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Science

A Homonuclear Molecule with a Permanent Electric Dipole Moment

Weibin Li, Thomas Pohl, Jan Michael Rost, Seth T. Rittenhouse, H. R. Sadeghpour, J. Nipper, Björn Butscher, Jonathan Balewski, Vera Bendkowsky, R. Löw, Tilman Pfau

Published Nov 24, 2011• 160 citations
Publisher

Abstract

Permanent electric dipole moments in molecules require a breaking of parity symmetry. Conventionally, this symmetry breaking relies on the presence of heteronuclear constituents. We report the observation of a permanent electric dipole moment in a homonuclear molecule in which the binding is based on asymmetric electronic excitation between the atoms. These exotic molecules consist of a ground-state rubidium (Rb) atom bound inside a second Rb atom electronically excited to a high-lying Rydberg state. Detailed calculations predict appreciable dipole moments on the order of 1 Debye, in excellent agreement with the observations.

DOI
10.1126/science.1211255

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•Jan 1, 2012•0 citations
Permanent electric dipole moments in molecules require a breaking of parity symmetry. Conventionally, this symmetry breaking relies on the presence of heteronuclear constituents. We report the observation of a permanent electric dipole moment in a homonuclear molecule in which the binding is based on asymmetric electronic excitation between the atoms. These exotic molecules consist of a ground-state rubidium (Rb) atom bound inside a second Rb atom electronically excited to a high-lying Rydberg state. Detailed calculations predict appreciable dipole moments on the order of 1 Debye, in excellent agreement with the observations.
The authors investigate long-range Cs2{}_{2}2​ molecules consisting of a ground-state and a Rydberg atom, and measure and calculate permanent molecular electric-dipole moments showing that the sign of the dipole moment is negative, reflecting a deficiency of Rydberg-electron density near the ground-state constituent
Cold and ultracold Rydberg atoms are in considerable vogue for their ability to creating strong interactions, stemming from their exaggerated and readily tunable properties. Ultralong-range Rydberg molecules have been predicted to form from the interaction of ultracold Rydberg atoms with ground-state atoms and polar molecules. In this work, we discuss and demonstrate how such molecules, which are homonuclear, form substantially large permanent electric dipole moments. A corollary benefit of such strong hybridisation is the realisation of high angular momentum degenerate Rydberg molecules (so-called trilobite molecules) with standard photoassociation techniques.
Permanent electric dipole moments are important for understanding symmetry breaking in molecular physics, control of chemical reactions, and realization of strongly correlated many-body quantum systems. However, large molecular permanent electric dipole moments are challenging to realize experimentally. We report the observation of ultralong-range Rydberg molecules with bond lengths of ~100 nanometers and kilo-Debye permanent electric dipole moments that form when an ultracold ground-state cesium (Cs) atom becomes bound within the electronic cloud of an extended Cs electronic orbit. The electronic character of this hybrid class of "trilobite" molecules is dominated by degenerate Rydberg manifolds, making them difficult to produce by conventional photoassociation. We used detailed coupled-channel calculations to reproduce their properties quantitatively. Our findings may lead to progress in ultracold chemistry and strongly correlated many-body physics.
The relativistic configuration interaction valence-bond method has been used to calculate permanent and transition electric dipole moments of the KRb heteronuclear molecule as a function of internuclear separation. The permanent dipole moment of the ground-state $X{}^{1}{\ensuremath{\Sigma}}^{+}$ potential is found to be 0.30(2) ea0{\mathrm{ea}}_{0}ea0​ at the equilibrium internuclear separation with excess negative charge on the potassium atom. For the $a{}^{3}{\ensuremath{\Sigma}}^{+}$ potential the dipole moment is an order of smaller magnitude (1 ${\mathrm{ea}}_{0}=8.47835{10}^{\ensuremath{-}30}\mathrm{Cm}).$ In addition, we calculate transition dipole moments between the two ground-state and excited-state potentials that dissociate to the K(4s)+Rb(5p)\mathrm{K}(4s)+\mathrm{Rb}(5p)K(4s)+Rb(5p) limits. Using this data we propose a way to produce singlet $X{}^{1}{\ensuremath{\Sigma}}^{+}$ KRb molecules by a two-photon Raman process starting from an ultracold mixture of doubly spin-polarized ground state K and Rb atoms. This Raman process is only allowed due to relativistic spin-orbit couplings and the absence of gerade-ungerade selection rules in heteronuclear dimers.
We report calculations of Born-Oppenheimer potential energy curves of the chromium-rubidium heteronuclear molecule (52Cr^{52}\mathrm{Cr}52Cr87Rb^{87}\mathrm{Rb}87Rb), and the long-range dispersion coefficient for the interaction between ground state Cr and Rb atoms. Our calculated van der Waals coefficient (C6=1770{C}_{6}=1770C6​=1770 a.u.) has an expected error of 33%3. The ground state ${}^{6}{\ensuremath{\Sigma}}^{+}$ molecule at its equilibrium separation has a permanent electric dipole moment of de(Re=3.34A˚)=2.90D{d}_{e}({R}_{e}=3.34\AA{})=2.90 Dde​(Re​=3.34A˚)=2.90D. We investigate the hyperfine and dipolar collisions between trapped Cr and Rb atoms, finding elastic to inelastic cross section ratio of 102{10}^{2}102--103{10}^{3}103.
Interactions between single atoms are fundamental to physics and to control them is an ultimate goal. The exaggerated properties of Rydberg atoms offer to met the technical challenges to isolate and control single interaction channels in ultracold gases. Here, I present experiments on two subjects related to interactions of Rydberg atoms in dense ultracold clouds. One subject concerns coherence in strongly interacting ensembles of atoms, where the interaction between Rydberg atoms is induced via Stark-tuned Förster resonances. Pulsed experiments, following the idea of Ramsey experiments, are used for high resolution spectroscopy of the Förster defect and phase sensitive detection. Coherent oscillations between pair states and an interaction-induced phase shift of Rydberg atoms are measured. These experiments are accompanied by calculations of the interaction strength and by simulations using the concept of a pair state interferometer. The simulations nicely reproduce the experimental findings and support the observation that the ensemble of atoms in the presence of interactions can be described and controlled coherently. The second subject of this thesis is the measurement of a permanent dipole moment in a homonuclear diatomic molecule that arises by the interaction between a Rydberg atom and a ground state atom. Usually parity symmetry prohibits a permanent dipole moment in diatomic molecules, but here the strong asymmetry between the constituents of the ultralong-range Rydberg molecule allows breaking parity symmetry. These molecules consist of one ground state atom bound inside the Rydberg electron wavefunction of a highly excited atom. Calculations predict dipole moments on the order of 1 Debye. Experimental proof is reported on the measurement of a linear Stark effect of these molecules, in excellent agreement with the calculations.