The Reversed Shield

Under ultrafast laser driving, a positron delays electron ionization in positronium hydride but slightly enhances it in positronium chloride. The same particle, the same interaction — opposite effects. The difference is entirely geometric.

In PsH, the positron’s looser binding means it responds faster to the laser field. Its rapid displacement partially shields the electron from the field’s full strength, delaying the electron’s escape. The positron acts as a buffer. In PsCl, the orbital energy alignment and effective potential shape conspire differently — the positron’s faster response slightly accelerates electron escape rather than retarding it. Same particle, same laser, same physics. Different container, opposite outcome.

The explanation traces to potential shape asymmetry. Electrons and positrons experience differently shaped effective potentials because their charges are opposite. In PsH, this asymmetry produces a shielding geometry. In PsCl, the different atomic environment reshapes the effective potential so that the positron’s motion destabilizes rather than protects. The distinction is not in the perturbation but in the landscape it acts on.

Both systems share one feature: the positronic response is always faster than the electronic response. The positron moves first. Whether that first motion helps or hurts the electron depends on the surrounding potential surface.

This is a clean case of context determining the sign of an interaction. The stabilizer and the destabilizer are the same object. Nothing about the positron changed — its charge, mass, and coupling are identical in both systems. What changed is the shape of the container. The same perturbation produces protection or exposure depending on the geometry of what surrounds it. Whether an interaction is beneficial is not a property of the interaction but of the space it inhabits.