecule-field interactions. We suggest that electric fields may affect chemical reactions through inducing nonadiabatic spin transitions and facilitate evaporative cooling of molecules in a magnetic trap.
The authors present a detailed study of low-temperature collisions between CaD molecules and He atoms in superimposed electric and magnetic fields with arbitrary orientations. Electric fields do not interact with the electron spin of the molecules directly but modify their rotational structure and, consequently, the spin-rotation interactions. The authors examine molecular Stark and Zeeman energy levels as functions of the angle between the fields and show that rotating fields may induce and shift avoided crossings between the Zeeman levels of the rotationally ground and rotationally excited states of the molecule. The dynamics of molecular collisions are extremely sensitive to external fields near these avoided crossings and it is shown that molecular collisions may be controlled by varying both the strength and the relative orientation of the fields. The effects observed in this study are due to interactions of the isolated molecules with external fields so the conclusions should be relevant for collisions of molecules with other atoms or collisions of molecules with each other. This study demonstrates that electric fields may be used to enhance or suppress spin-rotation interactions in molecules. The spin-rotation interactions induce nonadiabatic couplings between states of different total spins in systems of two open-shell species and it is suggested that electric fields might be used for controlling nonadiabatic spin transitions and spin-forbidden chemical reactions of cold molecules in a magnetic trap.
We use rigorous quantum mechanical theory to study collisions of magnetically oriented cold molecules in the presence of superimposed electric and magnetic fields. It is shown that electric fields suppress the spin-rotation interaction in rotationally excited 2Sigma molecules and inhibit rotationally elastic and inelastic transitions accompanied by electron spin reorientation. We demonstrate that electric fields enhance collisional spin relaxation in 3Sigma molecules and discuss the mechanisms for electric field control of spin-changing transitions in collisions of rotationally excited CaD(2Sigma) and ND(3Sigma) molecules with helium atoms. The propensities for spin depolarization in the rotationally excited molecules are analyzed based on the calculations of collision rate constants at T=0.5 K.
We explore the effects of microwave laser fields on spin-changing transitions in collisions of polar molecules in a $^{2}\ensuremath{\Sigma}$ electronic state with atoms in the presence of a static magnetic field. We use the dressed-state formalism of quantum optics to describe the interaction of the radiation field with the collision complex and perform rigorous scattering calculations to study spin-changing transitions in collisions of CaH molecules with He atoms at temperatures below 1 K. Our calculations demonstrate that collision-induced spin relaxation of $^{2}\ensuremath{\Sigma}$ molecules is enhanced near avoided crossings between different Zeeman levels. We show that the positions of the avoided crossings can be modified by varying the frequency and the intensity of the microwave laser field and discuss the mechanisms of microwave field control of spin-dependent interactions in cold open-shell molecules.
This article reviews the current state of the art in the field of cold and ultracold molecules and demonstrates that chemical reactions, inelastic collisions and dissociation of molecules at subkelvin temperatures can be manipulated with external electric or magnetic fields. The creation of ultracold molecules may allow for spectroscopy measurements with extremely high precision and tests of fundamental symmetries of nature, quantum computation with molecules as qubits, and controlled chemistry. The probability of chemical reactions and collisional energy transfer can be very large at temperatures near zero kelvin. The collision energy of ultracold atoms and molecules is much smaller than perturbations due to interactions with external electric or magnetic fields available in the laboratory. External fields may therefore be used to induce dissociation of weakly bound molecules, stimulate forbidden electronic transitions, suppress the effect of centrifugal barriers in outgoing reaction channels or tune Feshbach resonances that enhance chemical reactivity.
We present a rigorous study of cold and ultracold collisions of $\mathrm{Yb}\mathrm{F}(^{2}\ensuremath{\Sigma})$ molecules with He atoms in external electric and magnetic fields based on an accurate calculation of the interaction potential surface and quantum theory of atom-molecule scattering. We analyze the mechanisms of collisional depolarization of the electron and nuclear spins of YbF and demonstrate that the rate constants for elastic and inelastic collisions of YbF with He are sensitive to the magnitudes of the applied fields. Collisions of heavy polar molecules like YbF may thus be easily manipulated with external electromagnetic fields. We show that collisional spin relaxation of YbF molecules in rotationally excited states is suppressed by electric fields much more significantly than the spin relaxation in the ground rotational state. We explain this by the influence of electric-field-induced Feshbach resonances, which occur at much lower collision energies when the molecule is rotationally excited. Our results suggest that heavy polar molecules may be amenable to magnetic trapping in a buffer gas of He, which could greatly enhance the sensitivity of spectroscopic experiments to measure the electric dipole moment of the electron.
We use accurate quantum mechanical calculations to analyze the effects of parallel electric and magnetic fields on collision dynamics of OH(2pi) molecules. It is demonstrated that spin relaxation in 3He-OH collisions at temperatures below 0.01 K can be effectively suppressed by moderate electric fields of order 10 kV cm(-1). We show that electric fields can be used to manipulate Feshbach resonances in collisions of cold molecules. Our theoretical results can be tested in experiments with OH molecules in Stark decelerated molecular beams and electromagnetic traps. PACS numbers: 33.20.-t, 33.80.Ps.