1. Contribution from the Department of Physical Chemistry, University of Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany, Department of Physical Chemistry, Darmstadt University of Technology, Petersenstrasse 20, D-64287 Darmstadt, Germany, Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, and Department of Chemistry, Northwestern University, Evanston, Illinois 60208