Nonlinear Ionization of Organic Molecules in High Intensity Laser Fields

We use a series of 23 organic molecules to study ionization of complex media caused by their interaction with intense 40 fs, $0.8\mu \mathrm{m}$ pulses. All molecules reach saturated ionization at higher intensities than would be expected for atoms of the same ionization potential, reminiscent to what has been reported for dielectric breakdown with femtosecond pulses. Dependence of the ionization rate on the alignment of the molecule with the laser field is ruled out as the cause of the high saturation intensities. All molecules allow a significant range of intensities between the region of approximately $100\%$ ionization and before the second and subsequent electrons are removed.