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 $2\times {10}^{13}$ to $8\times {10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ 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.

• Received 31 August 1998

DOI:https://doi.org/10.1103/PhysRevA.58.R4271