The impact parameter dependence of the L-and K-vacancy production of target and projectile was measured in I-Ag collisions at 40 MeV and 63 MeV in a X-ray particle coincidence experiment. A large L-vacancy production is found for impact parameters b smaller than the L-shell radius with a pronounced increase for b<250 fm. The L-shell excitation is discussed in terms of 4f~ excitation and vacancy transfer in the 3pa, 3d~z, 3 da molecular orbitals through 3 p ~-3 p a and 3 d 6-3 drc-3 d o-rotational-couplings. The 2pa excitation probability is obtained from the summed K-vacancy production probability, and compared with the prediction of 2pa excitation in a multiple collision process. The latter is determined by folding the measured I L-shell excitation probability with the impact parameter dependence of the 2p~-2pa rotational coupling. The discrepancy observed at small impact parameters suggests an enhanced 2p~ and 2pa excitation at small impact parameters in a one-collision process. The L-and K-vacancy sharing probabilities are found to be impact parameter independent over a large impact parameter range.

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Abstract:The dependence of molecular X-ray emission on the azimuthal angle in heavy ion collisions has been measured as a function of the impact parameter in the collision systems F-AI and CI-CI at projectile energies of 20 and 48 MeV respectively. The values of the observed azimuthal anisotropies agree with dynamical calculations of molecular orbital X-ray emission, but they are at variance with predictions of the kinematic dipole model. Considerable theoretical progress in the description of molecular orbital (M0) X-ray emission has been made in the last years. Different approaches exist to explain the observed angular distribution of MO X-rays. These theories lead to different predictions for the dependence on the azimuthal angle @, i.e. the angle between the scattering plane and the detection plane of the X-rays.The kinematic dipole model proposed by Hartung and Fricke /1,2/ predicts a strong dependence of the X-ray emission on the azimuthal angle @ around the united atom energy E u. In contrast to this the calculations of Anholt, using the electron slip model /3/ give only a small effect, becoming sizeable only at impact parameters larger than the K-shell radius of the heavier collision partner.In order to check these different predictions a coincidence experiment was performed at the ETH EN-tandem accelerator. A 20 MeV F3 + and a 48 MeV C17+ beam were directed onto a 55 ~g/cm 2 selfsupporting AI target or a 50Dg/cm 2 NaCl-target on a 15~g/cm 2 carbon backing respectively.The emitted X-rays were detected with a 80 ~2 Si(Li)-detector placed at 90 ~ to the beam axis. To determine the scattering plane the Xrays were recorded in coincidence with the scattered ions, which were detected in a parallel plate avalanche counter.A system of collimators kept the diameter of the beam spot below 0.6 mm. The molecular X-ray intensity was approximately a factor of 105 smaller than the characteristic K-lines, therefore the characteristic X-rays were suppressed with a beryllium absorber to get a reasonable counting rate in the Si(Li)-detector. The anode of the avalanche counter was divided into 4 equal sectors with an aperture of • ~ to determine two scattering planes which were either perpendicular (@=90 ~ or parallel (@=0 ~ to the plane defined by the photon counter and the beam axis. The impact parameter regions were defined by an annular collimator placed in front of the counter, and by the distance between detector and target. The detector was aligned so that the counting rates in the 4 sectors were equal. A particle detector at a fixed angle of 30 ~ to the beam axis served as a monitor. The signals of the X-ray detector and Horizontal bars indicate the impact parameter ranges due to the finite angular resolution of the particle counter, vertical bars correspond to the statistical error.

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