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  • 1. Angelova, G.
    et al.
    Ziemann, V.
    Meseck, A.
    Salén, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Hamberg, Mathias
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Boedewadt, J.
    Khan, S.
    Winter, A.
    Schlarb, H.
    Loehl, F.
    Saldin, E.
    Schneidmiller, E.
    Yurkov, M.
    Observation of two-dimensional longitudinal-transverse correlations in an electron beam by laser-electron interactions2008In: Physical Review Special Topics. Accelerators and Beams, ISSN 1098-4402, E-ISSN 1098-4402, Vol. 11, no 7, article id 70702Article in journal (Refereed)
    Abstract [en]

    During the preparatory work for the optical-replica synthesizer experiment in the free-electron laser FLASH at DESY, we were able to superimpose a short, approximately 200 fs long pulse from a frequencydoubled mode-locked erbium laser with titanium-sapphire amplifier and an approximately 20 ps long electron bunch in an undulator. This induces an energy modulation in a longitudinal slice of the electron bunch. A magnetic chicane downstream of the undulator converts the energy modulation into a density modulation within the slice that causes the emission of coherent optical transition radiation from a silver-coated silicon screen. Varying the relative timing between electron and laser, we use a camera to record two-dimensional images of the slices as a function of the longitudinal position within the electron bunch.

  • 2. Berrah, Nora
    et al.
    Fang, Li
    Murphy, Brendan
    Osipov, Timur
    Ueda, Kiyoshi
    Kukk, Edwin
    Feifel, Raimund
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Salén, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Richter, Robert
    Prince, Kevin C.
    Bozek, John D.
    Bostedt, Christoph
    Wada, Shin-ichi
    Piancastelli, Maria N.
    Tashiro, Motomichi
    Ehara, Masahiro
    Double-core-hole spectroscopy for chemical analysis with an intense X-ray femtosecond laser2011In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 41, p. 16912-16915Article in journal (Refereed)
    Abstract [en]

    Theory predicts that double-core-hole (DCH) spectroscopy can provide a new powerful means of differentiating between similar chemical systems with a sensitivity not hitherto possible. Although DCH ionization on a single site in molecules was recently measured with double-and single-photon absorption, double-core holes with single vacancies on two different sites, allowing unambiguous chemical analysis, have remained elusive. Here we report that direct observation of double-core holes with single vacancies on two different sites produced via sequential two-photon absorption, using short, intense X-ray pulses from the Linac Coherent Light Source free-electron laser and compare it with theoretical modeling. The observation of DCH states, which exhibit a unique signature, and agreement with theory proves the feasibility of the method. Our findings exploit the ultrashort pulse duration of the free-electron laser to eject two core electrons on a time scale comparable to that of Auger decay and demonstrate possible future X-ray control of physical inner-shell processes.

  • 3. Frasinski, L. J.
    et al.
    Zhaunerchyk, V.
    Mucke, M.
    Squibb, R. J.
    Siano, M.
    Eland, J. H. D.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Sahlén, Martin
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Foucar, L.
    Ullrich, J.
    Motomura, K.
    Mondal, S.
    Ueda, K.
    Osipov, T.
    Fang, L.
    Murphy, B. F.
    Berrah, N.
    Bostedt, C.
    Bozek, J. D.
    Schorb, S.
    Messerschmidt, M.
    Glownia, J. M.
    Cryan, J. P.
    Coffee, R. N.
    Takahashi, O.
    Wada, S.
    Piancastelli, M. N.
    Richter, R.
    Prince, K. C.
    Feifel, R.
    Dynamics of Hollow Atom Formation in Intense X-Ray Pulses Probed by Partial Covariance Mapping2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 7, p. 073002-Article in journal (Refereed)
    Abstract [en]

    When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called partial covariance mapping'' to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

  • 4.
    Larsson, Mats
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Salén, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Feifel, R.
    Piancastelli, M. N.
    Fang, L.
    Murphy, B. F.
    Osipov, T.
    Berrah, N.
    Kukk, E.
    Ueda, K.
    Bozek, J. D.
    Bostedt, C.
    Wada, S.
    Richter, R.
    Feyer, V.
    Prince, K. C.
    Double core-hole formation in small molecules at the LCLS free electron laser2013In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 46, no 16, p. 164030-Article in journal (Refereed)
    Abstract [en]

    We have investigated nonlinear processes in small molecules by x-ray photoelectron spectroscopy using the Linac Coherent Light Source free electron laser, and by simulations. The main focus of the experiments was the formation of the two-site double core-hole (tsDCH) states in the molecules CO2, N2O and N-2. These experiments are described in detail and the results are compared with simulations of the photoelectron spectra. The double core-hole states, and in particular the tsDCH states, have been predicted to be highly sensitive to the chemical environment. The theory behind this chemical sensitivity is validated by the experiments. Furthermore, our simulations of the relative integrated intensities of the peaks associated with the nonlinear processes show that this type of simulation, in combination with experimental data, provides a useful tool for estimating the duration of ultra-short x-ray pulses.

  • 5. Liu, M.
    et al.
    Pauchard, T.
    Sjodin, M.
    Launila, O.
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Berg, L-E
    Time-resolved study of the A(2)Pi state of CaH by laser spectroscopy2009In: Journal of Molecular Spectroscopy, ISSN 0022-2852, E-ISSN 1096-083X, Vol. 257, no 1, p. 105-107Article in journal (Refereed)
    Abstract [en]

    A time-resolved experiment on the A(2)Pi state of gaseous calcium hydride has been performed by applying laser spectroscopic methods. The following zero-pressure lifetime was obtained for the CaH A(2)Pi state: tau(nu=0) = 33.2 (+/- 3.2) ns and tau(nu=1) = 33.7 (+/- 5.2) ns. The lifetime was found to be the same for the A(2)Pi(1/2) and Delta(2)Pi(3/2) states. 

  • 6. Mucke, M.
    et al.
    Zhaunerchyk, Vitali
    Stockholm University, Faculty of Science, Department of Physics. Uppsala University, Sweden; University of Gothenburg, Sweden.
    Frasinski, L. J.
    Squibb, R. J.
    Siano, M.
    Eland, J. H. D.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Salén, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    vab der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Foucar, L.
    Ullrich, J.
    Motomura, K.
    Mondal, S.
    Ueda, K.
    Osipov, T.
    Fang, L.
    Murphy, B. F.
    Berrah, N.
    Bostedt, C.
    Bozek, J. D.
    Schorb, S.
    Messerschmidt, M.
    Glownia, J. M.
    Cryan, J. P.
    Coffee, R. N.
    Takahashi, O.
    Wada, S.
    Piancastelli, M. N.
    Richter, R.
    Prince, K. C.
    Feifel, R.
    Covariance mapping of two-photon double core hole states in C2H2 and C2H6 produced by an x-ray free electron laser2015In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 17, article id 073002Article in journal (Refereed)
    Abstract [en]

    Few-photon ionization and relaxation processes in acetylene (C2H2) and ethane (C2H6) were investigated at the linac coherent light source x-ray free electron laser (FEL) at SLAC, Stanford using a highly efficient multi-particle correlation spectroscopy technique based on a magnetic bottle. The analysis method of covariance mapping has been applied and enhanced, allowing us to identify electron pairs associated with double core hole (DCH) production and competing multiple ionization processes including Auger decay sequences. The experimental technique and the analysis procedure are discussed in the light of earlier investigations of DCH studies carried out at the same FEL and at third generation synchrotron radiation sources. In particular, we demonstrate the capability of the covariance mapping technique to disentangle the formation of molecular DCH states which is barely feasible with conventional electron spectroscopy methods.

  • 7.
    Sahlén, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Schmidt, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Feifel, Raimund
    Stockholm University, Faculty of Science, Department of Physics.
    Piancastelli, M. N.
    Fang, L.
    Murphy, B.
    Osipov, T.
    Berrah, N.
    Kukk, E.
    Ueda, K.
    Bozek, J. D.
    Bostedt, C.
    Wada, S.
    Richter, R.
    Feyer, V.
    Prince, K. C.
    Experimental Verification of the Chemical Sensitivity of Two-Site Double Core-Hole States Formed by an X-Ray Free-Electron Laser2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 15, p. 153003-Article in journal (Refereed)
    Abstract [en]

    We have performed x-ray two-photon photoelectron spectroscopy using the Linac Coherent Light Source x-ray free-electron laser in order to study double core-hole (DCH) states of CO2, N2O, and N-2. The experiment verifies the theory behind the chemical sensitivity of two-site DCH states by comparing a set of small molecules with respect to the energy shift of the two-site DCH state and by extracting the relevant parameters from this shift.

  • 8.
    Salén, Peter
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kaminska, Magdalena
    Stockholm University, Faculty of Science, Department of Physics. Uppsala University, Sweden; Jan Kochanowski University, Poland.
    Squibb, Richard J.
    Richter, Robert
    Alagia, Michele
    Stranges, Stefano
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Eland, John H. D.
    Feifel, Raimund
    Zhaunerchyk, Vitali
    Selectivity in fragmentation of N-methylacetamide after resonant K-shell excitation2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 29, p. 15231-15240Article in journal (Refereed)
    Abstract [en]

    The fragmentation pattern of the peptide model system, N-methylacetamide, is investigated using ion time-of-flight (TOF) spectroscopy after resonant K-shell excitation. Corresponding near-edge X-ray absorption fine structure (NEXAFS) spectra recorded at high resolution at the C1s, N1s and O1s edges are presented. Analysis of the ion TOF data reveals a multitude of fragmentation channels and dissociation pathways. Comparison between the excitation of six different resonances in the vicinity of the C1s, N1s and O1s edges suggests evidence for site-selective bond breaking. In particular the breaking of the peptide bond and the N-C-alpha bond show a clear correlation with resonant excitation at the N1s edge. Also, stronger tendencies towards site-selective bond breaking are found for the generation of single ions compared with ion pairs. Analysis of angular distributions of ions from breakage of the peptide bond yields a fragmentation time of <400 fs.

  • 9.
    Viarbitskaya, Sviatlana
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kapshai, Valery
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Size dependence of second-harmonic generation at the surface of microspheres2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 5, p. 53850-Article in journal (Refereed)
    Abstract [en]

    The resonance-enhanced surface second-harmonic generation (SHG) from a suspension of polystyrene microspheres was investigated as a function of particle size in a range of the order of the fundamental wavelength for two different second-harmonic-enhancing dyes-malachite green and pyridine 1. The two dyes gave the same strongly modulated pattern of the forward second-harmonic scattering efficiency. Direct comparison to the nonlinear Rayleigh-Gans-Debye (NLRGD) and nonlinear Wentzel-Kramers-Brillouin (NLWKB) model predictions showed that the NLWKB model reproduces the overall trend in the size dependence but fails with respect to the strong modulations. The standard NLRGD model was found to fail altogether in the present particle size range, which was well beyond the observed upper particle size for which the NLRGD and NLWKB models give comparable results. A generalization of the NLRGD model to allow for dispersion and to use the particle refractive indices instead of those of the surrounding medium extended its applicability range by almost an order of magnitude in particle size. There is a pronounced maximal SHG efficiency for particles with a radius that is close to the fundamental wavelength inside the particle. The optically soft particle approximation is inadequate to describe the SHG in this particle size range, as refraction and reflection of the waves at the particle surface have a decisive influence. Dispersion of the media plays a negligible role for particle sizes up to about twice the optimal one for SHG.

  • 10. Zhaunerchyk, V.
    et al.
    Kamińska, Magdalena
    Stockholm University, Faculty of Science, Department of Physics. Uppsala University, Sweden; Jan Kochanowski University, Poland.
    Mucke, M.
    Squibb, R. J.
    Eland, J. H. D.
    Piancastelli, M. N.
    Frasinski, L. J.
    Grilj, J.
    Koch, M.
    McFarland, B. K.
    Sistrunk, E.
    Gühr, M.
    Coffee, R. N.
    Bostedt, C.
    Bozek, J. D.
    Salén, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Meulen, Peter v. d.
    Stockholm University, Faculty of Science, Department of Physics.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Foucar, L.
    Ullrich, J.
    Motomura, K.
    Mondal, S.
    Ueda, K.
    Richter, R.
    Prince, K. C.
    Takahashi, O.
    Osipov, T.
    Fang, L.
    Murphy, B. F.
    Berrah, N.
    Feifel, R.
    Disentangling formation of multiple-core holes in aminophenol molecules exposed to bright X-FEL radiation2015In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 48, no 24, article id 244003Article in journal (Refereed)
    Abstract [en]

    Competing multi-photon ionization processes, some leading to the formation of double core hole states, have been examined in 4-aminophenol. The experiments used the linac coherent light source (LCLS) x-ray free electron laser, in combination with a time-of-flight magnetic bottle electron spectrometer and the correlation analysis method of covariance mapping. The results imply that 4-aminophenol molecules exposed to the focused x-ray pulses of the LCLS sequentially absorb more than two x-ray photons, resulting in the formation of multiple core holes as well as in the sequential removal of photoelectrons and Auger electrons (so-called PAPA sequences).

  • 11. Zhaunerchyk, V.
    et al.
    Mucke, M.
    Salén, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    van der Meulen, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Kaminska, M.
    Squibb, R. J.
    Frasinski, L. J.
    Siano, M.
    Eland, J. H. D.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Foucar, L.
    Ullrich, J.
    Motomura, K.
    Mondal, S.
    Ueda, K.
    Osipov, T.
    Fang, L.
    Murphy, B. F.
    Berrah, N.
    Bostedt, C.
    Bozek, J. D.
    Schorb, S.
    Messerschmidt, M.
    Glownia, J. M.
    Cryan, J. P.
    Coffee, R. N.
    Takahashi, O.
    Wada, S.
    Piancastelli, M. N.
    Richter, R.
    Prince, K. C.
    Feifel, R.
    Using covariance mapping to investigate the dynamics of multi-photon ionization processes of Ne atoms exposed to X-FEL pulses2013In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 46, no 16, p. 164034-Article in journal (Refereed)
    Abstract [en]

    We report on a detailed investigation into the electron emission processes of Ne atoms exposed to intense femtosecond x-ray pulses, provided by the Linac Coherent Light Source Free Electron Laser (FEL) at Stanford. The covariance mapping technique is applied to analyse the data, and the capability of this approach to disentangle both linear and nonlinear correlation features which may be hidden on coincidence maps of the same data set is demonstrated. Different correction techniques which enable improvements on the quality of the spectral features extracted from the covariance maps are explored. Finally, a method for deriving characteristics of the x-ray FEL pulses based on covariance mapping in combination with model simulations is presented.

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