  We provide a basis for the laser intensity dependence of the momentum
distributions of electrons and ions arising from strong-field non-sequential
double ionization (NSDI) at intensities in the range $I=1-6.5 \times 10^{14}
W/cm^2$. To do this we use a completely classical method introduced previously
\cite{ho-etal05}. Our calculated results reproduce the features of experimental
observations at different laser intensities and depend on just two distinct
categories of electon trajectories.
