Atomic Physics of High Z-Systems

Abstract

On April 1*, 1976, the “Unilac” of the G.S.I. Darmstadt accelerated for the first time ^{2 3 8}U ions to an energy of 1.6 GeV (6.7 MeV/u), which is high enough to overcome the Coulomb barrier in U-U-collisions (6.3 MeV/u). This event opened a new field of atomic physics: The investigation of atomic processes in strong fields^{1) 2) 3)} with coupling strengths Zα>1. The strong fields are produced in heavy ion-atom collisions by superposition of the Coulomb fields of both colliding nuclei. Let us look at an U-U-encounter at a bombarding energy of 1.4 GeV, which corresponds to a relative velocity of ν = 0.11 × c. In a head on collision the distance of closest approach is 2a = 17 fm; at this distance the U-nuclei start to overlap with the outer tails of their wavefunctions. The collision time defined as the time during which the nuclei move ^±100 fm around the turning point is about 1.8 × 10^-19s. During this time the effective Coulomb-field outside a sphere of 100 fm radius originates from the sum of the charges Z_u = Z₁ + Z₂= 184 of both nuclei, or more accurately the electrons are exposed during the collision to a two center Coulombfield determined by Z₁, Z₂ and the internuclear distance ${\rm{\vec R}}$(t) which is time dependant corresponding to the motion on the hyperbolic orbits.