Impact parameter dependence of K-shell excitation probabilities in Ar-Ar-collisions

The impact parameter (b) dependence of the K-shell excitation probabilites (Psub(K)(b)) in the Ar on Ar collision system at 4.5, 8, 12 and 20 MeV impact energy have been measured over a wide b range (from the kinematic peak to far outside the adiabatic peak) for different projectile charge states. In agreement with the data of Lutz et al. and Nolte et al. a very strong increase of Psub(K)(b) towards very small b values (corresponding to scattering angles up to 40sup(o)) has been observed. From the measured b and projectile charge state dependences it has been derived for the systems investigated here the 2pπ-2psigma rotational coupling mechanism gives the dominant contribution to the K-shell excitation as well at the very small b (kinematic peak regime) as at the very large b (adiabatic peak regime). The analysis of the data shows that in disagreement with the general assumption of a b independent 2pπ vacancy probability P2sub(p)sub(π), the probability P2sub(p)sub(π) increases towards smaller b. At the high projectile energy the data confirm the results of Nolte et al. of large dynamically produced probability P2sub(p)sub(π)sup(d) and of strong contributions of direct 2psigma and 1ssigma excitation to the measured Psub(K)(b) at small impact parameters. (orig.)