Using ultrashort pulse multiphoton ionization, we launch a wave packet in the ${\mathrm{I}}_2^{2+}$ state that dissociates into the fragments ${\mathrm{I}}^{2+}$ and I. We measure its multiphoton ionization probability as a function of its internuclear distance with an intense delayed probe pulse. If the probe pulse is polarized parallel to the internuclear axis, we observe a peak in the ionization yield at a critical distance of 5–6 Å. No such enhancement is seen with perpendicular polarization in agreement with the model of enhanced ionization by electron localization.

• Received 12 February 1996

DOI:https://doi.org/10.1103/PhysRevLett.76.4140