  Polyatomic molecules in strong laser fields can undergo substantial nuclear
motion within tens of femtoseconds. Ion imaging methods based on dissociation
or Coulomb explosion therefore have difficulty faithfully recording the
geometry dependence of the field ionization that initiates the dissociation
process. Here we compare the strong-field double ionization and subsequent
dissociation of water (both H$_2$O and D$_2$O) in 10-fs and 40-fs 800-nm laser
pulses. We find that 10-fs pulses turn off before substantial internuclear
motion occurs, whereas rapid internuclear motion can take place during the
double ionization process for 40-fs pulses. The short-pulse measurements are
consistent with a simple tunnel ionization picture, whose predictions help
interpret the motion observed in the long-pulse measurements.
