| 1. |
- Ahrens, J, et al.
(författare)
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First measurement of the helicity-dependent (gamma)over-right-arrow(p)over-right-arrow -> p eta differential cross-section
- 2003
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Ingår i: European Physical Journal A. Hadrons and Nuclei. - : Springer Science and Business Media LLC. - 1434-6001. ; 17:2, s. 241-244
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Tidskriftsartikel (refereegranskat)abstract
- The helicity dependence of the (γ) over right arrow(p) over right arrow -+ peta reaction has been measured for the first time at a center-of-mass angle theta(eta)*= 70degrees in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam. and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis.
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| 2. |
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| 3. |
- Ahrens, J, et al.
(författare)
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Helicity dependence of the (gamma)over-right-arrow (p)over-right-arrow -> n pi(+) pi(0) reaction in the second resonance region
- 2003
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 551:1-2, s. 49-55
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Tidskriftsartikel (refereegranskat)abstract
- The helicity dependence of the total cross section for the (γ) over right arrow(p) over right arrownpi(+) pi(0) reaction has been measured for the first time at incident photon energies from 400 to 800 MeV The measurement was performed with the large acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, due to the intermediate production of the D-13 resonance. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 4. |
- Ahrens, J, et al.
(författare)
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Helicity dependence of the gamma p -> N pi channels and multipole analysis in the Delta region
- 2004
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Ingår i: European Physical Journal A. Hadrons and Nuclei. - : Springer Science and Business Media LLC. - 1434-6001. ; 21:2, s. 323-333
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Tidskriftsartikel (refereegranskat)abstract
- A high-quality double-polarization data set for the helicity dependence of the total and differential cross-sections for both gammap --> Npi channels in the Delta region has been obtained in the framework of the GDH experiment. The experiment, performed at the Mainz microtron MAMI, used a 4pi detection system, a circularly polarized photon beam, and a longitudinally polarized frozen-spin target. These data are included in the database to perform a multipole analysis to determine the properties of the Delta(1232)-resonance. For the resonant Delta(1232) multipoles we find a very good agreement with previous analyses, while the nonresonant ones show significant deviations.
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| 5. |
- Bergmann, U, et al.
(författare)
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High-resolution X-ray spectroscopy of rare events : a different look at local structure and chemistry
- 2001
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Ingår i: Journal of Synchrotron Radiation. - Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley, CA 94720 USA. Univ Calif Davis, Dept Appl Sci, Davis, CA 95616 USA. CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA. : MUNKSGAARD INT PUBL LTD. - 0909-0495 .- 1600-5775. ; 8, s. 199-203
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Tidskriftsartikel (refereegranskat)abstract
- The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal K beta fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the K beta spectrum resulting from valence-level and 'interatomic' ligand to metal transitions contain detailed information on the ligand-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorption fine structure (XAFS) spectra using hard ( 10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.
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| 6. |
- Dutz, H, et al.
(författare)
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Experimental check of the Gerasimov-Drell-Hearn sum rule for H-1
- 2004
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Ingår i: Physical Review Letters. - 1079-7114. ; 93:3
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Tidskriftsartikel (refereegranskat)abstract
- For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2-2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2-2.9 GeV yields [254+/-5(stat)+/-12(syst)] mub with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.
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| 7. |
- Dutz, H, et al.
(författare)
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First measurement of the Gerasimov-Drell-Hearn sum rule for H-1 from 0.7 to 1.8 GeV at ELSA
- 2003
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Ingår i: Physical Review Letters. - 1079-7114. ; 91:19: 192001
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Tidskriftsartikel (refereegranskat)abstract
- To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68-1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4pi detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9+/-2.4(stat)+/-2.2(syst)] mub.
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| 8. |
- Messinger, Johannes, 1963-, et al.
(författare)
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Absence of Mn-centered oxidation in the S2 → S3 Transition: : Implications for the mechanism of photosynthetic water oxidation
- 2001
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Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 123:32, s. 7804-7820
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Tidskriftsartikel (refereegranskat)abstract
- A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S-3 --> [S-4] --> So transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn K beta X-ray emission spectroscopy (K beta XES) to this problem for the first time. The K beta XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S-2 --> S3 transition, in contrast to the S-0 --> S-1 and S-1 --> S-2 transitions. does not involve a Mn-centered oxidation. On the basis of new structural data from the S-3-state, manganese mu -oxo bridge radical formation is proposed for the S-2 --> S-3 transition, and three possible mechanisms for the O-O bond formation are presented.
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