Query: Schorb

title: Nanoplasma dynamics of single large xenon clusters irradiated with superintense x ray pulses from the linac coherent light source free electron laser.
abstract: The plasma dynamics of single mesoscopic Xe particles irradiated with intense femtosecond x ray pulses exceeding 10(16) W/cm2 from the Linac Coherent Light Source free electron laser are investigated. Simultaneous recording of diffraction patterns and ion spectra allows eliminating the influence of the laser focal volume intensity and particle size distribution. The data show that for clusters illuminated with intense x ray pulses, highly charged ionization fragments in a narrow distribution are created and that the nanoplasma recombination is efficiently suppressed.

title: Ultra efficient ionization of heavy atoms by intense X ray free electron laser pulses
abstract: Researchers create high ionization states, up to Xe36+ using 1.5 keV free electron laser pulses. The higher than expected ionization may be due to transient resonance enhanced absorption and the effect may play an important role in interactions of intense X rays with high Z elements and radiation damage.

title: X ray optical cross correlator for gas phase experiments at the Linac Coherent Light Source free electron laser
abstract: X ray optical pump probe experiments at the Linac Coherent Light Source (LCLS) have so far been limited to a time resolution of 280 fs fwhm due to timing jitter between the accelerator based free electron laser (FEL) and optical lasers. We have implemented a single shot cross correlator for femtosecond x ray and infrared pulses. A reference experiment relying only on the pulse arrival time information from the cross correlator shows a time resolution better than 50 fs fwhm (22 fs rms) and also yields a direct measurement of the maximal x ray pulse length. The improved time resolution enables ultrafast pump probe experiments with x ray pulses from LCLS and other FEL sources.

title: Size dependent ultrafast ionization dynamics of nanoscale samples in intense femtosecond x ray free electron laser pulses.
abstract: All matter exposed to intense femtosecond x ray pulses from the Linac Coherent Light Source free electron laser is strongly ionized on time scales competing with the inner shell vacancy lifetimes. We show that for nanoscale objects the environment, i.e. nanoparticle size, is an important parameter for the time dependent ionization dynamics. The Auger lifetimes of large Ar clusters are found to be increased compared to small clusters and isolated atoms, due to delocalization of the valence electrons in the x ray induced nanoplasma. As a consequence, large nanometer sized samples absorb intense femtosecond x ray pulses less efficiently than small ones.

title: Ultrafast transitions from solid to liquid and plasma states of graphite induced by x ray free electron laser pulses.
abstract: We used photon pulses from an x ray free electron laser to study ultrafast x ray induced transitions of graphite from solid to liquid and plasma states. This was accomplished by isochoric heating of graphite samples and simultaneous probing via Bragg and diffuse scattering at high time resolution. We observe that disintegration of the crystal lattice and ion heating of up to 5 eV occur within tens of femtoseconds. The threshold fluence for Bragg peak degradation is smaller and the ion heating rate is faster than current x ray matter interaction models predict.

title: Explosions of xenon clusters in ultraintense femtosecond x ray pulses from the LCLS free electron laser.
abstract: Explosions of large Xe clusters 11,000) irradiated by femtosecond pulses of 850 eV x ray photons focused to an intensity of up to 10(17) W/cm(2) from the Linac Coherent Light Source were investigated experimentally. Measurements of ion charge state distributions and energy spectra exhibit strong evidence for the formation of a Xe nanoplasma in the intense x ray pulse. This x ray produced Xe nanoplasma is accompanied by a three body recombination and hydrodynamic expansion. These experimental results appear to be consistent with a model in which a spherically exploding nanoplasma is formed inside the Xe cluster and where the plasma temperature is determined by photoionization heating.

title: Transient X ray fragmentation: probing a prototypical photoinduced ring opening.
abstract: We report the first study of UV induced photoisomerization probed via core ionization by an x ray laser. We investigated x ray ionization and fragmentation of the cyclohexadiene hexatriene system at 850 eV during the ring opening. We find that the ion fragmentation patterns evolve over a picosecond, reflecting a change in the state of excitation and the molecular geometry: the average kinetic energy per ion fragment and H( ion count increase as the ring opens and the molecule elongates. We discuss new opportunities for molecular photophysics created by optical pump x ray probe experiments.

title: Angle resolved electron spectroscopy of laser assisted Auger decay induced by a few femtosecond x ray pulse.
abstract: Two color (x ray+infrared) electron spectroscopy is used for investigating laser assisted KLL Auger decay following 1s photoionization of atomic Ne with few femtosecond x ray pulses from the Linac Coherent Light Source. In an angle resolved experiment, the overall width of the laser modified Auger electron spectrum and its structure change significantly as a function of the emission angle. The spectra are characterized by a strong intensity variation of the sidebands revealing a gross structure. This variation is caused, as predicted by theory, by the interference of electrons emitted at different times within the duration of one optical cycle of the infrared dressing laser, which almost coincides with the lifetime of the Ne 1s vacancy.

title: Identification of twinned gas phase clusters by single shot scattering with intense soft x ray pulses
abstract: Scattering experiments on xenon nanoclusters with high intensity soft x ray laser pulses from the Free Electron LASer in Hamburg (FLASH) are performed to investigate different cluster morphologies in the gas phase. Three different types of scattering patterns can be identified. The most frequent pattern of concentric rings reflects the event of a single spherical cluster in focus. Fine interference rings similar to Newton rings appear when two clusters are illuminated at mm distance, revealing three dimensional information about the location of the clusters. Between 10 and 30% of all hits show a previously unknown twin cluster configuration with two clusters in direct contact. Simulations of scattering patterns for twin clusters with different sizes of the two particles, degree of fusion and orientation in space allow us to explain all the observed patterns.

title: Amorphous to crystalline phase transition in carbon induced by intense femtosecond x ray free electron laser pulses
abstract: We present the results of an experiment where amorphous carbon undergoes a phase transition induced by femtosecond 830 eV x ray free electron laser pulses. The phase transition threshold fluence is found to be 282 11 mJ/cm(2) Atomic force microscopy, photoelectron microscopy, and micro Raman spectroscopy give experimental evidence for the phase transition in terms of a volume expansion, graphitization, and change of local order of the irradiated sample area. The interaction is modeled by an accurate time dependent treatment of the ionization dynamics coupled to a two temperature model. At the phase transition fluence threshold the free electron density N e is found to be at maximum 9 x 10(20) cm( 3) while the ion (atom) temperature is found to be 1050 K, e.g. above the crystallization activation temperature reported in the literature. This low ionization rate and high atom temperature suggest a thermally activated phase transition.