Few-Photon Multiple Ionization of N2 by Extreme Ultraviolet Free-Electron Laser Radiation

Few-photon multiple ionization of N-2 was studied differentially in a reaction microscope using 44 eV, similar to 25 fs, intense (similar to 10(13) W/cm(2)) photon pulses from FLASH. Sequential ionization is observed to dominate. For various intermediate charge states N-2(n+) we find a considerable excess of photons absorbed compared to the minimum number that would energetically be required. Photoionization of aligned N-2(n+) ions, produced by photon absorption in sequential steps, is explored and few-photon absorption pathways are traced by inspecting kinetic energy releases and fragment-ion angular distributions.