Search

The advent of few femtosecond x-ray light sources brings promise of x-ray/optical pump-probe experiments that can measure chemical and structural changes in the 10-100 fs time regime. Widely distributed timing systems used at x-ray Free-Electron Laser facilities are typically limited to above 50 fs fwhm jitter in active x-ray/optical synchronization. The approach of single-shot timing measurements is used to sort results in the event processing stage. This has seen wide use to accommodate the insufficient precision of active stabilization schemes. In this article, we review the current technique for "measure-and-sort" at the Linac Coherent Light Source at the SLAC National Accelerator Laboratory. The relative arrival time between an x-ray pulse and an optical pulse is measured near the experimental interaction region as a spectrally encoded cross-correlation signal. The cross-correlation provides a time-stamp for filter-and-sort algorithms used for real-time sorting. Sub-10 fs rms resolution is common in this technique, placing timing precision at the same scale as the duration of the shortest achievable x-ray pulses. [ABSTRACT FROM AUTHOR]

We investigate the multiphoton ionization and the subsequent fragmentation of SF6 molecules with intense x-ray-free electron laser pulses at different photon energies. We observe highly charged molecular and atomic ions which were absent in previous experiments with conventional x-ray sources. The observation of fully stripped fluorine ions suggests sequential multiphoton ionization processes with intermediate 1s electron excitation. We measure the average momentum and kinetic energy of each fragment, the comparison of which implies many-body fragmentation pathways of the molecular ions. The observed non-monotonic dependence of the kinetic energy on the charge states of fluorine indicates the multiphoton ionization of 'isolated' atomic neutral fluorine or fluorine ions resultant from bond cleavages and also implies the fragmentation of highly charged molecular ions that are produced at a later time during a single pulse. [ABSTRACT FROM AUTHOR]

The morphology of micrometre-size particulate matter is of critical importance in fields ranging from toxicology to climate science, yet these properties are surprisingly difficult to measure in the particles' native environment. Electron microscopy requires collection of particles on a substrate; visible light scattering provides insufficient resolution; and X-ray synchrotron studies have been limited to ensembles of particles. Here we demonstrate an in situ method for imaging individual sub-micrometre particles to nanometre resolution in their native environment, using intense, coherent X-ray pulses from the Linac Coherent Light Source free-electron laser. We introduced individual aerosol particles into the pulsed X-ray beam, which is sufficiently intense that diffraction from individual particles can be measured for morphological analysis. At the same time, ion fragments ejected from the beam were analysed using mass spectrometry, to determine the composition of single aerosol particles. Our results show the extent of internal dilation symmetry of individual soot particles subject to non-equilibrium aggregation, and the surprisingly large variability in their fractal dimensions. More broadly, our methods can be extended to resolve both static and dynamic morphology of general ensembles of disordered particles. Such general morphology has implications in topics such as solvent accessibilities in proteins, vibrational energy transfer by the hydrodynamic interaction of amino acids, and large-scale production of nanoscale structures by flame synthesis. [ABSTRACT FROM AUTHOR]

Here we present the first angle-resolved, non-resonant (normal) Auger spectra for impulsively aligned nitrogen molecules. We have measured the angular pattern of Auger electron emission following K-shell photoionization by 1.1 keV photons from the Linac Coherent Light Source (LCLS). Using strong-field-induced molecular alignment to make molecular frame measurements is equally effective for both repulsive and quasi-bound final states. The capability to resolve Auger emission angular distributions in the molecular frame of reference provides a new tool for spectral assignments in congested Auger electron spectra that takes advantage of the symmetries of the final diction states. Based on our experimental results and theoretical predictions, we propose the assignment of the spectral features in the Auger electron spectrum. [ABSTRACT FROM AUTHOR]

Kinetic Boltzmann equations are used to describe electron emission spectra obtained after irradiation of noble-gas clusters with intense vacuum ultraviolet (VUV) radiation from a free-electron-laser (FEL). The experimental photoelectron spectra give a complementary and more detailed view of nonlinear processes within atoms and clusters in an intense laser field compared to mass spectroscopy data. Results from our model obtained in this study confirm the experimental and theoretical findings on the differing ionization scenarios at longer (100 nm) and shorter (32 nm) VUV radiation wavelengths. At the wavelength of 100 nm the thermoelectronic electron emission dominates the emission spectra. This indicates the plasma formation and the inverse bremsstrahlung (IB) heating of electrons inside the plasma. This effect is clearly visible for xenon (with the fitted temperature of 6-7 eV), and less visible for argon (with the fitted temperature of 2-3 eV). The two-photon-ionization rate for argon that initiates the cluster ionization, is much lower than the singlephotoionization rate for xenon. Also, more of the photoelectrons created within an argon cluster are able to leave it, as they are more energetic than those released from a xenon cluster. Therefore, the IB heating of plasma electrons in argon is less efficient than in xenon, as the density of the electrons remaining within the cluster is lower. At a wavelength of 32 nm the dominant ionization mechanism identified from the electron spectra of argon clusters is the direct multistep ionization. The signature of the thermalization of electrons is also observed. However, as the heating of electrons due to the inverse bremsstrahlung process is weak at these radiation wavelengths and pulse fluences, the increase of the electron temperature with the pulse intensity is mainly due to the increasing photoionization rate within the irradiated sample. [ABSTRACT FROM AUTHOR]

Abstract: We performed time-of-flight (TOF) spectroscopy on a jet of rare gas clusters using light pulses at λ FEL =32nm produced by the FLASH VUV freeelectron-laser (Hamburg, Germany) with a maximum peak power density of about 3×1013 W/cm2 at the sample. Energy deposition on the clusters was found to be strongly dependent on lambda and much more efficient in the VUV than in the IR and visible spectral range. We observed multiple ionization of rare gas atoms coming from clusters; the latter fragment completely upon absorption of a single pulse. We have also measured high quality photoelectron spectra. Small angle soft X-ray scattering (SAXS) from a single light pulse (λ FEL =32nm), was recorded with high signal to noise ratio for very large (N atoms 3×106) Argon clusters. [Copyright &y& Elsevier]

Abstract: We report on first experiments of rare gas cluster dynamics and spectroscopy using light from the DESY free electron laser at 13eV. The energy absorption is found to be very efficient and the clusters fragment completely. Non-linear effects appear at a peak power density threshold much lower than for visible light. The data are consistent with a three-step ionization process, but the nature of the first step is still controversial. [Copyright &y& Elsevier]

Photoemission spectroscopy measurements have been performed on films of individual germanium nanocrystals. The Ge 3d core-level photoemission reveals mainly bulk-like coordinated atoms and structural disorder on the nanocrystal surface. The Ge 3d plasmon excitation exhibits a reduced bulk- and an increased surface-plasmon contribution underlining the importance of the surface region to the overall particle electronic structure. In the Ge nanocrystal valence-band photoemission features similar to amorphous-Ge are found, further indicating structural disorder in the surface region of the nanocrystals. [Copyright &y& Elsevier]

Intense laser induced plasma dynamics in sub-micron scale helium droplets are monitored by femtosecond time-resolved X-ray coherent diffractive imaging. Anisotropic surface softening and strongly anisotropic shrinking of the plasma core are observed.

The structure of KH2PO4 single crystals (so-called KDP) irradiated with ∼3 ns, 355 nm laser pulses with fluences above the laser-induced breakdown threshold is studied by a combination of Raman scattering, photoluminescence, and soft x-ray absorption spectroscopies. We compare spectra from the as-grown material, surface and bulk laser-induced damage sites, as well as from KPO3 references. Results show that irradiation with fluences above the laser-induced breakdown threshold leads to decomposition of KDP at surface damage sites but not at bulk damage sites. New spectroscopic features are attributed to dehydration products. For the laser irradiation conditions used in this study, the decomposed near-surface layer absorbs photons at ∼3.4 eV (364 nm). These results may explain the recently reported fact that surface laser damage sites in KDP crystals tend to grow with subsequent exposure to high-power laser pulses, while bulk damage sites do not. [ABSTRACT FROM AUTHOR]

To investigate the impact of damp heat treatments on the electronic and chemical structure of Cu(In,Ga)(S,Se)[sub 2]-based thin film solar cells, we have performed a detailed soft x-ray emission study of the ZnO/Culn(S,Se)[sub 2] and ZnO/CdS/Culn(S,Se)[sub 2] interfaces. By comparing the sulfur L[sub 2,3] emission spectra of pristine and damp-heat treated samples, we find a sulfate formation at the ZnO/Culn(S,Se)[sub 2] and the ZnO/CdS interface. The intensity behavior as a function of ZnO film thickness further reveals a diffusion of sulfur atoms into the ZnO film, leading to the formation of zinc sulfate in the ZnO window layer of damp-heat-treated Cu(In,Ga)(S,Se)[sub 2]-based solar cells. [ABSTRACT FROM AUTHOR]