Rydberg atoms are (usually alkali) atoms in a highly excited state, where the principal quantum number n can range from the order of 10 up to several hundred. The sizes of such Rydberg atoms are several orders of magnitude larger than the atomic ground state, and their astonishingly large polarizabilities and electric dipole moments make them unusually sensitive to external fields.

One of the group’s projects examines how external fields can be used to control the motion of the Rydberg electron and its role in binding of distant ground state atoms. However, the inclusion of an external field introduces non-perturbative effects, such as the Stark effect in the case of an external electric field. Our research in this field has improved existing theoretical methods for treating the Stark effect of photoionization processes in Rydberg atoms. From an experimental viewpoint, photoionization in an electric field enables a study of the quantum nature of the electronic probability density. The main goal of this project is to design novel theoretical frameworks which address the underlying physics of highly correlated system, and to interpret the emergent resonant features in the alkali Rydberg spectra while identifying the fundamental processes which cause them.

In another project, this group examines the behavior of a single Rydberg atom excited in a Bose-Einstein condensate (BEC). By virtue of the excited electron's interaction with a nearby neutral atom, the Rydberg atom and the neutral atom can form a long-range diatomic molecule. If the Rydberg electron's angular momentum is high enough, the molecular state, dubbed a "trilobite" state, possesses an enormous permanent electric dipole moment. We have extended the theory describing these molecules to include the rich physics pervasive throughout the periodic table due to multichannel interactions in Rydberg spectra. The multichannel behavior of calcium provides favorable conditions for the association of trilobite molecules, while in silicon, channel interactions create multi-scale binding possibilities. Additionally, we have investigated the effects of the condensate density on these molecular states and are exploring the polyatomic molecules that can form in dense gases along with their signatures in line broadening and spectral line shapes.

Figure: An exotic polyatomic Rydberg molecule formed by a Rydberg atom at the center of a cube of neutral atoms. The electron probability, represented by the colored points extending radially outward, is maximized at each of the perturbing ground state atoms and close to the Rydberg core exhibits complicated interference nodal patterns.

Relevant Publications:

M. T. Eiles and C. H. Greene, “Ultracold long-range Rydberg molecules with complex multichannel spectra”, Phys. Rev. Lett. 115, 193201 (2015)

P. Giannakeas, F. Robicheaux, and C. H. Greene, “Photoionization microscopy in terms of local-frame-transformation theory”, Phys. Rev. A 91, 043424 (2015)