"The Distant Decay"

Intermolecular Coulombic decay — where an excited atom transfers its excess energy to ionize a neighbor — was understood as a condensed-phase process. In clusters and liquids, molecules sit close enough for efficient energy transfer. In dilute gases, the distances are too large.

The mechanism works in dilute gases. But it is a fundamentally different mechanism.

In clusters, the transfer proceeds through overlap of electronic wavefunctions — a short-range process that decays exponentially with distance. In gases, the transfer proceeds through a long-range channel that operates efficiently despite separation. The same phenomenon — energy transfer leading to ionization — has two distinct physical regimes, not one process that weakens with distance.

The structural insight: a process that appears to weaken gradually with distance may actually transition to a qualitatively different mechanism at large separations. The condensed-phase and gas-phase instances of intermolecular Coulombic decay share an outcome (neighbor ionization) but not a mechanism. Extrapolating the short-range physics to long range would predict negligible efficiency; the actual mechanism at long range is different and efficient.

When a process “shouldn’t work” at large distance, the question is not whether it works but whether the mechanism has changed.