Nonlinear single and double ionization of molecules by strong laser pulses

We present a theoretical treatment for the single and double ionization of molecules in strong laser pulses, including the effect of electron correlat ions. Applied to a model N-2 molecule, our simulations show that double ion ization occurs via a direct two-electron mechanism at moderate laser intens ities (N-2 ((h) over bar omega) under right arrow N-2(2+) + 2e(-)) while a step-by-step sequential ionization process dominates at higher intensity (N -2 ((h) over bar omega) under right arrow N-2(+) + e(-) ((h) over bar omega ) under right arrow N-2(2+) + 2e(-)). At intermediate intensity, these two mechanisms have a comparable contribution to the total double-ionization yi eld. This phenomenon is directly reflected in the spatial distributions of N-2(+) and N-2(2+) in the focal volume of the laser pulse. In addition we s how that at high intensity the singly and doubly charged molecular ions are created in well separated regions of space.