1. School of EECS; Oregon State University; Corvallis; Oregon; 97331-5501; USA
2. Department of Chemistry; Oregon State University; Corvallis; OR; 97331-4003; USA
3. Hewlett-Packard Company; Corvallis; OR; 97330-4239; USA
4. Department of Engineering; Cambridge University; Cambridge; CB2 1PZ; UK