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Physical Review A

Microwave-laser-field modification of molecular collisions at low temperatures

Sergey Alyabyshev, Timur V. Tscherbul, Roman V. Krems

Published Jun 12, 2009• 17 citations
Publisher

Abstract

We present a rigorous study of low-temperature collisions of polar molecules and atoms in a microwave laser field. Our calculations show that inelastic relaxation of molecules in a microwave cavity may occur through collision-induced absorption of photons stimulated by the anisotropy of the atom-molecule interaction potential. Our results indicate that molecular collisions at temperatures below 1 K can be efficiently manipulated with microwave laser fields of moderate strength.

DOI
10.1103/physreva.79.060703

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We present a rigorous study of low-temperature collisions of polar molecules and atoms in a microwave laser field. Our calculations show that inelastic relaxation of molecules in a microwave cavity may occur through collision-induced absorption of photons stimulated by the anisotropy of the atom-molecule interaction potential. Our results indicate that molecular collisions at temperatures below 1 K can be efficiently manipulated with microwave laser fields of moderate strength.
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.
The research field of cold and ultracold atoms and molecules is rapidly growing and expanding into different areas of research such as quantum information science and condensed matter physics. The success of this field is due to the possibility of precisely controlling and manipulating atoms and molecules at low temperatures. The progress in this field relies on the development of new methods for controlling collisional dynamics and interactions of particles with electromagnetic fields. This Thesis describes research on modification of the collisional dynamics of ultracold atoms and molecules by external laser and microwave fields as well as new methods for the detection of weak electromagnetic fields. First, we study the scattering of atoms and molecules confined in a 2D geometry by optical lattices. In particular we develop a theory for scattering in 2D and derive the equations for the threshold dependence of the collision cross sections. We show that inelastic processes and chemical reactions can be suppressed under strong confinement in one dimension and can be controlled by varying the orientation of the external field with respect to the plane of confinement. Next, we present a rigorous theory of low-temperature molecular collisions in the presence of a microwave field. The microwave field can theoretically be used to trap and control polar molecules. The molecular collisions may lead to trap loss and decoherence. We develop a rigorous quantum theory for molecular scattering in the presence of microwave fields. We study inelastic, spin-changing molecular collisions and Feshbach resonances in the presence of microwave fields. We demonstrate that inelastic collisions accompanied by absorption of microwave photons can be significant. The detection of weak electromagnetic fields is very important for various applications ranging from fundamental measurements to biomagnetic imaging, and for tests of microwave chips. We present a method for the detection of weak electromagnetic fields in a wide range of frequencies from sub-kHz to THz with ultracold polar molecules. We show that using ultracold molecules one can achieve the sensitivity of two orders of magnitude larger than with a similar method based on ultracold Rb atoms.
Collisions at ultracold temperatures between ${}^{1}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Sigma}$-state diatomic polar molecules in a circularly polarized microwave field are theoretically analyzed. We demonstrate that elastic and inelastic collisions of polar molecules at ultracold temperatures are affected by an external microwave field and that they are enhanced at resonance frequencies. We show that the collision dynamics of polar molecules in a microwave field is largely determined by the long-range dipole-dipole interaction. Through this interaction, two polar molecules can resonantly exchange internal energy by undergoing a transition between neighboring rotational levels. This process may stabilize the system and enable the successive evaporative cooling of molecules. This paper discusses fermionic polar molecules at ultralow temperatures (1 $\ensuremath{\mu}$K).
Collisions of molecules in a thermal gas are difficult to control. Thermal motion randomizes molecular encounters and diminishes the effects of external radiation or static electromagnetic fields on intermolecular interactions. The effects of the thermal motion can be reduced by cooling molecular gases to low temperatures. At temperatures near or below 1 K, the collision energy of molecules becomes less significant than perturbations due to external fields. At the same time, inelastic scattering and chemical reactions may be very efficient in low-temperature molecular gases. The purpose of this article is to demonstrate that collisions of molecules at temperatures below 1 K can be manipulated by external electromagnetic fields and to discuss possible applications of cold controlled chemistry. The discussion focuses on molecular interactions at cold (0.001-2 K) and ultracold (<0.001 K) temperatures and is based on both recent theoretical and experimental work. The article concludes with a summary of current challenges for theory and experiment in the research of cold molecules and cold chemistry.
Collisions of molecules in a thermal gas are difficult to control. Thermal motion randomizes molecular encounters and diminishes the effects of external radiation or static electromagnetic fields on intermolecular interactions. The effects of the thermal motion can be reduced by cooling molecular gases to low temperatures. At temperatures near or below 1 K, the collision energy of molecules becomes less significant than perturbations due to external fields. At the same time, inelastic scattering and chemical reactions may be very efficient in low-temperature molecular gases. The purpose of this article is to demonstrate that collisions of molecules at temperatures below 1 K can be manipulated by external electromagnetic fields and to discuss possible applications of cold controlled chemistry. The discussion focuses on molecular interactions at cold (0.001-2 K) and ultracold (<0.001 K) temperatures and is based on both recent theoretical and experimental work. The article concludes with a summary of current challenges for theory and experiment in the research of cold molecules and cold chemistry.
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.