Ultralong-range Rydberg Molecules of Ytterbium Demonstrate Novel Interactions and Potential Applications

Ytterbium atoms, with their unique two-electron structure, represent a promising new frontier in ultracold atom research, yet fundamental aspects of their behaviour remain poorly understood. Tangi Legrand, Xin Wang, and Milena Simić, from Purdue University, alongside Florian Pausewang, Wolfgang Alt, and Eduardo Uruñuela, now present the first comprehensive investigation into ultralong-range Rydberg molecules formed from ytterbium atoms. Their work reveals crucial details about how electrons interact with ytterbium, demonstrating that the ytterbium anion exists only as a fleeting, unstable state. By meticulously analysing these molecules and comparing the results with theoretical models, the team establishes a powerful new method for probing electron-atom interactions and paves the way for advanced Rydberg experiments using divalent atoms like ytterbium.

Rydberg Molecules and Green’s Function Methods

This research encompasses a comprehensive investigation of Rydberg molecules, ultracold physics, and atomic/molecular interactions. Studies establish the theoretical framework for understanding Rydberg molecules, utilizing Green’s function methods to describe interactions and resonances, and employing Siegert pseudo-states to analyze scattering. Angular momentum theory provides essential background for describing molecular structure, while calculations of atomic polarizabilities determine the strength of long-range interactions. Multichannel Quantum Defect Theory offers a method for analyzing Rydberg states and their interactions.

The core of this research focuses on the formation and properties of Rydberg molecules, particularly ultralong-range Rydberg molecules (ULRMs) formed by interactions between Rydberg and ground-state atoms or molecules. Investigations extend to macrodimers and giant polyatomic Rydberg molecules, exploring rotational hybridization, charge-dipole interactions, and spin effects. Researchers are actively developing methods for controlling molecular orientation within these structures. Studies explore how Rydberg molecules interact with other atoms, molecules, or fields, focusing on Rydberg blockade, loss rates in Bose-Einstein condensates, and electron scattering resonances.

Observations of density shifts in Rydberg gases and the creation of Rydberg polarons in Bose gases further expand understanding of these interactions. Experimental techniques, including trapping, cooling, photoassociation spectroscopy, and quantum gas microscopy, are employed to create and study Rydberg molecules. Theoretical calculations focus on potential energy surfaces, dipole moments, and molecular dynamics simulations to predict Rydberg molecule behavior. Research extends to specific systems, including ytterbium, cesium, and rubidium, with a strong emphasis on ytterbium due to its favorable properties. Key themes include the dominant role of ultralong-range Rydberg molecules, the control of molecular structure and orientation, and the application of Rydberg molecules to many-body physics and quantum simulation. Spectroscopy and dynamics of Rydberg states remain a significant area of investigation, reflecting the vibrant and rapidly evolving field of Rydberg molecule research.

Ytterbium Rydberg Molecule Potential Energy Curves

Scientists conducted a detailed investigation of ultralong-range Rydberg molecules (ULRMs) formed with ytterbium, extending their analysis across a wide range of binding energies and principal quantum numbers. Two computational approaches were employed to derive potential energy curves (PECs) describing the Rydberg atom interacting with a ground-state perturber. One method involved diagonalizing the molecular Hamiltonian, while the other utilized a closed-form solution of the Coulomb Green’s function. Combining these approaches allowed for a qualitative survey of PEC structure and the generation of accurate PECs for vibrational spectrum analysis.

Calculations, incorporating electron-atom scattering phase shifts, revealed two categories of PECs distinguished by the quantum defect of the unperturbed Rydberg state. Detailed analysis revealed a dependence of quantum defects on both orbital and spin angular momentum. Close inspection of avoided crossings confirmed the nearly degenerate doublet of butterfly states. This methodology enabled the extraction of low-energy electron-ytterbium scattering phase shifts, establishing ytterbium ULRMs as a powerful probe of electron-ytterbium interactions.

Ytterbium Rydberg Molecules, Binding Energy Spectra Revealed

Scientists have conducted the first comprehensive investigation of ultralong-range Rydberg molecules (ULRMs) formed with ytterbium, revealing detailed insights into electron-atom interactions. Spectroscopic measurements were taken across a wide range of binding energies and principal quantum numbers, providing an unprecedented view of these weakly-bound molecular structures. Utilizing the Coulomb Green’s function formalism, they computed Born-Oppenheimer molecular potentials, achieving quantitative agreement with high-resolution molecular spectra. These calculations and experiments enabled the extraction of crucial low-energy electron-Yb scattering phase shifts, including the zero-energy s-wave scattering length and the positions of p-wave shape resonances. The data strongly supports the conclusion that a stable Yb anion does not exist, demonstrating that the Yb atom can only support a metastable resonance state for negative ions. The study establishes Yb ULRMs as a powerful probe of electron-Yb interactions, providing essential groundwork for future Rydberg experiments with divalent atoms and expanding the understanding of long-range molecular physics in ultracold atomic systems.

Ytterbium Rydberg Molecules, Spectra and Interactions

This research presents a comprehensive investigation of ultralong-range Rydberg molecules (ULRMs) formed with ytterbium, a divalent atom increasingly used in ultracold atom experiments. Scientists successfully combined experimental measurements with theoretical modelling, using the Coulomb Green’s function formalism to accurately describe the interactions between a Rydberg atom and a ground-state ytterbium atom. This approach yielded detailed molecular potentials and vibrational spectra that closely match experimental data. The work extends beyond simply confirming theoretical models, providing new insights into the fundamental properties of ytterbium itself.

By analysing the ULRM spectra, the team extracted crucial information about electron-ytterbium scattering, including the zero-energy scattering length and the positions of shape resonances. Importantly, these findings strongly suggest that a stable negative ytterbium ion cannot form, existing only as a short-lived resonance state. Furthermore, the sensitivity of the ULRM spectra to atomic defects allowed for a refinement of existing quantum defect values. The authors acknowledge the approximations within the Born-Oppenheimer framework and the complexity of modelling collisional dynamics at ultracold temperatures. Future research directions include exploring the use of ytterbium ULRMs as a platform for studying few-body physics and investigating the potential for creating novel quantum states of matter. This work establishes ytterbium ULRMs as a valuable tool for probing electron-atom interactions and lays the groundwork for advanced Rydberg atom experiments with divalent species.