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  • 1.
    Adiels, L
    et al.
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Antonelli, A
    Backenstoss, G.
    Institute for Physics, University of Basle.
    Beltrami, I
    Bergström, I.
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Bloch, P
    Burgun, G
    DPhPe, CENSaclay.
    Carius, S.
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Chardalas, M
    Charalambous, S.
    Dept. of Nuclear Physics, Univerity of Tessaloniki.
    Dedoussis, S
    Derré, J
    Dousse, J. C.
    University of Fribourg.
    Dris, M
    Faure, J L
    Fetscher, W
    Fry, J R
    Filokyprou, G
    Gabathuler, Erwin
    Gamet, R
    Garreta, D
    Geralis, J
    Gerber, H J
    Guyot, C
    Hatzifotiadou, D.
    Hayman, P
    Hugi, M
    Institute for Physics, University of Basle.
    Kern, J
    Kerek, A.
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Kesseler, G
    Kochowski, Claude
    Kokinias, P
    Kostarakis, P
    Kuzminski, J
    Lefas, C
    Lindblad, T
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Mall, U
    Marel, Gérard
    Mason, P
    Milshtein, A I
    Nakada, Tatsuya
    Nilsson, A
    Stockholm University, Faculty of Science, Department of Physics, The Manne Siegbahn Laboratory.
    Papadopoulos, I M
    Pavlopoulos, P.
    Institute for Physics, University of Basle.
    Pauli, E
    Rickenbach, R
    Schaller, L A
    Schopper, A
    Tauscher, L.
    Institute for Physics, University of Basle.
    Tröster, D.
    Institute for Physics, University of Basle.
    Tzamouranis, Yu
    Watson, E. J.
    CERN.
    Zevgolatakos, S
    Study of CP violation in a tagged neutral kaon beam.1986In: Topical European Meeting on the Quark Structure of Matter in the Rhine Valley, 1986Conference paper (Other academic)
  • 2. Ai, Yue-jie
    et al.
    Tian, Guangjun
    Liao, Rong-zhen
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Zhang, Qiong
    Fang, Wei-hai
    Luo, Yi
    Intrinsic Property of Flavin Mononucleotide Controls its Optical Spectra in Three Redox States2011In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 12, no 16, p. 2899-2902Article in journal (Refereed)
  • 3.
    Ali, Safdar
    Stockholm University, Faculty of Science, Department of Physics.
    Electron - Ion Recombination Data for Plasma Applications: Results from Electron Beam Ion Trap and Ion Storage Ring2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis contains results of electron-ion recombination processes in atomic ions relevant for plasma applications. The measurements were performed at the Stockholm Refrigerated Electron Beam Ion Trap (R-EBIT) and at the CRYRING heavy-ion storage ring. Dielectronic recombination (DR) cross sections, resonant strengths, rate coefficients and energy peak positions in H-like and He-like S are obtained for the first time from the EBIT measurements. Furthermore, the experimentally obtained DR resonant strengths are used to check the behaviour of a scaling formula for low Z, H-and He-like iso-electronic sequences and to update the fitting parameters. KLL DR peak positions for initially He- to B-like Ar ions are obtained experimentally from the EBIT measurements. Both the results from highly charged sulfur and argon are compared with the calculations performed with a distorted wave approximation.

    Absolute recombination rate coefficients of B-like C, B-like Ne and Be-like F ions are obtained for the first time with high energy resolution from storage ring measurements. The experimental results are compared with the intermediate coupling AUTOSTRUCTURE calculations. Plasma rate coefficients of each of these ions are obtained by convoluting the energy dependent recombination spectra with a Maxwell-Boltzmann energy distribution in the temperature range of 103-106 K. The resulting plasma rate coefficients are presented and compared with the calculated data available in literature.

  • 4.
    Ali, Safdar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mahmood, Sultan
    Stockholm University, Faculty of Science, Department of Physics.
    Orban, Istvan
    Stockholm University, Faculty of Science, Department of Physics.
    Tashenov, Stanislav
    Stockholm University, Faculty of Science, Department of Physics.
    Li, Y. M.
    Wu, Z.
    Schuch, Reinhold
    Stockholm University, Faculty of Science, Department of Physics.
    Electron-ion recombination of H- and He-like sulfur2011In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 44, no 22, p. 225203-Article in journal (Refereed)
    Abstract [en]

    Electron-ion recombination of sulfur ions with electrons in the energy range of 1.6-3 keV was studied at the Stockholm Refrigerated Electron Beam Ion Trap. We obtained the KLn dielectronic recombination (DR) cross sections up to n = 5 for H-like and He-like sulfur ions by observing the x-rays from the trapped ions. A fully relativistic distorted wave approximation method was used for calculating the DR cross sections, while the resonance energies were obtained with a multiconfiguration Dirac-Fock approach using the GRASP II code. The calculations agree with the experimental results within the experimental error bars. Additionally, the obtained total DR resonance strengths were used to check the behaviour of a scaling formula for low-Z, He-like iso-electronic sequence (Watanabe et al 2001 J. Phys. B: At. Mol. Opt. Phys. 34 5095).

  • 5.
    Ali, Safdar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mahmood, Sultan
    Stockholm University, Faculty of Science, Department of Physics.
    Orban, Istvan
    Stockholm University, Faculty of Science, Department of Physics.
    Tashenov, Stanislav
    Stockholm University, Faculty of Science, Department of Physics.
    Li, Y. M.
    Wu, Z.
    Schuch, Reinhold
    Stockholm University, Faculty of Science, Department of Physics.
    Photo-recombination studies at R-EBIT with a Labview control and data = quisition system2011In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 6, p. C01016-Article in journal (Refereed)
    Abstract [en]

    Equipment at the Stockholm Refrigerated Electron Beam Ion Trap (R-EBIT) was developed for photo-recombination studies. A LabView-based event mode data acquisition and R-EBIT control system was implemented. The energies of KLL dielectronic recombination resonances in Li- to C-like argon ions were determined and compared with theoretical calculations performed using a distorted wave approximation. The theoretical and experimental peak positions for Li-, Be-, and C-like argon ions agree within the error bars. For B-like argon we observe an energy shift of 9 eV between the experimentally obtained peak position and the calculated result.

  • 6.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Reply to 'Experimental bound entanglement?'2010In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 6, p. 827-827Article in journal (Other (popular science, discussion, etc.))
  • 7.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adán
    Departamento de Física Aplicada II, Universidad de Sevilla.
    Danielsen, Lars Eirik
    Department of Informatics, University of Bergen.
    López-Tarrida, Antonio
    Departamento de Física Aplicada II, Universidad de Sevilla.
    Portillo, José
    Departamento de Matemática Aplicada I, Universidad de Sevilla.
    Experimental fully contextual correlationsArticle in journal (Refereed)
    Abstract [en]

    Quantum correlations are contextual yet, in general, nothing prevents the existence of even morecontextual correlations. We identify and test a simple noncontextual inequality in which the quantumviolation cannot be improved by any hypothetical post-quantum resource, and use it to experimentally obtain correlations in which the maximum noncontextual content, defined as the maximumfraction of noncontextual correlations, is less than 0.06. Our correlations are experimentally generated from the outcomes of sequential compatible measurements on a four-state quantum system encoded in the polarization and path of a single photon.

  • 8.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Sadiq, Muhamad
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental bound entanglement through a Pauli channel2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, article id 1966Article in journal (Refereed)
    Abstract [en]

    Understanding the characteristics of a quantum systems when affected by noise is one of the biggest challenges for quantum technologies. The general Pauli error channel is an important lossless channel for quantum communication. In this work we consider the effects of a Pauli channel on a pure four-qubit state and simulate the Pauli channel experimentally by studying the action on polarization encoded entangled photons. When the noise channel acting on the photons is correlated, a set spanned by four orthogonal bound entangled states can be generated. We study this interesting case experimentally and demonstrate that products of Bell states can be brought into a bound entangled regime. We find states in the set of bound entangled states which experimentally violate the CHSH inequality while still possessing a positive partial transpose.

  • 9. Andersson, E
    et al.
    Stenrup, M
    Eland, J H D
    Hedin, L
    Berglund, M
    Karlsson, L
    Larson, Åsa
    Stockholm University, Faculty of Science, Department of Physics.
    Ågren, H
    Rubensson, J. -E.
    Feifel, R
    Single-photon core-valence double ionization of molecular oxygen2008In: Phys. Rev. A, Vol. 78, p. 023409-Article in journal (Refereed)
    Abstract [en]

    Single-photon core-valence double ionization of molecular oxygen has been studied using a magnetic bottle time-of-flight electron coincidence spectrometer. The K−1V−1 double ionization electron spectrum of O2 is reported and is assigned with the aid of ab initio calculations. A direct comparison of the core-valence double ionization electron spectra with the conventional valence band photoelectron spectrum is made. The lowest core-valence double ionization energy is found to be 571.6 eV and is associated with a 3 dicationic state.

  • 10. Andresen, G. B.
    et al.
    Ashkezari, M. D.
    Baquero-Ruiz, M.
    Bertsche, W.
    Bowe, P. D.
    Butler, E.
    Cesar, C. L.
    Chapman, S.
    Charlton, M.
    Deller, A.
    Eriksson, S.
    Fajans, J.
    Friesen, T.
    Fujiwara, M. C.
    Gill, D. R.
    Gutierrez, A.
    Hangst, J. S.
    Hardy, W. N.
    Hayden, M. E.
    Humphries, A. J.
    Hydomako, R.
    Jenkins, M. J.
    Jonsell, Svante
    Stockholm University, Faculty of Science, Department of Physics.
    Jorgensen, L. V.
    Kurchaninov, L.
    Madsen, N.
    Menary, S.
    Nolan, P.
    Olchanski, K.
    Olin, A.
    Povilus, A.
    Pusa, P.
    Robicheaux, F.
    Sarid, E.
    Nasr, S. Seif el
    Silveira, D. M.
    So, C.
    Storey, J. W.
    Thompson, R. I.
    van der Werf, D. P.
    Wurtele, J. S.
    Yamazaki, Y.
    Trapped antihydrogen2010In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 468, no 7324, p. 673-676Article in journal (Refereed)
    Abstract [en]

    Antimatter was first predicted1 in 1931, by Dirac. Work with high-energy antiparticles is now commonplace, and anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen, the bound state of an antiproton and a positron, has been produced2, 3 at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature’s fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom (about two parts in 1014 for the frequency of the 1s-to-2s transition4), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter5. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 107 antiprotons and 7 × 108 positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4 ± 1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen.

  • 11. Andresen, G. B.
    et al.
    Ashkezari, M. D.
    Baquero-Ruiz, M.
    Bertsche, W.
    Bowe, P. D.
    Butler, E.
    Cesar, C. L.
    Chapman, S.
    Charlton, M.
    Fajans, J.
    Friesen, T.
    Fujiwara, M. C.
    Gill, D. R.
    Hangst, J. S.
    Hardy, W. N.
    Hayano, R. S.
    Hayden, M. E.
    Humphries, A.
    Hydomako, R.
    Jonsell, Svante
    Stockholm University, Faculty of Science, Department of Physics.
    Kurchaninov, L.
    Lambo, R.
    Madsen, N.
    Menary, S.
    Nolan, P.
    Olchanski, K.
    Olin, A.
    Povilus, A.
    Pusa, P.
    Robicheaux, F.
    Sarid, E.
    Silveira, D. M.
    So, C.
    Storey, J. W.
    Thompson, R. I.
    van der Werf, D. P.
    Wilding, D.
    Wurtele, J. S.
    Yamazaki, Y.
    Evaporative Cooling of Antiprotons to Cryogenic Temperatures2010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 1, p. 013003-Article in journal (Refereed)
    Abstract [en]

    We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature as low as 9 K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise CPT test on trapped antihydrogen is a long-standing goal.

  • 12. Araujo, Mateus
    et al.
    Quintino, Marco Tulio
    Cavalcanti, Daniel
    Santos, Marcelo Franca
    Cabello, Adan
    Stockholm University, Faculty of Science, Department of Physics.
    Cunha, Marcelo Terra
    Tests of bell inequality with arbitrarily low photodetection efficiency and homodyne measurements2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 86, no 3, p. 030101-Article in journal (Refereed)
    Abstract [en]

    We show that hybrid local measurements combining homodyne measurements and photodetection provide violations of a Bell inequality with arbitrarily low photodetection efficiency. This is shown in two different scenarios: when one part receives an atom entangled to the field mode to be measured by the other part and when both parts make similar photonic measurements. Our findings promote the hybrid measurement scenario as a candidate for loophole-free Bell tests beyond previous expectations.

  • 13.
    Argenti, Luca
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Ionization branching ratio control with a resonance attosecond clock2010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, p. 053002-Article in journal (Refereed)
    Abstract [en]

    We investigate the possibility to monitor the dynamics of autoionizing states in real-time and control the yields of different ionization channels in helium by simulating extreme ultraviolet (XUV) pump IR-probe experiments focused on the N=2 threshold. The XUV pulse creates a coherent superposition of doubly excited states which is found to decay by ejecting electrons in bursts. Prominent interference fringes in the photoelectron angular distribution of the 2s and 2p ionization channels are observed, along with significant out-of-phase quantum beats in the yields of the corresponding parent ions.

  • 14. Argenti, Luca
    et al.
    Pazourek, Renate
    Feist, Johannes
    Nagele, Stefan
    Liertzer, Matthias
    Persson, Emil
    Burgdoerfer, Joachim
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Photoionization of helium by attosecond pulses: Extraction of spectra from correlated wave functions2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 87, no 5, p. 053405-Article in journal (Refereed)
    Abstract [en]

    We investigate the photoionization spectrum of helium by attosecond XUV pulses both in the spectral region of doubly excited resonances as well as above the double ionization threshold. In order to probe for convergence, we compare three techniques to extract photoelectron spectra from the wave packet resulting from the integration of the time-dependent Schrodinger equation in a finite-element discrete variable representation basis. These techniques are projection on products of hydrogenic bound and continuum states, projection onto multichannel scattering states computed in a B-spline close-coupling basis, and a technique based on exterior complex scaling implemented in the same basis used for the time propagation. These methods allow one to monitor the population of continuum states in wave packets created with ultrashort pulses in different regimes. Applications include photo cross sections and anisotropy parameters in the spectral region of doubly excited resonances, time-resolved photoexcitation of autoionizing resonances in an attosecond pump-probe setting, and the energy and angular distribution of correlated wave packets for two-photon double ionization.

  • 15.
    Baggesen, J.C.
    et al.
    Department of Physics and Astronomy, Aarhus University.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Madsen, L.
    Department of Physics and Astronomy, Aarhus University.
    Theory of attosecond absorption spectroscopy in krypton2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, p. 013415-Article in journal (Refereed)
    Abstract [en]

    A theory for time-domain attosecond pump–attosecond probe photoabsorption spectroscopy is formulated and related to the atomic response. The theory is illustrated through a study of attosecond absorption spectroscopy in krypton. The atomic parameters entering the formulation such as energies and Auger widths, as well as wave functions and dipole coupling matrix elements, are determined by accurate many-body structure calculations. We create a hole in a valence shell by an attosecond pump, couple an inner-shell electron to the hole by an attosecond probe, and then monitor the formation of the hole in this manner. In a second example, a hole is created in an inner shell by the first pulse, and the second probe pulse couples an even more tightly bound state to that hole. The hole decays in this example by Auger electron emission, and the absorption spectroscopy follows the decay of the hole and the associated coherences in real time.

  • 16.
    Baym, Gordon
    et al.
    Univ Illinois, Dept Phys, Urbana, IL , USA.
    Pethick, Christopher
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Univ Copenhagen, Niels Bohr Int Acad, Niels Bohr Inst, Copenhagen , Denmark.
    Normal mass density of a superfluid Fermi gas at unitarity2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, no 4, p. 043631-Article in journal (Refereed)
    Abstract [en]

    We calculate the normal mass density of a paired Fermi gas at unitarity. The dominant contribution near the superfluid transition is from fermionic quasiparticle excitations and is, thus, sensitive to the pairing gap. A comparison with the recent experiment of Sidorenkov et al. [Nature (London) 498, 78 (2013)] suggests that the superfluid gap near the transition temperature is larger than the BCS value, but the data do not permit a quantitative inference of the gap. Calculations of the quenched moment of inertia of a BCS superfluid in a harmonic trap are in reasonable agreement with the earlier experiment of Riedl et al.

  • 17. Behringer, Hans
    et al.
    Eichhorn, Ralf
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Brownian dynamics simulations with hard-body interactions: Spherical particles2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 16, p. 164108-Article in journal (Refereed)
    Abstract [en]

    A novel approach to account for hard-body interactions in (overdamped) Brownian dynamics simulations is proposed for systems with non-vanishing force fields. The scheme exploits the analytically known transition probability for a Brownian particle on a one-dimensional half-line. The motion of a Brownian particle is decomposed into a component that is affected by hard-body interactions and into components that are unaffected. The hard-body interactions are incorporated by replacing the affected component of motion by the evolution on a half-line. It is discussed under which circumstances this approach is justified. In particular, the algorithm is developed and formulated for systems with space-fixed obstacles and for systems comprising spherical particles. The validity and justification of the algorithm is investigated numerically by looking at exemplary model systems of soft matter, namely at colloids in flow fields and at protein interactions. Furthermore, a thorough discussion of properties of other heuristic algorithms is carried out.

  • 18.
    Bengtsson, Jakob
    Stockholm University, Faculty of Science, Department of Physics.
    On the use of the uniform complex scaling-method for studying time-dependent systems2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    During the last few decades, laser technology has gone through a series of revolutionary improvements. With light pulses in the sub-femtosecond regime now experimentally available, an explicit time-dependent Hamilton operator is indispensable for a proper theoretical description of the interactions of atoms with such pulses. However, these theoretical studies are computationally very demanding, which motivates the search for new numerical methods and algorithms to approach time-dependent problems. This thesis contributes to this research field, with the main focus on the use of complex-scaled Hamilton operators. Thus, the formalism of complex scaling is studied in the context of its application to explicitly time-dependent atomic systems.

    Both non-relativistic and relativistic dynamics are investigated. The numerical advantages and the possibilities to extract physical quantities from complex-scaled wave functions are discussed. Of special interest is the ability to treat resonance states. These are multiply excited electronic states with sufficient energy to decay through Auger transitions to the surrounding continuum. With complex scaling, the Hamilton operator is non-Hermitian and such resonance states can be obtained as eigenstates.

    To analyze the non-bound part of the wave function requires essentially the construction of a second wave function; the left state vector. This additional wave function is, however, not easily constructed numerically in the complex scaling-method. To circumvent some of the numerical problems, we have proposed three different methods. These methods are based on Floquet theory, a propagation on a complex time-grid and time-dependent perturbation theory. By carefully investigating the numerical properties of the left state vector, we have thus studied the non-bound part of the system.

  • 19.
    Bengtsson, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Modifying H− resonance asymmetries with short light pulses2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 5, p. 053413-Article in journal (Refereed)
    Abstract [en]

    We present a method, based on time-dependent perturbation theory and complex rotation, to treat the interaction of a short light pulse with a correlated atomic system. The pulse is built from two short and weak pulses with Gaussian envelopes that are centred at two different frequencies.The method is applied to the negative hydrogen ion in the vicinity of a doubly excited resonanceand it is shown that the two light pulses can be used to alter the Fano profile of a resonance.

  • 20.
    Bengtsson, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Selstø, Sølve
    Stockholm University, Faculty of Science, Department of Physics.
    Solution of the time-dependent Schrödinger equation using uniform complex scaling2008In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 78, no 3, p. 032502-Article in journal (Refereed)
    Abstract [en]

    The formalism of complex rotation of the radial coordinate is studied in the context of time-dependent systems. The applicability of this method is discussed and illustrated with numerical examples involving atoms exposed to electromagnetic field pulses. Complex rotation proves to be an efficient tool to obtain ionization probabilities and rates. Although, in principle, any information about the system may be obtained from the rotated wave function by transforming it back to its unrotated form, a good description of the ionized part of the wave function is generally subject to numerical challenges. It is, however, found that the combination of complex rotation and Floquet formalism offers an alternative and promising possibility to retrieve the physical information.

  • 21.
    Bengtsson, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Selstø, Sølve
    Wave functions associated with time-dependent, complex-scaled Hamiltoniansevaluated on a complex time grid2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 1, p. 013419-Article in journal (Refereed)
    Abstract [en]

    We solve the time-dependent Schr ̈ dinger equation with the method of uniform complex scaling and investigateothe possibility to evaluate the solution on a complex time grid. With this approach it is possible to calculateproperties that relate directly to the continuum part of the complex scaled wave function, such as the photoelectronspectrum after photoabsorption.

  • 22. Berggren, A. -M
    et al.
    Aldahan, A.
    Possnert, G.
    Hansson, Margareta
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Steen-Larsen, H. C.
    Storm, A. Sturevik
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Murad, A.
    Variability of Be-10 and delta O-18 in snow pits from Greenland and a surface traverse from Antarctica2013In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 294, p. 568-572Article in journal (Refereed)
    Abstract [en]

    To examine temporal variability of Be-10 in glacial ice, we sampled snow to a depth of 160 cm at the NEEM (North Greenland Eemian Ice Drilling) drilling site in Greenland. The samples span three years between the summers of 2006 and 2009. At the same time, spatial variability of Be-10 in glacial ice was explored through collection of the upper similar to 5 cm of surface snow in Antarctica during part of the Swedish-Japanese traverse from Svea to Syowa station during the austral summer in 2007-2008. The results of the Greenlandic 1 Be snow suggested variable concentrations that apparently do not clearly reflect the seasonal change as indicated by the delta O-18 data. The Be-10 concentration variability most likely reflects also effects of aerosol loading and deposition pathways, possibly in combination with post-depositional processes. The Antarctic traverse data expose a negative correlation between Be-10 and delta O-18, while there are weaker but still significant correlations to altitude and distance to the coast (approximated by the distance to the 70th latitude). These relationships indicate that geographical factors, mainly the proximity to the coast, may strongly affect 1 Be concentrations in snow in Queen Maud Land, Antarctica.

  • 23.
    Bergqvist, Mikael
    Stockholm University, Faculty of Science.
    Light transport and positioning simulations for the design of a cylindrical SPECT camera1998Doctoral thesis, comprehensive summary (Other academic)
  • 24. Björck, M.
    et al.
    Soroka, I. L.
    Stockholm University, Faculty of Science, Department of Physics. Uppsala University, Sweden.
    Chacon-Carillo, C.
    Andersson, G.
    The asymmetric interface structure of bcc Fe82Ni18/Co superlattices as revealed by neutron diffraction2007In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 515, no 08-jul, p. 3619-3623Article in journal (Refereed)
    Abstract [en]

    The interface structure of Fe82Ni18/Co (001) Superlattices has been studied with a combination of X-ray and neutron diffraction. The analysis 'reveals highly asymmetric interfaces with total interface widths of 10 +/- 1 ML(monolayers) for Fe82Ni18 on Co and a maximum interface width of 1 ML for Co on Fe82Ni18. In addition it is concluded that there is no detectable long range B2-type chemical order occurring in the interface region. These results are also discussed in the context of previously measured magnetic mornents of the same system.

  • 25.
    Blomberg, Mattias
    Stockholm University, Faculty of Science, Department of Physics.
    Redox Reactions of NO and O2 in Iron Enzymes: A Density Functional Theory Study2006Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the present thesis the density functional B3LYP has been used to study reactions of NO and O2 in redox active enzymes.

    Reduction of nitric oxide (NO) to nitrous oxide (N2O) is an important part in the bacterial energy conservation (denitrification). The reduction of NO in three different bimetallic active sites leads to the formation of hyponitrous acid anhydride (N2O22-). The stability of this intermediate is crucial for the reaction rate. In the two diiron systems, respiratory and scavenging types of NOR, it is possible to cleave the N-O bond, forming N2O, without any extra protons or electrons. In a heme-copper oxidase, on the other hand, both a proton and an electron are needed to form N2O.

    In addition to being an intermediate in the denitrification, NO is a toxic agent. Myoglobin in the oxy-form reacts with NO forming nitrate (NO3 -) at a high rate, which should make this enzyme an efficient NO scavenger. Peroxynitrite (ONOO-) is formed as a short-lived intermediate and isomerizes to nitrate through a radical reaction.

    In the mechanism for pumping protons in cytochrome oxidase, thermodynamics, rather than structural changes, might guide protons to the heme propionate for further translocation.

    The dioxygenation of arachidonic acid in prostaglandin endoperoxide H synthase forms the bicyclic prostaglandin G2, through a cascade of radical reactions. The mechanism proposed by Hamberg and Samuelsson is energetically feasible.

  • 26. Borglund, Niclas
    Methods for improvement of data acquisition and analysis in electron energy loss spectroscopy2002Doctoral thesis, comprehensive summary (Other academic)
  • 27.
    Brandefelt, Nicklas
    Stockholm University, Faculty of Science, Department of Physics.
    Resonant States in Negative Ions2001Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Resonant states are multiply excited states in atoms and ions that have enough energy to decay by emitting an electron. The ability to emit an electron and the strong electron correlation (which is extra strong in negative ions) makes these states both interesting and challenging from a theoretical point of view. The main contribution in this thesis is a method, which combines the use of B splines and complex rotation, to solve the three-electron Schrödinger equation treating all three electrons equally. It is used to calculate doubly excited and triply excited states of 4S symmetry with even parity in He-. For the doubly excited states there are experimental and theoretical data to compare with. For the triply excited states there is only theoretical data available and only for one of the resonances. The agreement is in general good. For the triply excited state there is a significant and interesting difference in the width between our calculation and another method. A cause for this deviation is suggested. The method is also used to find a resonant state of 4S symmetry with odd parity in H2-. This state, in this extremely negative system, has been predicted by two earlier calculations but is highly controversial.

    Several other studies presented here focus on two-electron systems. In one, the effect of the splitting of the degenerate H(n=2) thresholds in H-, on the resonant states converging to this threshold, is studied. If a completely degenerate threshold is assumed an infinite series of states is expected to converge to the threshold. Here states of 1P symmetry and odd parity are examined, and it is found that the relativistic and radiative splitting of the threshold causes the series to end after only three resonant states. Since the independent particle model completely fails for doubly excited states, several schemes of alternative quantum numbers have been suggested. We investigate the so called DESB (Doubly Excited Symmetry Basis) quantum numbers in several calculations. For the doubly excited states of He- mentioned above we investigate one resonance and find that it cannot be assigned DESB quantum numbers unambiguously. We also investigate these quantum numbers for states of 1S even parity in He. We find two types of mixing of DESB states in the doubly excited states calculated. We also show that the amount of mixing of DESB quantum numbers can be inferred from the value of the cosine of the inter-electronic angle. In a study on Li- the calculated cosine values are used to identify doubly excited states measured in a photodetachment experiment. In particular a resonant state that violates a propensity rule is found.

  • 28.
    Brinne Roos, Johanna
    Stockholm University, Faculty of Science, Department of Physics.
    Reaction dynamics on highly excited states2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis I have performed theoretical studies on the reaction dynamics in few-atom molecules. In particular, I have looked at reaction processes in which highly excited resonant states are involved. When highly excited states are formed, the dynamics becomes complicated and approximations normally used in chemical reaction studies are no longer applicable.To calculate the potential energy curve for some of these states as a function of internuclear distance, a combination of structure calculations and scattering calculations have to be performed, and the reaction dynamics on the potentials has been studied using both time-independent and time-dependent methods.The processes that have been studied and which are discussed in this thesis are ion-pair formation in electron recombination with H3+, dissociative recombination and ion-pair formation of HF+, mutual neutralization in H++F- collisions and dissociative recombination of BeH+. Isotope effects in these reactions have also been investigated. Our calculated cross sections are compared with experimentally measured cross sections for these reactions.

  • 29.
    Brinne Roos, Johanna
    et al.
    Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Stockholm.
    Larson, Åsa
    Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Stockholm.
    Orel, Ann
    Department of Applied Science, University of California, Davis.
    Electron collisions with H3+: Ion-pair formation2007In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, no 4, p. 042703-1-042703-9Article in journal (Refereed)
    Abstract [en]

    The formation of the ion pair H2++H in electron recombination with H3+ is studied. The diabatic potentials and electronic couplings are extracted from ab initio electron scattering calculations as well as quantum chemistry calculations. In order to describe this reaction we include six coupled electronic states and propagate wave packets in two dimensions using the multiconfiguration time-dependent Hartree method. Also, the cross section for ion-pair formation in electron recombination with D3+ is calculated. The cross section for this isotopomer is found to be about a factor of 3 smaller than the cross section for H3+.

  • 30.
    Brinne Roos, Johanna
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Larson, Åsa
    Stockholm University, Faculty of Science, Department of Physics.
    Orel, Ann
    Department of Applied Science, University of California, Davis.
    Resonant ion-pair formation in electron recombination with HF+Manuscript (Other (popular science, discussion, etc.))
    Abstract [en]

    The cross section for resonant ion-pair formation in the collision of low-energy electrons with HF+ is calculated by the solution of the time-dependent Schrödinger equation with multiple coupledstates using a wave packet method. A diabatization procedure is proposed to obtain the electroniccouplings between quasidiabatic potentials of 1Σ+ symmetry for HF. By including these couplingsbetween the neutral states, the cross section for ion-pair formation increases with about two ordersof magnitude compared with the cross section for direct dissociation. Qualitative agreement with themeasured cross section is obtained. The oscillations in the calculated cross section are analyzed. Thecross section for ion-pair formation in electron recombination with DF+ is calculated to determinethe effect of isotopic substitution.

  • 31.
    Brinne Roos, Johanna
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Larson, Åsa
    Stockholm University, Faculty of Science, Department of Physics.
    Orel, Ann E.
    Dept of Applied Science US Davis.
    Dissociative recombination of HF+2008In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 78, no 2, p. 022508-1-022508-12Article in journal (Refereed)
    Abstract [en]

    The direct mechanism of dissociative recombination of HF+ is studied usingboth time-dependent and time-independent methods, where the dynamics on 30resonant states is explored. The relevant electronic states are calculatedab initio by combining electron scattering calculations with multireference configurationinteraction structure calculations. For collision energies between 0.04 and 10  eV,we obtain qualitative agreement with experiment. At 1.9  eV there isa sharp threshold in both the experimental and theoretical crosssections that can be explained by the opening of newasymptotic limits. The measured cross section below 0.04  eV is notreproduced due to the neglect of the electronic couplings betweenthe neutral states. We examine the validity of the localapproximation for treating autoionization from the resonant states included inthis study.

  • 32.
    Brinne Roos, Johanna
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Mats
    Stockholm University, Faculty of Science, Department of Physics.
    Larson, Åsa
    Stockholm University, Faculty of Science, Department of Physics.
    Orel, Ann
    Deptartment of Applied Science, University of California, Davis.
    Dissociative recombination of BeH+Manuscript (Other academic)
    Abstract [en]

    The cross section for dissociative recombination of BeH+ is calculated by solution of the timedependent Schrödinger equation in the local complex potential approximation. The effects of couplings between resonant states and the Rydberg states converging to the ground state of the ionare studied. The relevant potentials, couplings and autoionization widths are extracted using abinitio electron scattering and structure calculations, followed by a diabatization procedure. Thecalculated cross sections shows a sizable magnitude at low energy, followed by a high-energy peakcentered around 1 eV. The electronic couplings between the neutral states induce oscillations in thecross section. Analytical forms for the cross sections at low collision energies are given.

  • 33.
    Broström, Lars
    Stockholm University, Faculty of Science, Department of Physics.
    Atomic and molecular properties probed by selective excitation of fast ion beams1995Doctoral thesis, comprehensive summary (Other academic)
  • 34.
    Bäckström, Erik
    Stockholm University, Faculty of Science, Department of Physics.
    LIfetimes measurement states in ions at an ion storage ring2011Licentiate thesis, comprehensive summary (Other academic)
  • 35.
    Bäckström, Erik
    Stockholm University, Faculty of Science, Department of Physics.
    Time-Resolved Spectroscopy and Intensity Measurements of Singly Charged Ions2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is mainly concerned with investigations of spontaneous radiatively decaying states in both negative and positive singly charged ions. When possible, the measured lifetime of the state has been combined with branching fractions in order to derive the absolute transition probability (A-value) between different quantum states. The radiative transition probability between two quantum states is a fundamental atomic property. Knowledge of this property can be used as a diagnostic tool in, for example, abundance and temperature determinations with applications in many fields, e.g. astronomy, plasma physics, atomic physics etc.

    The focus of the experiments has been on lifetime measurements of long-lived metastable states. Lifetimes of long-lived metastable states are interesting in both theoretical aspects as well as the challenge it poses to the experimentalist. To perform such experiments, impact from the surrounding environment on the stored ions has to be kept to a minimum for extended periods of time. The metastable lifetimes presented here have been measured with time-resolved laser spectroscopic techniques in two different types of ion storage rings. One of them is a new type of unique cryogenically cooled storage ring made of purely electrostatic ion optical elements. As is demonstrated in this thesis, this device opens up a completely new time domain where lifetime measurements now can be performed. In addition, this thesis includes a discussion and preliminary studies of weak interactions otherwise limited by magnetic fields and/or thermal radiation from the environment. When available, the results have been compared to previous measurements and theoretical calculations which enables an evaluation of different methods and theoretical models.

  • 36.
    Bäckström, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Gurell, Jonas
    Stockholm University, Faculty of Science, Department of Physics.
    Royen, Peder
    Stockholm University, Faculty of Science, Department of Physics.
    Mannervik, Sven
    Stockholm University, Faculty of Science, Department of Physics.
    Norlin, L.
    Blackwell-Whitehead, R.
    Hartman, H.
    Nilsson, H.
    The FERRUM project: metastable lifetimes in Cr II2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 420, no 2, p. 1636-1639Article in journal (Refereed)
    Abstract [en]

    Parity forbidden radiative transitions from metastable levels are observed in spectra of low-density astrophysical plasmas. These lines are used as probes of the physical conditions, made possible due to the long lifetime of their upper level. In a joint effort, the FERRUM project aims to obtain new and accurate atomic data for the iron-group elements, and part of this project concerns forbidden lines. The radiative lifetimes of the metastable energy levels 3 d4(a 3 D)4 s c4 D 5/2 and 3 d4(a 3 D)4 s c4 D 7/2 of singly ionized chromium have been measured. The experiment has been performed at the ion storage ring CRYRING. We employed a laser-probing technique developed for measuring long lifetimes. In this article, we present the lifetimes of these levels to be t5/2= 1.28(16) s and t7/2= 1.37(7) s, respectively. A comparison with previous theoretical work shows good agreement and the result is discussed in a theoretical context.

  • 37.
    Bäckström, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Hanstorp, D.
    Hole, Odd Magnar
    Stockholm University, Faculty of Science, Department of Physics.
    Kaminska, Magdalena
    Stockholm University, Faculty of Science, Department of Physics. Jan Kochanowski University, Poland.
    Nascimento, Rodrigo F.
    Stockholm University, Faculty of Science, Department of Physics.
    Blom, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Björkhage, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Källberg, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Löfgren, Patrik
    Stockholm University, Faculty of Science, Department of Physics.
    Reinhed, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Rosén, Stefan
    Stockholm University, Faculty of Science, Department of Physics.
    Simonsson, Ansgar
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Mannervik, Sven
    Stockholm University, Faculty of Science, Department of Physics.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Cederquist, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Storing keV negative ions for an hour: The lifetime of the metastable 2P1/2 level in 32S−2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 14, article id 143003Article in journal (Refereed)
    Abstract [en]

    We use a novel electrostatic ion storage ring to measure the radiative lifetime of the upper level in the 3p 5  P 2  o 1/2 →3p 5  P 2  o 3/2   spontaneous radiative decay in S −  32   to be 503±54  sec . This is by orders of magnitude the longest lifetime ever measured in a negatively charged ion. Cryogenic cooling of the storage ring gives a residual-gas pressure of a few times 10 −14   mbar at 13 K and storage of 10 keV sulfur anions for more than an hour. Our experimental results differ by 1.3σ  from the only available theoretical prediction.

  • 38.
    Bäckström, Erik
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, H.
    Engström, L.
    Hartman, H.
    Mannervik, Sven
    Stockholm University, Faculty of Science, Department of Physics.
    Experimentally determined oscillator strengths in Rh II2013In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 46, no 20, p. 205001-Article in journal (Refereed)
    Abstract [en]

    This paper presents new experimentally determined branching fractions and oscillator strengths (log gf) for lines originating from 17 levels belonging to 5 terms of the first excited odd configuration 4d(7)(D-4) 5p in Rh II. The intensity calibrated spectra of Rh II have been recorded with a Fourier transform spectrometer between 25000 and 45000 cm(-1) (2200-4000 angstrom). In this region, 49 lines have been identified and measured. By combining the branching fractions obtained from the spectra with previously measured lifetimes, log gf values are reported. The new results are compared with previous theoretical work.

  • 39. Calabrese, C.
    et al.
    Vigorito, A.
    Feng, G.
    Favero, L. B.
    Maris, A.
    Melandri, S.
    Geppert, Wolf D.
    Stockholm University, Faculty of Science, Department of Physics.
    Caminati, W.
    Laboratory rotational spectrum of acrylic acid and its isotopologues in the 6-18.5 GHz and 52-74.4 GHz frequency ranges2014In: Journal of Molecular Spectroscopy, ISSN 0022-2852, E-ISSN 1096-083X, Vol. 295, p. 37-43Article in journal (Refereed)
    Abstract [en]

    In order to facilitate the detection of acrylic acid in space, for which a possible mechanism of formation is proposed, we extended the measurements of the rotational spectrum of this molecule to the 6-18.5 GHz (time domain Fourier transform) and 52-74.4 GHz (frequency domain) ranges in supersonic expansions. 77 new lines were assigned to the s-cis conformer and 83 new lines to the s-trans conformer. In addition, the rotational spectra of the three single C-13 isotopologues have been measured in natural abundance for both conformers. High resolution measurements of the carboxylic deuterated isotopologues allowed for the determination of the deuterium nuclear quadrupole coupling constants. All the spectroscopic experimental parameters were compared to the ones obtained with quantum chemical methods at the MP2(fc)/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels of calculation.

  • 40.
    Carette, Thomas
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Dahlström, J. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Argenti, L.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Multiconfigurational Hartree-Fock close-coupling ansatz: Application to the argon photoionization cross section and delays2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 87, no 2, p. 023420-Article in journal (Refereed)
    Abstract [en]

    We present a robust, ab initio method for addressing atom-light interactions and apply it to photoionization of argon. We use a close-coupling ansatz constructed on a multiconfigurational Hartree-Fock description of localized states and B-spline expansions of the electron radial wave functions. In this implementation, the general many-electron problem can be tackled thanks to the use of the ATSP2K libraries [C. Froese Fischer et al., Comput. Phys. Commun. 176, 559 (2007)]. In the present contribution, we combine this method with exterior complex scaling, thereby allowing for the computation of the complex partial amplitudes that encode the whole dynamics of the photoionization process. The method is validated on the 3s3p(6)np series of resonances converging to the 3s extraction. Then, it is used for computing the energy dependent differential atomic delay between 3p and 3s photoemission, and agreement is found with the measurements of Guenot et al. [Phys. Rev. A 85, 053424 (2012)]. The effect of the presence of resonances in the one-photon spectrum on photoionization delay measurements is studied. DOI: 10.1103/PhysRevA.87.023420

  • 41.
    Carette, Thomas
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University Libre Brussels .
    Godefroid, M. R.
    Isotope shift on the chlorine electron affinity revisited by an MCHF/CI approach2013In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 46, no 9, p. 095003-Article in journal (Refereed)
    Abstract [en]

    Today, the electron affinity is experimentally well known for most of the elements and is a useful guideline for developing ab initio computational methods. However, the measurements of isotope shifts on the electron affinity are limited by both resolution and sensitivity. In this context, theory is of great help to further our knowledge and understanding of atomic structures, even though correlation plays a dominant role in negative ions' properties and, particularly, in the calculation of the specific mass shift contribution. This study solves the longstanding discrepancy between calculated and measured specific mass shifts on the electron affinity of chlorine.

  • 42.
    Carette, Thomas
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nemouchi, Messaoud
    Li, Jiguang
    Godefroid, Michel
    Relativistic effects on the hyperfine structures of 2 p(4)(P-3)3 p D-2(o), D-4(o), and P-4(o) in F-19 I2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, no 4, p. 042501-Article in journal (Refereed)
    Abstract [en]

    The hyperfine interaction constants of the 2p(4)(P-3)3p D-2(3/2,5/2)o, D-4(1/2-7/2)o, and P-4(1/2-5/2)o levels in neutral fluorine are investigated theoretically. Large-scale calculations are carried out using the multiconfiguration Hartree-Fock (MCHF) and Dirac-Hartree-Fock (MCDHF) methods. In the framework of the MCHF approach, the relativistic effects are taken into account in the Breit-Pauli approximation using nonrelativistic orbitals. In the fully relativistic approach, the orbitals are optimized using the Dirac-Coulomb Hamiltonian with correlation models inspired by the nonrelativistic calculations. Higher-order excitations are captured through multireference configuration interaction calculations including the Breit interaction. In a third (intermediate) approach, the Dirac-Coulomb-Breit Hamiltonian matrix is diagonalized in a relativistic configuration space built with nonrelativistic MCHF radial functions converted into Dirac spinors using the Pauli approximation. The magnetic dipole hyperfine-structure constants calculated with the three relativistic models are consistent and reveal unexpectedly large effects of relativity for 2D(5/2)(o), P-4(3/2)o, and P-4(5/2)o. The agreement with the few available experimental values is satisfactory. The strong J dependence of relativistic corrections on the hyperfine constants is investigated through the detailed analysis of the orbital, spin-dipole, and contact relative contributions calculated with the nonrelativistic magnetic dipole operator.

  • 43.
    Chang, Liu
    Stockholm University, Faculty of Science, Department of Physics.
    Ghost in the shell: Studies on subsurface oxygen in oxide-derived copper nanocube catalysts2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    With the passage of time and the advancement of our industrial civilization, environmental concerns have become more and more recognized since the 1990s. Carbon dioxide reduction reactions are capable of converting carbon dioxide into valuable hydrocarbons and reducing the carbon emission from the combustion of fossil fuels. This is a promising direction for sustainable energy resources given that the scarcity of fossil fuels is becoming more threatening to the survival of mankind. In recent years, oxide-derived metal nanostructures have been synthesized and show unique catalytic features. Recently, Sloan et al. synthesized a novel oxide-derived copper nanocube structure, which showed a high selectivity toward ethylene over methane and low overpotentials. In this work, the presence of subsurface oxygen in the catalyst surface is tested with density functional theory (DFT) calculations, as a complement to experimental x-ray photoelectron spectroscopy. Due to limitations on the scale of modeling with DFT, the results indicate a very low stability of subsurface oxygen, which give rise to a question if subsurface oxygen would be stable with a reasonably large cluster model. Self-consistent charge density functional tight binding (SCC-DFTB) is adopted to investigate a nanocube model. In this model, a manually reduced cuprious oxide nanocube is constructed and investigated. Subsurface oxygen atoms close to facets are found to be more stable inside. A higher degree of disorder is proposed to be the cause of this difference in stabilizing subsurface oxygen atoms between the slab and nanocube models. The presence of subsurface oxygen enhances the adsorption of CO on the Cu(100) surface, increasing the likelihood for adsorbed CO molecules to dimerize, which is the rate determining step for ethylene production on Cu(100) under low-overpotential conditions. With subsurface electronegative atoms such as oxygen or fluorine, it is also found that the d-band scaling relation could be broken.

  • 44. Chen, Chen
    et al.
    Huang, Congcong
    Waluyo, Iradwikanari
    Nordlund, Dennis
    Weng, Tsu-Chien
    Sokaras, Dimosthenis
    Weiss, Thomas
    Bergmann, Uwe
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Solvation structures of protons and hydroxide ions in water2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 15, p. 154506-Article in journal (Refereed)
    Abstract [en]

    X-ray Raman spectroscopy (XRS) combined with small-angle x-ray scattering (SAXS) were used to study aqueous solutions of HCl and NaOH. Hydrated structures of H+ and OH- are not simple mirror images of each other. While both ions have been shown to strengthen local hydrogen bonds in the hydration shell as indicated by XRS, SAXS suggests that H+ and OH- have qualitatively different long-range effects. The SAXS structure factor of HCl (aq) closely resembles that of pure water, while NaOH (aq) behaves similar to NaF (aq). We propose that protons only locally enhance hydrogen bonds while hydroxide ions induce tetrahedrality in the overall hydrogen bond network of water.

  • 45. Chen, Jing-Ling
    et al.
    Su, Hong-Yi
    Wu, Chunfeng
    Deng, Dong-Ling
    Cabello, Adan
    Stockholm University, Faculty of Science, Department of Physics.
    Kwek, L. C.
    Oh, C. H.
    Quantum contextuality for a relativistic spin-1/2 particle2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 87, no 2, p. 022109-Article in journal (Refereed)
    Abstract [en]

    The quantum predictions for a single nonrelativistic spin-1/2 particle can be reproduced by noncontextual hidden variables. Here we show that quantum contextuality for a relativistic electron moving in a Coulomb potential naturally emerges if relativistic effects are taken into account. The contextuality can be identified through the violation of noncontextuality inequalities. We also discuss quantum contextuality for the free Dirac electron as well as the relativistic Dirac oscillator. DOI: 10.1103/PhysRevA.87.022109

  • 46.
    Chen, Tao
    Stockholm University, Faculty of Science, Department of Physics.
    Ions colliding with molecules and molecular clusters: fragmentation and growth processes2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this work we will discuss fragmentation and molecular growth processes in collisions of Polycyclic Aromatic Hydrocarbon (PAH) molecules, fullerenes, or their clusters with atoms or atomic ions. Simple collision models as well as molecular structure calculations are used to aid the interpretations of the present and other experimental results. Fragmentation features at center-of-mass collision energies around 10 keV are dominated by interactions between the fast ion/atom and the electron cloud in the molecules/clusters (electronic stopping processes). This electronic excitation energy is rapidly distributed on the vibrational degrees of freedom of the molecule or of the molecules in a cluster and may result in fragmentation. Here, the fragmentation is statistical and favors the lowest-energy dissociation channels which are losses of intact molecules from clusters, H- and C2H2-losses from isolated PAHs, and C2-loss from fullerene monomers. We will also discuss the possibility of formation of molecular H2 direct from native PAHs which reach high enough energies when interacting with ions, electrons, or photons.

    For the experiments at lower center of mass collision energies (~100 eV) a single atom may be knocked out in close atom-atom interaction. Such non-statistical fragmentation are due to nuclear stopping processes and gives highly reactive fragments which may form covalent bonds with other molecules in a cluster on very short time scales (picoseconds). This process may be important when considering the formation of new species. For collision between 12 keV Ar2+ and clusters of pyrene (C16H10) molecules, new molecules, e.g. C17H10+, C30H18+, C31H19+, etc are detected. We also observe molecular fusion processes for He and Ar ions colliding with clusters of C60 molecules. These and related molecular fusion processes may play a key role for understanding molecular growth processes under certain astrophysical conditions.

  • 47.
    Chen, Tao
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Gatchell, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Stockett, Mark H.
    Stockholm University, Faculty of Science, Department of Physics.
    Rudy, Delaunay
    Domaracka, Alicja
    Micelotta, Elisabetta R.
    Tielens, Alexander G. G. M.
    Rousseau, Patrick
    Adoui, Lamri
    Huber, Bernd A.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Cederquist, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Formation of H2 from internally heated polycyclic aromatic hydrocarbons: Excitation energy dependence2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 14, article id 144305Article in journal (Refereed)
    Abstract [en]

    We have investigated the effectiveness of molecular hydrogen (H-2) formation from Polycyclic Aromatic Hydrocarbons (PAHs) which are internally heated by collisions with keV ions. The present and earlier experimental results are analyzed in view of molecular structure calculations and a simple collision model. We estimate that H-2 formation becomes important for internal PAH temperatures exceeding about 2200 K, regardless of the PAH size and the excitation agent. This suggests that keV ions may effectively induce such reactions, while they are unlikely due to, e.g., absorption of single photons with energies below the Lyman limit. The present analysis also suggests that H-2 emission is correlated with multi-fragmentation processes, which means that the [PAH-2H](+) peak intensities in the mass spectra may not be used for estimating H-2-formation rates.

  • 48.
    Dahlström, J. M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Carette, Thomas
    Stockholm University, Faculty of Science, Department of Physics.
    Lindroth, Eva
    Stockholm University, Faculty of Science, Department of Physics.
    Diagrammatic approach to attosecond delays in photoionization2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 86, no 6, p. 061402-Article in journal (Refereed)
    Abstract [en]

    We study laser-assisted photoionization by attosecond pulses using a time-independent formalism based on diagrammatic many-body perturbation theory. Our aim is to provide an ab initio route to the delays for this above-threshold ionization process, which is essential for a quantitative understanding of attosecond metrology. We present correction curves for characterization schemes of attosecond pulses, such as streaking, that account for the delayed atomic response in ionization from neon and argon. We also verify that photoelectron delays from many-electron atoms can be measured using similar schemes if, instead, the so-called continuum-continuum delay is subtracted. Our method is general and it can be extended also to more complex systems and additional correlation effects can be introduced systematically. DOI: 10.1103/PhysRevA.86.061402

  • 49.
    Dahlström, J. M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    L'Huillier, A.
    Maquet, A.
    Introduction to attosecond delays in photoionization2012In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 45, no 18, p. 183001-Article in journal (Refereed)
    Abstract [en]

    This tutorial presents an introduction to the interaction of light and matter on the attosecond timescale. Our aim is to detail the theoretical description of ultra-short time delays and to relate these to the phase of extreme ultraviolet (XUV) light pulses and to the asymptotic phaseshifts of photoelectron wave packets. Special emphasis is laid on time-delay experiments, where attosecond XUV pulses are used to photoionize target atoms at well-defined times, followed by a probing process in real time by a phase-locked, infrared laser field. In this way, the laser field serves as a 'clock' to monitor the ionization event, but the observable delays do not correspond directly to the delay associated with single-photon ionization. Instead, a significant part of the observed delay originates from a measurement induced process, which obscures the single-photon ionization dynamics. This artefact is traced back to a phaseshift of the above-threshold ionization transition matrix element, which we call the continuum-continuum phase. It arises due to the laser-stimulated transitions between Coulomb continuum states. As we shall show here, these measurement-induced effects can be separated from the single-photon ionization process, using analytical expressions of universal character, so that eventually the attosecond time delays in photoionization can be accessed.

  • 50.
    Dahlström, J. Marcus
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Guénot, Diego
    Klünder, Kathrin
    Gisselbrecht, Matthieu
    Mauritsson, Johan
    L'Huillier, Anne
    Maquet, Alfred
    Taïeb, Richard
    Theory of attosecond delays in laser-assisted photoionization2013In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 414, p. 53-64Article in journal (Refereed)
    Abstract [en]

    We study the temporal aspects of laser-assisted extreme ultraviolet (XUV) photoionization using attosecond pulses of harmonic radiation. The aim of this paper is to establish the general form of the phase of the relevant transition amplitudes and to make the connection with the time-delays that have been recently measured in experiments. We find that the overall phase contains two distinct types of contributions: one is expressed in terms of the phase-shifts of the photoelectron continuum wavefunction while the other is linked to continuum–continuum transitions induced by the infrared (IR) laser probe. Our formalism applies to both kinds of measurements reported so far, namely the ones using attosecond pulse trains of XUV harmonics and the others based on the use of isolated attosecond pulses (streaking). The connection between the phases and the time-delays is established with the help of finite difference approximations to the energy derivatives of the phases. The observed time-delay is a sum of two components: a one-photon Wigner-like delay and an universal delay that originates from the probing process itself.

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