Multi-photon ionization of molecular nitrogen by femtosecond soft x-ray FEL pulses

At the new free-electron laser (FEL) for vacuum ultraviolet (VUV) and soft x-ray radiation FLASH of the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, multi-photon double ionization of molecular nitrogen has been observed and studied by ion time-of-flight spectroscopy. The experiments have been performed at the microfocus beamline BL2 with photon pulses of 25 fs duration and irradiance levels up to 2 x 10(13) W cm(-2) at a photon energy of 38 eV, i.e. well above the first ionization/dissociation thresholds of the target. A new and important aspect of our experiments has been the reliable determination of absolute photon numbers per pulse with the help of a calibrated setup allowing the investigation of nonlinear effects by quantitative measurements. Results obtained are discussed in terms of a sequential two-photon ionization scheme.