Geometric Dependence of Strong Field Enhanced Ionization in D₂O

We have studied strong-field enhanced dissociative ionization of D$_{2}$O in 40 fs, 800 nm laser pulses with focused intensities of $< 1 - 3 \times 10^{15}W/cm^2$ by resolving the charged fragment momenta with respect to the laser polarization. We observe dication dissociation into OD$^{+}$/D$^{+}$ dominates when the polarization is out of the plane of the molecule, whereas trication dissociation into O$^{+}$/D$^{+}$/D$^{+}$ is strongly dominant when the polarization is aligned along the D-D axis. Dication dissociation into O/D$^{+}$/D$^{+}$, and O$^{+}$/D$_2$$^{+}$ is not seen, nor is there any significant fragmentation into multiple ions when the laser is polarized along the C$_{2v}$ symmetry axis of the molecule. Even below the saturation intensity for OD$^{+}$/D$^{+}$, the O$^{+}$/D$^{+}$/D$^{+}$ channel has higher yield. By analyzing how the laser field is oriented within the molecular frame for both channels, we show that enhanced ionization is driving the triply charged three body breakup, but is not active for the doubly charged two body breakup. We conclude that laser-induced distortion of the molecular potential suppresses multiple ionization along the C$_{2v}$ axis, but enhances ionization along the D-D direction.