Yes—but the exception is a specially prepared rubidium Rydberg molecule, not an ordinary molecule made polar just because its atoms interact. In 2011, researchers observed a permanent dipole in a molecule of two rubidium atoms, one in its ground state and one excited to a high-lying Rydberg state. The unusual electronic state breaks the symmetry expected for identical atoms and produces a dipole of about 1 Debye.
Why are identical-atom molecules usually considered nonpolar?
A homonuclear molecule contains two nuclei of the same element. In the conventional picture, exchanging those identical nuclei leaves the molecule unchanged. That symmetry prevents a permanent electric dipole along the molecular axis: the positive and negative charge distributions cannot have a lasting separation in the molecule-fixed frame.
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This is a statement about the molecule’s symmetry and electronic state, not a rule that identical atoms can never form an unusual state with asymmetric charge. A permanent body-fixed dipole also does not mean the molecule points in a fixed direction in the laboratory when no external field is applied.
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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →How did the rubidium Rydberg molecule become polar?
One atom was excited, the other remained in its ground state
The molecule reported in 2011 was a special ultracold rubidium dimer: one atom occupied a high-lying Rydberg state, while the other remained in its ground state. It was not an ordinary stable diatomic molecule. Earlier work had shown that scattering between a Rydberg electron and a nearby ground-state atom can create an attraction that binds the ground-state atom within the Rydberg electron’s extended wavefunction.
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The second atom distorted the Rydberg electron density
In the 2011 system, interaction with the ground-state atom mixed a nearly degenerate electronic manifold into the Rydberg state. That admixture localized some electron density near the perturbing atom, making the electronic distribution asymmetric even though both nuclei were rubidium. The resulting dipole is permanent in the molecule-fixed frame.
Li and colleagues reported a linear Stark response as evidence of the dipole and compared spectroscopy with detailed calculations. Their paper described a moment on the order of 1 Debye, with calculations in excellent agreement with the observations. The paper’s abstract and publication record summarize the finding; the authors’ full paper is available at their hosted PDF.
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How large is the dipole, and how does it compare with trilobite molecules?
The 1-Debye result belongs to the 2011 low-angular-momentum Rydberg state. It should not be confused with later pure trilobite Rydberg molecules, which occupy high-angular-momentum electronic manifolds and have dipoles in the kilo-Debye range.
| Rydberg molecular state | Electronic character | Reported dipole scale | Context |
|---|---|---|---|
| 2011 homonuclear rubidium molecule | Rydberg state with admixture of a nearly degenerate electronic manifold | On the order of 1 Debye | Permanent body-fixed dipole inferred from a linear Stark response and supported by calculations |
| Pure trilobite rubidium molecules | High-angular-momentum Rydberg states | Kilo-Debye range | A distinct molecular regime; a 2023 study reports two vibrational series |
The larger trilobite values do not revise the measured scale for the 2011 state: these are different electronic states. The 2023 study, “Exploring the vibrational series of pure trilobite Rydberg molecules”, reports the later trilobite work.
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Does this overturn the rule that homonuclear molecules are nonpolar?
It refines the usual symmetry explanation rather than overturning it. The familiar rule applies when the molecule’s electronic state retains the symmetry that makes its two ends equivalent. In this Rydberg dimer, asymmetric electronic excitation and interaction with the ground-state atom alter that state’s electron distribution. The result is an exception tied to a particular prepared state—not evidence that all homonuclear molecules are polar.
The essential distinction is between identical nuclei and a symmetric electronic state. The rubidium nuclei are identical; the electronic distribution in this state is not.
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