Influence of Molecular Structure on Double Ionization of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ by High Intensity Ultrashort Laser Pulses

The electron momentum correlation after nonsequential double ionization of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ in ultrashort light pulses at light intensities near $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ has been investigated. The experimental results reveal distinctive differences between the molecular species and between molecules and atoms of similar ionization threshold. We provide evidence that recollision double ionization is the essential mechanism and trace the origin of the differences back to the symmetry of the orbitals occupied by the valence electrons.

Figure 1

Measured electron sum-momentum distributions (connected squares) after double ionization of (a) ${\mathrm{N}}_2$ at $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, (b) ${\mathrm{O}}_2$ at $1.1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, and (c) ${\mathrm{O}}_2$ at $1.7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The distributions are shown projected on an axis parallel to the laser beam polarization vector. The arrows indicate the “cutoff” momentum of $4\sqrt{U_p}$ which is expected for NSDI due to recollision. For ${\mathrm{O}}_2$, the dotted lines show calculated spectra if sequential double ionization is assumed. The light gray lines in (a) and (c) show the result of our NSDI model.

Figure 2

The measured (a), (b) and corresponding calculated (c), (d) electron momentum correlation for the momentum components parallel to the light beam polarization vector: (a), (c) ${\mathrm{N}}_2$ at $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and (b), (d) ${\mathrm{O}}_2$ at $1.7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.