The help of Dr. J. Struckmeier from GSI with the modification of the PARMILA-program has been indispensable and is greatly appreciated. One of the authors (S. Ivanov) is on leave from the Sofia University;
Bulgaria;
and acknowledges a fellowship from the International Atomic Energy Agency. Furthermore we like to thank Professor J. Los of the FOM-Institute for Atomic and Molecular Physics for many stimulating discussions. The work described here was performed as part of the research program of the association agreement between the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and EURATOM;
with financial support from the Nederlandse Organisatie voor Zuiver-Wetenshappelijk Onderzoek (ZWO);
the Neder- landse Ministerie van Onderwi[!text type='js']js[!/text] en Wetenschappen;
and EURATOM;