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Träfflista för sökning "WFRF:(Wahlström Claes Göran) srt2:(2000-2004)"

Sökning: WFRF:(Wahlström Claes Göran) > (2000-2004)

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1.
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2.
  • Albert, F, et al. (författare)
  • Laser produced X-ray source in the 10-60 keV range at 1 kHz. Modified irradiation schemes in order to reach medical imaging quality
  • 2001
  • Ingår i: Journal de Physique IV: Proceedings. - EDP Sciences. - 1764-7177. ; 11:PR2, s. 429-432
  • Tidskriftsartikel (refereegranskat)abstract
    • By tightly focusing ultra-short pulses from a Ti:sapphire terawatt laser onto a high-Z metallic target, hard x-ray pulses of short duration are produced. In most of our previous work concerning x-rays, a 150 mJ laser pulse with a 110 A duration has been used. Using mostly tin and tantalum targets, hard x-rays in the 10-60 keV range have been produced and used in differential absorption imaging around the K-alpha absorption edge of a contrast agent and also in imaging employing gated viewing for suppression of scattered radiation. In order to increase the x-ray yield (shortening the acquisition time) an increase in the laser repetition rate is desirable while still staying in the K-alpha energy regime. We have used a I kHz repetition-rate laser delivering 35 fs pulses in order to work towards these goals. We have clear evidence of hard x-ray generation above 30 keV, even for low laser pulse energies. We also studied the effect of a fs prepulse. The medical imaging capability of the source was explored. The use of a prepulse has been optimized in order to improve the image quality as well as the overall x-ray generation yield.
3.
  • Blagoev, K, et al. (författare)
  • Radiative lifetimes of 6sns S-3(1) and 6snd D-3(1) excited states of HgI
  • 2002
  • Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - American Physical Society (APS). - 1050-2947. ; 66:3
  • Tidskriftsartikel (refereegranskat)abstract
    • Natural radiative lifetimes have been measured along the 6sns S-3(1) (n=7-10) and 6snd D-3(1) (n=6-11) Rydberg series of neutral mercury using time-resolved laser-induced fluorescence spectroscopy in a laser-produced mercury plasma. The states of Hg I investigated in the present work were populated from the ground state by a stepwise excitation process. Relativistic Hartree-Fock calculations, including intravalence interactions and core-polarization effects, have been performed and the theoretical lifetimes have been compared with the measurements.
4.
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5.
  • Cacciani, P, et al. (författare)
  • Isotope dependent predissociation in the C-1 Sigma(+) ,v=0 and v=1 states of CO
  • 2001
  • Ingår i: European Physical Journal D. Atomic, Molecular, Optical and Plasma Physics1998-01-01+01:00. - EDP Sciences. - 1434-6060. ; 15:1, s. 47-56
  • Tidskriftsartikel (refereegranskat)abstract
    • Rotationally resolved spectral lines in the C-X(1,0) band of carbon monoxide are investigated under high resolution using a coherent vacuum ultraviolet laser source, continuously tunable near 107 nm. Transition frequencies are determined by calibrating against a reference standard of iodine lines, recorded with saturation spectroscopy in the visible range, yielding an absolute accuracy of 0.003 cm(-1) in the vacuum ultraviolet. Improved molecular constants for the excited state are clarified and Mo effects of perturbation are found at the present level of accuracy. Line broadening measurements result in information on the excited state lifetime of the (CC)-C-1 Sigma (+),v = 1 state fur five natural isotopomers: of cal bon monoxide: tau((CO)-C-12-O-17) = 280 ps, tau((CO)-C-12-O-18) = 210 ps. tau((CO)-C-13-O-16) = 295 ps, tau((OO)-O-13-O-17) = 160 ps and tau((CO)-C-13-O-18) = 150 Ps. Within the accuracy of the present measurements: Ilo effects: of J-dependent lifetimes: were observed, for neither of the isotopomers. In addition direct time domain measurements: of the lifetime of the C(1)Sigma (+),v = 0 and v = 1 states of the main isotopomer are performed ill a pump-mode experiment using a picosecond VUV-laser, yielding tau((CO)-C-12-O-16) = 1780 ps for v = 0 and tau((CC)-C-12-C-16)) = 625 ps for v = 1. For C(1)Sigma (+), v = 0 in (CO)-C-12-O-16 and (CO)-C-13-O-16 the same lifetime is found; this lifetime matches experimental values of the oscillator strength and hence supports previous results showing pure radiative decay ill this state; the error margins however do not exclude some low level of predissociation. The measurements indicate that the C(1)Sigma (+),v = 0 state of the (CO)-C-13-O-18 isotopomer is predissociated with an estimated yield of 17% (i.e. above the level of predissociation for (CO)-C-12-O-16.) From the combined data predissociation yields: upon excitation of the C(1)Sigma (+),v = 1 state are derived, lying in the range 0.84-0.91. for the five less abundant isotopomers; for the main (CO)-C-12-O-16 isotopomer a strongly deviating predissociation yield of 0.65 is deduced.
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6.
  • Cacciani, P, et al. (författare)
  • Predissociation of the 4p pi L-1 Pi Rydberg state of carbon monoxide
  • 2002
  • Ingår i: Chemical Physics. - Elsevier. - 0301-0104. ; 282:1, s. 63-73
  • Tidskriftsartikel (refereegranskat)abstract
    • Time-domain and frequency-domain spectroscopic experiments have been performed on the (4ppi)L(1)Pi, v = 0 Rydberg state in three isotopomers of carbon monoxide. Accurate values for the excited state lifetimes of the f-parity components have been determined: tau((CO)-C-12-O-16) = 1.08 +/- 0.05 ns, tau((CO)-C-13-O-16) = 72 +/- 10 ps and tau((CO)-C-13-O-18) = 29 +/- 6 ps. The spectral resolution in the frequency-domain experiment goes as far as the limit imposed by the natural lifetime; Q-branch lines, or f-parity components of the heavier isotopes, are resolved for the first time. Highly accurate transition frequencies are determined in a molecular beam experiment using comparison and interpolation with a saturated iodine reference standard. The results reveal a number of perturbations and predissociation mechanisms, displaying a high degree of complexity in the energetic region of the 4p Rydberg states of CO with strong isotopic effects. (C) 2002 Elsevier Science B.V. All rights reserved.
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7.
  • Delfin, C, et al. (författare)
  • Influence of laser pulse duration on relativistic channels
  • 2002
  • Ingår i: Physics of Plasmas. - American Institute of Physics. - 1070-664X. ; 9:3, s. 937-940
  • Tidskriftsartikel (refereegranskat)abstract
    • A high-power (10 TW) laser is employed to generate relativistic channels in an underdense plasma. The lengths of the channels are measured by imaging the Thomson-scattered light, and the gas densities are determined through the forward Raman scattered light. The laser-pulse parameters are varied and their impact on the channel formation is studied. It is found that increasing the laser pulse duration in many cases produces longer channels, even as this implies reducing the laser peak power. A theoretical discussion is presented, proposing an explanation of the experimental results. (C) 2002 American Institute of Physics.
8.
  • Descamps, D, et al. (författare)
  • Extreme ultraviolet interferometry measurements with high-order harmonics
  • 2000
  • Ingår i: Optics Letters. - Optical Society of America. - 0146-9592. ; 25:2, s. 135-137
  • Tidskriftsartikel (refereegranskat)abstract
    • We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2-10(20) electrons/cm(3) are measured. (C) 2000 Optical Society of America. OCIS codes: 190.0190, 190.4160, 190.7110, 120.3180, 140.7240, 350.4500.
9.
  • Descamps, D, et al. (författare)
  • Two- and three-photon ionization of rare gases using femtosecond harmonic pulses generated in a gas medium
  • 2001
  • Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - American Physical Society (APS). - 1050-2947. ; 64:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The fifth harmonic of a 60-fs 10-Hz terawatt titanium:sapphire laser generated in Xe gas is used to ionize Xe and Kr atoms by nonresonant two-photon absorption and At atoms through a quasiresonant three-photon process. At relatively low harmonic yields, the variation of the number of Kr and Xe ions with the number of harmonic photons is quadratic. At high harmonic yields, it deviates from a quadratic behavior, which we attribute to the influence of ionization in the harmonic process distorting the harmonic wave front. The two-photon ionization cross sections in Xe and Kr at 160 nm are found to be comparable.
10.
  • Gahn, C, et al. (författare)
  • Generating positrons with femtosecond-laser pulses
  • 2000
  • Ingår i: Applied Physics Letters. - American Institute of Physics. - 0003-6951. ; 77:17, s. 2662-2664
  • Tidskriftsartikel (refereegranskat)abstract
    • Utilizing a femtosecond table-top laser system, we have succeeded in converting via electron acceleration in a plasma channel, low-energy photons into antiparticles, namely positrons. The average intensity of this source of positrons is estimated to be equivalent to 2x10(8) Bq and it exhibits a very favorable scaling for higher laser intensities. The advent of positron production utilizing femtosecond laser pulses may be the forerunner to a table-top positron source appropriate for applications in material science, and fundamental physics research like positronium spectroscopy. (C) 2000 American Institute of Physics. [S0003-6951(00)00143-1].
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