The Roaming Threshold

Radiation damage begins with a single event: an ionizing photon knocks an electron out of an atom. What happens next depends on the atom’s neighbors. In electron-transfer-mediated decay (ETMD), the excited atom stabilizes by stealing an electron from a neighbor, releasing energy that ionizes a third atom. The process creates low-energy electrons — the kind that break bonds in DNA and shatter water molecules in living cells.

The standard picture treats ETMD as an electronic process. The atoms are assumed stationary while the electron transfer occurs. The geometry is fixed; the decay probability is calculated from static configurations.

Researchers at the Fritz Haber Institute studied a neon atom weakly bound to two krypton atoms — a NeKr2 trimer. They hit the neon with soft X-rays at synchrotron facilities BESSY II and PETRA III, then used a COLTRIMS reaction microscope to reconstruct the exact arrangement of all three atoms at the moment the decay occurred. They tracked the system for up to one picosecond after ionization — an eternity on the atomic timescale.

The atoms moved. Substantially. One krypton drifted closer to the neon, the other pulled away. The geometry at the moment of decay was systematically different from the geometry at the moment of ionization. The system rearranged itself before it decayed, and the rearrangement determined both the timing and the outcome.

Ab initio simulations tracked thousands of possible atomic pathways and calculated the decay probability along each one. The probability was not constant. It varied with the geometry — rising sharply when certain interatomic distances fell below a threshold, dropping when the atoms were far apart. The atoms were not passive bystanders. They were participants, and their motion steered the process.

The through-claim: decay is not an electronic event that happens to a molecular structure. It is a coupled electronic-nuclear event where the nuclear motion selects which electronic pathway occurs. The structure moves first, and the decay follows the structure. The atoms are not a stage. They are actors — and the play doesn’t start until they reach their marks.