Single and double ionization of diatomic molecules in strong laser fields

Strong-field single ionization and double ionization of two diatomic molecules, ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2},$ are studied and compared to Xe and Ar, using an intense ultrashort pulse Ti:sapphire laser in the $2\ifmmode\times\else\texttimes\fi{}{10}^{13}$ to $8\ifmmode\times\else\texttimes\fi{}{10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ intensity range. ${\mathrm{N}}_{2}$ behaves like a structureless atom for both single and double ionization. The recently reported suppression of the ${\mathrm{O}}_{2}^{+}$ ion yield compared to ${\mathrm{Xe}}^{+}$ is confirmed in our experiment, but we show that the suppression is not due to dissociative recombination. Rather, we conclude that the ionization rate of ${\mathrm{O}}_{2}$ is below that predicted by tunneling ionization. We extend the study to the double ionization of ${\mathrm{O}}_{2}$ and find a distinctly reduced nonsequential double-ionization rate. We find evidence that electronic structure influences strong-field tunneling ionization in molecules.