6 July 2026

Photo: AG Schmelcher

(a) Born-Oppenheimer potential energy curves for the n =18 ULRM. The black curve highlights the 1P1 potential curve whose oscillations are deep enough to support vibrational states. The red box marks the first butterfly well, magnified in the inset with the calculated vibrational states. (b) Electron density distribution of the spin-singlet butterfly molecule

In a recent collaboration between researchers in Kaiserslautern, Hamburg, Dresden, and Boston [1], the scientists were able to successfully create and detect the singlet-butterfly molecules. These ultralong-range Rydberg molecules (ULRM), with P-wave spin-singlet character (1P1), pave the way to the creation of long-sought-after states of matter: ultracold anions and ultracold neutral plasmas.

The creation of  ultracold anions and neutral plasmas remain as key challenges in the frontier of modern ultracold physics, due to free electron heating mechanisms and the lack of laser-cooling transitions in alkali anions. A previous work from the authors [2] proposed a new scheme to achieve these states by charge transfer from a singlet-butterfly ULRM, via radiative transition. Given the anion’s 1Σ singlet molecular symmetry, it is imperative that the ULRM needs a 1P1 character to facilitate such a transfer. This work [1]  reports the first ever observation of the spin-singlet butterfly molecule in a cold Rubidium gas. The n=18 Rydberg molecule, bound in a potential well with a predominant 1P1 electronic character, is photoassociated through the small Rydberg f-state admixture in its electronic state. The measured binding energies, vibrational splitting, spectral profiles, and dipole moments are in excellent agreement with the theoretical prediction, confirming its electronic configuration.

References:

[1] M. Exner, R. Srikumar, R. Blättner, P. Schmelcher, H.R. Sadeghpour, M.T. Eiles, H. Ott
Observation of Spin-Singlet Butterfly Rydberg Molecules in an Ultracold Atomic Rb Gas
Physical Review Letters 136, 243002 (2026)

[2] F. Hummel, P. Schmelcher, H. Ott and H.R. Sadeghpour
An ultracold heavy Rydberg system formed from ultra-long-range molecules bound in a stairwell potential
New Journal Physics  22, 063060 (2020)