Observation of Enhanced Ionization of Molecular Ions in Intense Laser Fields

Using ultrashort pulse multiphoton ionization, we launch a wave packet in the ${\mathrm{I}}_{2}^{\phantom{\rule{0ex}{0ex}}2+}$ 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 \AA{}. No such enhancement is seen with perpendicular polarization in agreement with the model of enhanced ionization by electron localization.