A homonuclear molecule with a permanent dipole moment

Traditional wisdom tells us that homonuclear molecules cannot have a permanent dipole moment. However, when one of the atoms in a diatomic molecule is in a highly excited Rydberg state, the exchange symmetry between the two atoms is broken. In a new article published today in Science [1], ITAMP postdoc Seth Rittenhouse and scientist Hossein Sadeghpour in collaboration with groups from the Max-Planck-Institut für Physik komplexer Systeme and the University of Stuttgart, we describe the first direct measurement of a homonuclear, diatomic molecule with a permanent electric dipole moment.

Giant Rydberg molecules, first predicted over a decade ago [2], can bond courtesy of frequent scattering of the Rydberg electron off of a ground state atom. These molecules were later observed spectroscopically in s-wave dominated states [3]. However, because the electronic distribution was assumed to be isotropic, it was thought that this type of molecule would not have any polar behavior.

In our new work, we show that in truth a very small amount of the so-called "trilobite" state is admixed to the molecular electronic state resulting in appreciable dipole moments (on the order od 1 Debye). This prediction is born out by the observation of a linear Stark shift a small fields in high precision spectroscopic measurements of the Stark map of homonuclear rubidium Rydberg molecules.

(a) The electron density for a Giant homonuclear molecule is show in cylindrical coordinates. (b) The trilobite-like electron density is shown after the isotropic amplitude has been subtracted. In both figures the Rb Rydberg core is at z = ρ = 0, the while the ground state atom is located z = 1900 atomic units (figure from Ref. [1]).


References:
[1] W. Li, T. Pohl, J. M. Rost, S. T. Rittenhouse, H. R. Sadeghpour, J. Nipper, B. Butscher, J. B. Balewski, V. Bendkowsky, R. Low and T. Pfau, Science 334, 1110 (2011).

[2] H. R. Sadeghpour, A. S. Dickinson and C. H. Greene, Phys. Rev Lett. 85, 2458 (2000).

[3] V. Bendkowsky et al., Nature 458, 1005 (2009).