How to Catch an Invisible Electron express
An extra electron in a molecule is like a passenger barely hanging onto the step of a departing train. Such a 'passenger' turns the particle into an anion — a negative ion. Studying it is tough: the bond is weak, the electron cloud is diffuse, and any jolt can knock the electron off.
A team of researchers took on diatomic anions of metals — alkali (lithium, sodium, potassium, rubidium, cesium, francium) and alkaline-earth (beryllium, magnesium, calcium, strontium, barium, radium). That's 57 molecules in total. Using powerful computational methods, they built potential energy curves — graphs showing how energy changes as atoms approach or move apart. Then they calculated electric dipole moments and polarizabilities — a molecule's ability to deform under an electric field.
But the real surprise was in the excited states. Some of them turned out to cross with the ground state of the neutral molecule. It's as if the train's step momentarily lined up with the platform — making it easier for the electron to hop on or off. Such crossings can enhance resonant electron capture by the neutral molecule and the subsequent decay of the anion. This discovery could be useful in experiments with mixtures of ultracold molecules and Rydberg atoms — atoms with an electron that has wandered very far away.
Original abstract
as written by the authors arXiv:2608.05067v1
Studies of anions are inherently more challenging than investigations of neutrals and cations because of the diffuse and weakly bound character of an anionic electron. Here, we present a comprehensive computational examination of ground-state diatomic molecular anions composed of alkali-metal (Li, Na, K, Rb, Cs, Fr) and alkaline-earth-metal (Be, Mg, Ca, Sr, Ba, Ra) atoms. We study 21 alkali-metal diatomic anions in the X$$^{2}\Sigma^{+}$$ electronic state and 36 alkali-metal--alkaline-earth-metal diatomic anions in the X$$^{1}\Sigma^{+}$$ electronic state. The calculations employ a hierarchy of the coupled cluster methods, combined with large Gaussian basis sets and small-core relativistic energy-consistent pseudopotentials for heavier elements. We compute potential energy curves, permanent electric dipole moments, and static polarizabilities, and we assess convergence and uncertainties of our results. Additionally, using the multireference configuration interaction and equation-of-motion electron-attachment coupled cluster methods, we investigate excited electronic states of alkali-metal molecular anions, including valence-bound and dipole-bound states. We predict crossings between ground neutral and excited anionic states, which may enhance resonant electron attachment and subsequent anion dissociation. This finding may be relevant for experiments with mixtures of ultracold ground-state alkali-metal molecules and Rydberg atoms.
Alkali metal anions are unstable on their own — the extra electron is held only by a weak interaction. Even room temperature is enough to 'shake it off'.