Angle-resolved and internuclear-separation-resolved measurements of the single ionization rate of neutral I${}_2$ have been obtained. By launching a wave packet in the $B{}^3\Pi {{}_u}^{+}$ state ($B$ state) of I${}_2$ with a 50-fs tunable pump pulse we can measure the ionization rate as a function of internuclear separation as the wave packet evolves in the $B$ state. Moreover, since the ground to $B$-state optical transition dipole moment is parallel to the internuclear axis, the $B$-state subpopulation of the I${}_2$ thermal ensemble will have a high degree of alignment, allowing for angular measurements. The $B$ state shows the well-known effect of enhanced ionization at a critical separation, $R$${}_c$, with an enhancement factor of 22 when the ionizing field is polarized along the internuclear axis and the enhanced ionization decreases when the angle between the field and the axis increases from 0 ${}^{\circ }$ to 90 ${}^{\circ }$, finally disappearing at 90 ${}^{\circ }$. These results on the enhanced ionization of the $B$ state of I${}_2$ give the most precise determination of $R$${}_c$ for any molecule and agree extremely well with the prediction from a simple model of electron localization.

• Received 12 August 2011

DOI:https://doi.org/10.1103/PhysRevA.84.043427