Single and double ionization of diatomic molecules in strong laser fields

Strong-field single ionization and double ionization of two diatomic molecules, O₂ and N₂, are studied and compared to Xe and Ar, using an intense ultrashort pulse Ti:sapphire laser in the 2×10¹³ to 8×10¹⁴W/cm² intensity range. N₂ behaves like a structureless atom for both single and double ionization. The recently reported suppression of the O₂⁺ ion yield compared to 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 O₂ is below that predicted by tunneling ionization. We extend the study to the double ionization of O₂ and find a distinctly reduced nonsequential double-ionization rate. We find evidence that electronic structure influences strong-field tunneling ionization in molecules.