1. Department of Chemistry, University of California, Berkeley, California 94720-1460; Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200 Australia; and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 5, 1-1-1 Higushi, Tsukuba, Ibaraki 305-8565, Japan