1. Institut für Physikalische und Theoretische Chemie der TU Braunschweig, Hans-Sommer-Strasse 10, D-38106 Braunschweig, Germany, Institute of Electronics, Bulgarian Academy of Sciences, Boulevard Tzarigradsko Shoussee 72, 1784 Sofia, Bulgaria, and Alphion Corporation, 196 Princeton Hightstown Road, Princeton Junction, New Jersey 08550