Trilobite Molecules Measured
A Rydberg atom is one with a highly excited electron, and this electron can interact with a second atom in its ground state to form a weakly bound Rydberg molecule. One example of such a bound state is a “trilobite Rydberg molecule,” so-called because the electron’s probability distribution resembles the long-extinct sea creature. Markus Exner at the Technical University of Kaiserslautern, Germany, and colleagues have now obtained high-resolution measurements of the vibrational spectra of trilobite Rydberg molecules formed from pairs of rubidium atoms [1]. The results offer a rigorous test of theoretical models.
The excited electron in a Rydberg molecule has a principal quantum number n of at least 20. In a trilobite Rydberg molecule, the electron also has a high angular momentum . Previous spectroscopic studies unavoidably mixed high- trilobite Rydberg molecules with low- Rydberg molecules, making precise spectra difficult to extract.
Exner and colleagues created a pure sample of high- trilobite Rydberg molecules by first confining and cooling a cloud of rubidium atoms in the ground state, then promoting them to a high-n, high- state using a three-photon excitation process. The third of the three sequential photons came from a laser whose originally resonant frequency was detuned in increments so that the resulting molecules formed in various discrete vibrational states. For varying degrees of detuning, the researchers counted the number of trilobite Rydberg molecules that had been created, allowing them to map the energies of the allowed states.
The researchers found that their experiment was in broad agreement with a recently developed theoretical model. However, subtle high-order effects, which originate in electron–atom interactions at low energies, are not yet accounted for in the model.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- M. Exner et al., “High precision spectroscopy of trilobite Rydberg molecules,” Phys. Rev. Lett. 134, 223401 (2025).