Intermolecular Coulombic decay is an energy transfer process: an excited atom or molecule relaxes by transferring its energy to a neighboring system, which ionizes. The process is ultrafast — femtoseconds — and has been studied extensively in clusters, fluids, and condensed systems where molecules sit close together. Weak bonding (van der Waals, hydrogen bonds) keeps the units near enough for efficient energy transfer. The proximity is supposed to be essential.
Falkowski, Kuleff, and Cederbaum (arXiv:2603.08414, March 2026) demonstrate that intermolecular Coulombic decay operates efficiently in gases, where the distances between molecules are orders of magnitude larger than in clusters.
The mechanism is different from the one that operates in weakly bound systems. In clusters, the energy transfer relies on the overlap between the excited state of the donor and the ionization continuum of the acceptor at short range. In gases, the transfer occurs at long range through a different coupling — Coulombic interaction between the transition density of the donor and the bound-to-continuum transition of the acceptor, which falls off more slowly with distance. The transfer rate is lower per pair, but in a gas there are many more potential acceptor molecules within the relevant range. The product of rate and count can make the process competitive even at gas-phase separations.
The result is unexpected because the field assumed that proximity was necessary, not just convenient. Nearly all investigations of ICD targeted weakly bound systems specifically because tight binding seemed to be a precondition for efficient transfer. The assumption was reasonable — the process was discovered in clusters, characterized in clusters, and applied in clusters. But the assumption was wrong. The mechanism that operates at long range was always available. Nobody calculated it because nobody expected it to matter.
The practical consequence: any atomic or molecular gas with sufficient excitation energy can drive ionization of neighboring molecules through ICD. The process doesn't need clusters. It doesn't need condensation. It needs only excited donors and ground-state acceptors at any separation where the Coulombic coupling is nonzero — which, in principle, is all separations. The mechanism was hidden by the assumption that limited the search to places where it was already known to exist.
Falkowski, Kuleff, and Cederbaum, "Hitherto unrecognized intermolecular Coulombic decay mechanism in gases," arXiv:2603.08414 (March 2026).